I rarely deal with valve/tube amps and am more familiar with radio
output-matching transformers primary-windings having DC resistance of
windings of hundreds of ohms.
Been in a shed for years and no known history so treading cautiously.
Measuring the DC resistance of each side to centre tap of this Marshall amp,
shows only about 15 ohm each way.
Amp is 100W o/p using 4 EL34 , two paralleled anodes going to each side of
this impedance matcher.
Does 15 ohm look right for each of these primary coils ?
Output resistance of about 2.5 and 4.5 for 4 and 8 ohm settings seems fine.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

I will check a couple of my amps tonight and see what the primary
_resistance_ is on the output transformers. One uses a pair of EL-34s
each channel, the other a pair of 6550s each channel. They are
audio-amps, not guitar amps, but the readings should be similar....

Peter Wieck
Wyncote, PA

Peter Wieck wrote in message
oups.com...
I will check a couple of my amps tonight and see what the primary
_resistance_ is on the output transformers. One uses a pair of EL-34s
each channel, the other a pair of 6550s each channel. They are
audio-amps, not guitar amps, but the readings should be similar....

Peter Wieck
Wyncote, PA


Yes, thanks. The only ones I've got lying around are small low wattage radio
receiver type ones with range of 600 to 800 ohms on high voltage side. I
could not find anything googling. It had obviously been overdriven as
charred but functional gate resistors.

Peter Wieck wrote in message
oups.com...
I will check a couple of my amps tonight and see what the primary
_resistance_ is on the output transformers. One uses a pair of EL-34s
each channel, the other a pair of 6550s each channel. They are
audio-amps, not guitar amps, but the readings should be similar....

Peter Wieck
Wyncote, PA


Probably doesn't have any shorted turns and is OK.
I put a 1KHz AC LCR meter on the coils and it comes out to 32 Henries for
each half.
I wonder what guage of wire to give only 15 ohms for all those turns.

My thoughts are that a guitar amp which spends most of its time driven
to distortion will have transformers able to take considerable abuse
without failure. So, I expect it to be much lower than an audio amp
(which I will check anyway).

Peter Wieck
Wyncote, PA

Peter Wieck wrote in message
ups.com...
My thoughts are that a guitar amp which spends most of its time driven
to distortion will have transformers able to take considerable abuse
without failure. So, I expect it to be much lower than an audio amp
(which I will check anyway).

Peter Wieck
Wyncote, PA


I found this in the archives, putting
matching transformer henries ohms
as keywords in google usenet, on a guitar group

"Primary winding, by direct meter measurement with a Fluke 8600-A:
DCR Blue side to center tap = 44.91 ohms
DCR Brown side to center tap = 51.03 ohms "

I assume DCR is DC resistance of that matching transformer.
Mine was measuring 14 and 15.5 ohms which seemed a
bit unbalanced but no worse than this example

n cook wrote:



I assume DCR is DC resistance of that matching transformer.
Mine was measuring 14 and 15.5 ohms which seemed a
bit unbalanced but no worse than this example


One side of center will have more DC resistance because more wire is
required for the same number of windings.

GL,
Bill

"n cook" wrote in message
...
Peter Wieck wrote in message
ups.com...
My thoughts are that a guitar amp which spends most of its time driven
to distortion will have transformers able to take considerable abuse
without failure. So, I expect it to be much lower than an audio amp
(which I will check anyway).

Peter Wieck
Wyncote, PA


I found this in the archives, putting
matching transformer henries ohms
as keywords in google usenet, on a guitar group

"Primary winding, by direct meter measurement with a Fluke 8600-A:
DCR Blue side to center tap = 44.91 ohms
DCR Brown side to center tap = 51.03 ohms "

I assume DCR is DC resistance of that matching transformer.
Mine was measuring 14 and 15.5 ohms which seemed a
bit unbalanced but no worse than this example


This may sound like a silly suggestion but .....

Why not just ring Jim Marshall, and ask ? They're only up in Milton Keynes,
and are a pretty friendly company.

FWIW, for a paralleled pair of EL34s in this application, I'd say that the
readings you have obtained are fair. If it has been standing for a long
time, I would strongly advise bringing the power up slowly on a variac. The
main resevoir and smoothing caps, may not be in a state to accept HT right
away, and might need reforming.

If you are a reader of " Television " magazine, look out for the new one
that will be coming out at the end of March. It's going to be called "
Technology @ Home ", and when I get round to it, you can read my ramblings
on repairs to valved group and PA amps.

Arfa

"n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
--- on the heady topic of "Marshall 4140 Amp question"

Hi,

That 15 ohms is about right for a high power output transformer.

The first thing is to check the input grid coupling caps for leakage.
Then the screen grid resistors for opens. Then remove the finals and
apply power briefly to measure that the grid negative bias is healthy.
Check the tubes. Mount them back in. Then on to actual running tests.
Beware of the B+, high DC with ++joules is quite painful and lethal.
Do discharge all supply electros first thing before working inside!

A*s*i*m*o*v


nc From: "n cook"
nc Xref: core-easynews rec.antiques.radio+phono:359961
nc sci.electronics.repair:355619

nc I rarely deal with valve/tube amps and am more familiar with radio
nc output-matching transformers primary-windings having DC resistance of
nc windings of hundreds of ohms.
nc Been in a shed for years and no known history so treading cautiously.
nc Measuring the DC resistance of each side to centre tap of this
nc Marshall amp, shows only about 15 ohm each way.
nc Amp is 100W o/p using 4 EL34 , two paralleled anodes going to each
nc side of this impedance matcher.
nc Does 15 ohm look right for each of these primary coils ?
nc Output resistance of about 2.5 and 4.5 for 4 and 8 ohm settings seems
nc fine.
nc --
nc Diverse Devices, Southampton, England
nc electronic hints and repair briefs , schematics/manuals list on
nc http://home.graffiti.net/diverse:graffiti.net/

.... "'Blues,' for all you people who paid to get in." - Pete Townshend

"Asimov" wrote in message
...
"n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
--- on the heady topic of "Marshall 4140 Amp question"

Hi,

That 15 ohms is about right for a high power output transformer.

The first thing is to check the input grid coupling caps for leakage.
Then the screen grid resistors for opens. Then remove the finals and
apply power briefly to measure that the grid negative bias is healthy.
Check the tubes. Mount them back in. Then on to actual running tests.
Beware of the B+, high DC with ++joules is quite painful and lethal.
Do discharge all supply electros first thing before working inside!

A*s*i*m*o*v


nc From: "n cook"
nc Xref: core-easynews rec.antiques.radio+phono:359961
nc sci.electronics.repair:355619

nc I rarely deal with valve/tube amps and am more familiar with radio
nc output-matching transformers primary-windings having DC resistance of
nc windings of hundreds of ohms.
nc Been in a shed for years and no known history so treading cautiously.
nc Measuring the DC resistance of each side to centre tap of this
nc Marshall amp, shows only about 15 ohm each way.
nc Amp is 100W o/p using 4 EL34 , two paralleled anodes going to each
nc side of this impedance matcher.
nc Does 15 ohm look right for each of these primary coils ?
nc Output resistance of about 2.5 and 4.5 for 4 and 8 ohm settings seems
nc fine.
nc --
nc Diverse Devices, Southampton, England
nc electronic hints and repair briefs , schematics/manuals list on
nc http://home.graffiti.net/diverse:graffiti.net/

... "'Blues,' for all you people who paid to get in." - Pete Townshend


I agree with Asimov regarding leaky grid coupling caps, but I am not a fan
of running tubed / valved group amps without the output bottles in place, no
matter how briefly ( but if you must, take note that Asimov does say
briefly - IMO 40 seconds max ). Without the output stage drawing current
from the HT line, this can rise easily by 20% or more, from its nominal
running value, which can stress the power supply caps, and any other
decouplers along the way on the HT line.

This is not quite such a bad thing if the amp employs semiconductor HT
rectifiers, as the HT rail will be high of its nominal running value until
the outputs warm up anyway, and this factor is designed in when the voltage
ratings for the caps are chosen by the designer.

However, if the amp employs a thermionic rectifier, its output will come up
slowly, as will the load imposed by the outputs as they warm up. This
results in an HT rail that comes up to a value, and stays there. If you now
run the amp up with the outputs removed, the output of the reccy will come
up with no load on it, which is an unexpected condition for the designer,
and the resulting unloaded voltage may well come very close to, or even
exceed the voltage ratings of some caps on the HT line.

You should be able to check the bias voltage with the outputs in place.
Marshall generally tend to use a negative supply of around 50v for the bias
source, potted down and fed to the outputs' control grids via a few
resistors.

If a seperate bias supply were missing or low, this would more than likely
result in excess output stage current demand, with consequent lowering of
the HT rail. Glowing anodes are the order of the day here, but I would
rather have that, as the outputs will stand this for a considerable time
without damage, rather than have exploding or shorting electrolytics,
brought on by voltage stress.

Arfa

What is the problem with the amp?

--
==========================
Jeff Stielau
Shoreline Electronics Repair
344 East Main Street
Clinton,CT 06413
860-399-1861
860-664-3535 (fax)

========================

"n cook" wrote in message
...
I rarely deal with valve/tube amps and am more familiar with radio
output-matching transformers primary-windings having DC resistance of
windings of hundreds of ohms.
Been in a shed for years and no known history so treading cautiously.
Measuring the DC resistance of each side to centre tap of this Marshall
amp,
shows only about 15 ohm each way.
Amp is 100W o/p using 4 EL34 , two paralleled anodes going to each side of
this impedance matcher.
Does 15 ohm look right for each of these primary coils ?
Output resistance of about 2.5 and 4.5 for 4 and 8 ohm settings seems
fine.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"Arfa Daily" bravely wrote to "All" (19 Jan 06 01:49:16)
--- on the heady topic of " Marshall 4140 Amp question"

AD From: "Arfa Daily"
AD Xref: core-easynews sci.electronics.repair:355730

AD lowering of the HT rail. Glowing anodes are the order of the day here,
AD but I would rather have that, as the outputs will stand this for a
AD considerable time without damage, rather than have exploding or
AD shorting electrolytics, brought on by voltage stress.


Arfa,

High voltage electros have a surge rating about 20% higher than the
wvdc specified on the can. A brief surge less than the few seconds it
takes to get a reading will not usually be a problem. OTOH leaving the
tubes in runs the real risk of having a large current burn out a
transformer winding. Which risk would you prefer to take; ending up
with on your workbench: a smoking rectifier with a zapped electro, or
an open winding in a cherished rare vintage audio output transformer?

A*s*i*m*o*v

.... Transformer designers take turns doing it.

Arfa Daily wrote in message
...

"Asimov" wrote in message
...
"n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
--- on the heady topic of "Marshall 4140 Amp question"
... "'Blues,' for all you people who paid to get in." - Pete Townshend


I agree with Asimov regarding leaky grid coupling caps, but I am not a fan
of running tubed / valved group amps without the output bottles in place,
no
matter how briefly ( but if you must, take note that Asimov does say
briefly - IMO 40 seconds max ). Without the output stage drawing current
from the HT line, this can rise easily by 20% or more, from its nominal
running value, which can stress the power supply caps, and any other
decouplers along the way on the HT line.

This is not quite such a bad thing if the amp employs semiconductor HT
rectifiers, as the HT rail will be high of its nominal running value until
the outputs warm up anyway, and this factor is designed in when the
voltage
ratings for the caps are chosen by the designer.

However, if the amp employs a thermionic rectifier, its output will come
up
slowly, as will the load imposed by the outputs as they warm up. This
results in an HT rail that comes up to a value, and stays there. If you
now
run the amp up with the outputs removed, the output of the reccy will come
up with no load on it, which is an unexpected condition for the designer,
and the resulting unloaded voltage may well come very close to, or even
exceed the voltage ratings of some caps on the HT line.

You should be able to check the bias voltage with the outputs in place.
Marshall generally tend to use a negative supply of around 50v for the
bias
source, potted down and fed to the outputs' control grids via a few
resistors.

If a seperate bias supply were missing or low, this would more than likely
result in excess output stage current demand, with consequent lowering of
the HT rail. Glowing anodes are the order of the day here, but I would
rather have that, as the outputs will stand this for a considerable time
without damage, rather than have exploding or shorting electrolytics,
brought on by voltage stress.

Arfa



Amp is 1975 , from electrolytics, and uses Si rectifiers.
Amp was in a damp shed for maybe 15 years. Doesn't look too bad for that ,
after blowing out the cocoony bug nests etc (1KW Martingdale ).
Charred/burnt 1.5K grid resistors and blown HT fuses.
All valves ( all marked Marshall) checked out good on Avo CT160 - I'd
forgotten how problematic , with high current valves, to get the initial
zero on the meter before rotating the SET mA/V.
I always power up kit left idle for a long term with a variac + current
meter + thermal trip.
Is it still perceived wisdom in such circumstances to power up valve amps
with full speaker load on output with all valves in place and to power up
transistor amps intitially without load ?

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"Asimov" wrote in
message
...
"n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
--- on the heady topic of "Marshall 4140 Amp question"
... "'Blues,' for all you people who paid to get in." - Pete Townshend


I agree with Asimov regarding leaky grid coupling caps, but I am not a
fan
of running tubed / valved group amps without the output bottles in place,
no
matter how briefly ( but if you must, take note that Asimov does say
briefly - IMO 40 seconds max ). Without the output stage drawing current
from the HT line, this can rise easily by 20% or more, from its nominal
running value, which can stress the power supply caps, and any other
decouplers along the way on the HT line.

This is not quite such a bad thing if the amp employs semiconductor HT
rectifiers, as the HT rail will be high of its nominal running value
until
the outputs warm up anyway, and this factor is designed in when the
voltage
ratings for the caps are chosen by the designer.

However, if the amp employs a thermionic rectifier, its output will come
up
slowly, as will the load imposed by the outputs as they warm up. This
results in an HT rail that comes up to a value, and stays there. If you
now
run the amp up with the outputs removed, the output of the reccy will
come
up with no load on it, which is an unexpected condition for the designer,
and the resulting unloaded voltage may well come very close to, or even
exceed the voltage ratings of some caps on the HT line.

You should be able to check the bias voltage with the outputs in place.
Marshall generally tend to use a negative supply of around 50v for the
bias
source, potted down and fed to the outputs' control grids via a few
resistors.

If a seperate bias supply were missing or low, this would more than
likely
result in excess output stage current demand, with consequent lowering of
the HT rail. Glowing anodes are the order of the day here, but I would
rather have that, as the outputs will stand this for a considerable time
without damage, rather than have exploding or shorting electrolytics,
brought on by voltage stress.

Arfa



Amp is 1975 , from electrolytics, and uses Si rectifiers.
Amp was in a damp shed for maybe 15 years. Doesn't look too bad for that ,
after blowing out the cocoony bug nests etc (1KW Martingdale ).
Charred/burnt 1.5K grid resistors and blown HT fuses.
All valves ( all marked Marshall) checked out good on Avo CT160 - I'd
forgotten how problematic , with high current valves, to get the initial
zero on the meter before rotating the SET mA/V.
I always power up kit left idle for a long term with a variac + current
meter + thermal trip.
Is it still perceived wisdom in such circumstances to power up valve amps
with full speaker load on output with all valves in place and to power up
transistor amps intitially without load ?


That's my usual recommendation.

Arfa

Arfa Daily wrote in message
...

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"Asimov" wrote in
message
...
"n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
--- on the heady topic of "Marshall 4140 Amp question"
... "'Blues,' for all you people who paid to get in." - Pete
Townshend


I agree with Asimov regarding leaky grid coupling caps, but I am not a
fan
of running tubed / valved group amps without the output bottles in
place,
no
matter how briefly ( but if you must, take note that Asimov does say
briefly - IMO 40 seconds max ). Without the output stage drawing
current
from the HT line, this can rise easily by 20% or more, from its nominal
running value, which can stress the power supply caps, and any other
decouplers along the way on the HT line.

This is not quite such a bad thing if the amp employs semiconductor HT
rectifiers, as the HT rail will be high of its nominal running value
until
the outputs warm up anyway, and this factor is designed in when the
voltage
ratings for the caps are chosen by the designer.

However, if the amp employs a thermionic rectifier, its output will
come
up
slowly, as will the load imposed by the outputs as they warm up. This
results in an HT rail that comes up to a value, and stays there. If you
now
run the amp up with the outputs removed, the output of the reccy will
come
up with no load on it, which is an unexpected condition for the
designer,
and the resulting unloaded voltage may well come very close to, or even
exceed the voltage ratings of some caps on the HT line.

You should be able to check the bias voltage with the outputs in place.
Marshall generally tend to use a negative supply of around 50v for the
bias
source, potted down and fed to the outputs' control grids via a few
resistors.

If a seperate bias supply were missing or low, this would more than
likely
result in excess output stage current demand, with consequent lowering
of
the HT rail. Glowing anodes are the order of the day here, but I would
rather have that, as the outputs will stand this for a considerable
time
without damage, rather than have exploding or shorting electrolytics,
brought on by voltage stress.

Arfa



Amp is 1975 , from electrolytics, and uses Si rectifiers.
Amp was in a damp shed for maybe 15 years. Doesn't look too bad for that
,
after blowing out the cocoony bug nests etc (1KW Martingdale ).
Charred/burnt 1.5K grid resistors and blown HT fuses.
All valves ( all marked Marshall) checked out good on Avo CT160 - I'd
forgotten how problematic , with high current valves, to get the initial
zero on the meter before rotating the SET mA/V.
I always power up kit left idle for a long term with a variac + current
meter + thermal trip.
Is it still perceived wisdom in such circumstances to power up valve
amps
with full speaker load on output with all valves in place and to power
up
transistor amps intitially without load ?


That's my usual recommendation.

Arfa



On the slow variac power up, I usually power up to about 80 per cent mains
with no valves in and then add the valves and then go low to 100 percent.

"Asimov" wrote in message
...
"Arfa Daily" bravely wrote to "All" (19 Jan 06 01:49:16)
--- on the heady topic of " Marshall 4140 Amp question"

AD From: "Arfa Daily"
AD Xref: core-easynews sci.electronics.repair:355730

AD lowering of the HT rail. Glowing anodes are the order of the day
here,
AD but I would rather have that, as the outputs will stand this for a
AD considerable time without damage, rather than have exploding or
AD shorting electrolytics, brought on by voltage stress.


Arfa,

High voltage electros have a surge rating about 20% higher than the
wvdc specified on the can. A brief surge less than the few seconds it
takes to get a reading will not usually be a problem. OTOH leaving the
tubes in runs the real risk of having a large current burn out a
transformer winding. Which risk would you prefer to take; ending up
with on your workbench: a smoking rectifier with a zapped electro, or
an open winding in a cherished rare vintage audio output transformer?

A*s*i*m*o*v

... Transformer designers take turns doing it.

Hi Asimov

Please don't get me wrong - I'm not challenging what you say. Merely
pointing out that there's two schools of thought on this. I agree that high
voltage electros should stand +20% on their indicated voltage rating, but
that is when they are youthful and in good condition ( unlike me these days
.... !! ). The point is that any measurements done with the tubes removed,
MUST be done quickly - which is why I said 40 seconds maximum in my post.

As far as the smoking reccy and zapped electro versus the open transformer
go, personally, having been in the vicinity of many exploding electrolytics
in the old days, I would rather risk the transformer damage, as I can't
remember ever having seen a transformer - power or output - in a high power
group amplifier, damaged from an output tube or bias problem. Remember that
old electro cans, at best had a ' pip ' in the rubber seal at the bottom,
and at worst, nothing to assist with venting, unlike the proper bungs or
rupture lines designed into modern ones. This is why they had a tendency to
go off like hand grenades, spewing boric acid soaked paper everywhere.

In contrast, if the output stage is drawing anything like enough current to
damage a transformer - and, IME, these will happily drip wax for at least 5
minutes without failure - then the HT fuse should ideally blow, or the surge
limiter / smoothing resistor smoke, glow, and go open.

It's probably just a matter of opinion and experience, which you obviously
have. I'm just making sure that those who might be reading, and have less
experience than you and I, understand the possible dangers and cosequences
d:~}

Arfa

After a check yesterday evening, I got ~100 ohms on the primary side
across the entire winding (ST-70), I did not disconnect any wires to
measure. So, based on a guitar-amp, I would expect your measurement to
be just fine. I did not get a chance to measure the Scott as I had less
time than I thought.

However, this evening I will have more time, and I will use the Fluke
meter rather than the R/S glove-box unit.

Peter Wieck
Wyncote, PA

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"Asimov" wrote in
message
...
"n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
--- on the heady topic of "Marshall 4140 Amp question"
... "'Blues,' for all you people who paid to get in." - Pete
Townshend


I agree with Asimov regarding leaky grid coupling caps, but I am not a
fan
of running tubed / valved group amps without the output bottles in
place,
no
matter how briefly ( but if you must, take note that Asimov does say
briefly - IMO 40 seconds max ). Without the output stage drawing
current
from the HT line, this can rise easily by 20% or more, from its
nominal
running value, which can stress the power supply caps, and any other
decouplers along the way on the HT line.

This is not quite such a bad thing if the amp employs semiconductor HT
rectifiers, as the HT rail will be high of its nominal running value
until
the outputs warm up anyway, and this factor is designed in when the
voltage
ratings for the caps are chosen by the designer.

However, if the amp employs a thermionic rectifier, its output will
come
up
slowly, as will the load imposed by the outputs as they warm up. This
results in an HT rail that comes up to a value, and stays there. If
you
now
run the amp up with the outputs removed, the output of the reccy will
come
up with no load on it, which is an unexpected condition for the
designer,
and the resulting unloaded voltage may well come very close to, or
even
exceed the voltage ratings of some caps on the HT line.

You should be able to check the bias voltage with the outputs in
place.
Marshall generally tend to use a negative supply of around 50v for the
bias
source, potted down and fed to the outputs' control grids via a few
resistors.

If a seperate bias supply were missing or low, this would more than
likely
result in excess output stage current demand, with consequent lowering
of
the HT rail. Glowing anodes are the order of the day here, but I would
rather have that, as the outputs will stand this for a considerable
time
without damage, rather than have exploding or shorting electrolytics,
brought on by voltage stress.

Arfa



Amp is 1975 , from electrolytics, and uses Si rectifiers.
Amp was in a damp shed for maybe 15 years. Doesn't look too bad for
that
,
after blowing out the cocoony bug nests etc (1KW Martingdale ).
Charred/burnt 1.5K grid resistors and blown HT fuses.
All valves ( all marked Marshall) checked out good on Avo CT160 - I'd
forgotten how problematic , with high current valves, to get the
initial
zero on the meter before rotating the SET mA/V.
I always power up kit left idle for a long term with a variac + current
meter + thermal trip.
Is it still perceived wisdom in such circumstances to power up valve
amps
with full speaker load on output with all valves in place and to power
up
transistor amps intitially without load ?


That's my usual recommendation.

Arfa



On the slow variac power up, I usually power up to about 80 per cent mains
with no valves in and then add the valves and then go low to 100 percent.


That's a good way of doing it, and avoiding the potential pitfalls that I
have pointed out regarding over-volting the smoothers. About the only slight
difference that I would normally do with an amp of this type that had been
standing unused for a long time, would be to first come up to about 30% on
the variac, and leave it running like this for 10 minutes, then come up to
50% for 20 minutes to half an hour, before going up to 80% to do the checks.
This will ensure that there are no issues with the caps reforming.

Arfa

"n cook" bravely wrote to "All" (19 Jan 06 09:11:40)
--- on the heady topic of " Marshall 4140 Amp question"

nc From: "n cook"
nc Xref: core-easynews sci.electronics.repair:355765

nc Amp is 1975 , from electrolytics, and uses Si rectifiers.
nc Amp was in a damp shed for maybe 15 years. Doesn't look too bad for
nc that , after blowing out the cocoony bug nests etc (1KW Martingdale ).
nc Charred/burnt 1.5K grid resistors and blown HT fuses.
nc All valves ( all marked Marshall) checked out good on Avo CT160 - I'd
nc forgotten how problematic , with high current valves, to get the
nc initial zero on the meter before rotating the SET mA/V.
nc I always power up kit left idle for a long term with a variac +
nc current meter + thermal trip.
nc Is it still perceived wisdom in such circumstances to power up valve
nc amps with full speaker load on output with all valves in place and to
nc power up transistor amps intitially without load ?


The burnt 1.5K grid resistors might indicate the last user tried to
operate the amp without a load and after the smoke out set it aside.
You are a wise man. Why did you need our help again?

A*s*i*m*o*v

.... The current limits placed are based on resistance

Arfa Daily wrote in message
...

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"Asimov" wrote in
message
...
"n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
--- on the heady topic of "Marshall 4140 Amp question"
... "'Blues,' for all you people who paid to get in." - Pete
Townshend


I agree with Asimov regarding leaky grid coupling caps, but I am not
a
fan
of running tubed / valved group amps without the output bottles in
place,
no
matter how briefly ( but if you must, take note that Asimov does say
briefly - IMO 40 seconds max ). Without the output stage drawing
current
from the HT line, this can rise easily by 20% or more, from its
nominal
running value, which can stress the power supply caps, and any other
decouplers along the way on the HT line.

This is not quite such a bad thing if the amp employs semiconductor
HT
rectifiers, as the HT rail will be high of its nominal running value
until
the outputs warm up anyway, and this factor is designed in when the
voltage
ratings for the caps are chosen by the designer.

However, if the amp employs a thermionic rectifier, its output will
come
up
slowly, as will the load imposed by the outputs as they warm up.
This
results in an HT rail that comes up to a value, and stays there. If
you
now
run the amp up with the outputs removed, the output of the reccy
will
come
up with no load on it, which is an unexpected condition for the
designer,
and the resulting unloaded voltage may well come very close to, or
even
exceed the voltage ratings of some caps on the HT line.

You should be able to check the bias voltage with the outputs in
place.
Marshall generally tend to use a negative supply of around 50v for
the
bias
source, potted down and fed to the outputs' control grids via a few
resistors.

If a seperate bias supply were missing or low, this would more than
likely
result in excess output stage current demand, with consequent
lowering
of
the HT rail. Glowing anodes are the order of the day here, but I
would
rather have that, as the outputs will stand this for a considerable
time
without damage, rather than have exploding or shorting
electrolytics,
brought on by voltage stress.

Arfa



Amp is 1975 , from electrolytics, and uses Si rectifiers.
Amp was in a damp shed for maybe 15 years. Doesn't look too bad for
that
,
after blowing out the cocoony bug nests etc (1KW Martingdale ).
Charred/burnt 1.5K grid resistors and blown HT fuses.
All valves ( all marked Marshall) checked out good on Avo CT160 - I'd
forgotten how problematic , with high current valves, to get the
initial
zero on the meter before rotating the SET mA/V.
I always power up kit left idle for a long term with a variac +
current
meter + thermal trip.
Is it still perceived wisdom in such circumstances to power up valve
amps
with full speaker load on output with all valves in place and to
power
up
transistor amps intitially without load ?


That's my usual recommendation.

Arfa



On the slow variac power up, I usually power up to about 80 per cent
mains
with no valves in and then add the valves and then go low to 100
percent.


That's a good way of doing it, and avoiding the potential pitfalls that I
have pointed out regarding over-volting the smoothers. About the only
slight
difference that I would normally do with an amp of this type that had been
standing unused for a long time, would be to first come up to about 30% on
the variac, and leave it running like this for 10 minutes, then come up to
50% for 20 minutes to half an hour, before going up to 80% to do the
checks.
This will ensure that there are no issues with the caps reforming.

Arfa



I've replaced the burnt stuff and its back working.
Unfortunately after about 10 minutes hum makes an appearance and after half
an hour becomes excessive.
I stuck a piece of 20 to 35 degree C thermochromic paper to each of the 4
can electrolytics and one is heating up.
I assume its normal for each of the 4 EL34 to have a blue glow observable
through some of the holes in the internal metal-work as well as normal
orange heater glow ?

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"Asimov" wrote in
message
...
"n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
--- on the heady topic of "Marshall 4140 Amp question"
... "'Blues,' for all you people who paid to get in." - Pete
Townshend


I agree with Asimov regarding leaky grid coupling caps, but I am
not
a
fan
of running tubed / valved group amps without the output bottles in
place,
no
matter how briefly ( but if you must, take note that Asimov does
say
briefly - IMO 40 seconds max ). Without the output stage drawing
current
from the HT line, this can rise easily by 20% or more, from its
nominal
running value, which can stress the power supply caps, and any
other
decouplers along the way on the HT line.

This is not quite such a bad thing if the amp employs semiconductor
HT
rectifiers, as the HT rail will be high of its nominal running
value
until
the outputs warm up anyway, and this factor is designed in when the
voltage
ratings for the caps are chosen by the designer.

However, if the amp employs a thermionic rectifier, its output will
come
up
slowly, as will the load imposed by the outputs as they warm up.
This
results in an HT rail that comes up to a value, and stays there. If
you
now
run the amp up with the outputs removed, the output of the reccy
will
come
up with no load on it, which is an unexpected condition for the
designer,
and the resulting unloaded voltage may well come very close to, or
even
exceed the voltage ratings of some caps on the HT line.

You should be able to check the bias voltage with the outputs in
place.
Marshall generally tend to use a negative supply of around 50v for
the
bias
source, potted down and fed to the outputs' control grids via a few
resistors.

If a seperate bias supply were missing or low, this would more than
likely
result in excess output stage current demand, with consequent
lowering
of
the HT rail. Glowing anodes are the order of the day here, but I
would
rather have that, as the outputs will stand this for a considerable
time
without damage, rather than have exploding or shorting
electrolytics,
brought on by voltage stress.

Arfa



Amp is 1975 , from electrolytics, and uses Si rectifiers.
Amp was in a damp shed for maybe 15 years. Doesn't look too bad for
that
,
after blowing out the cocoony bug nests etc (1KW Martingdale ).
Charred/burnt 1.5K grid resistors and blown HT fuses.
All valves ( all marked Marshall) checked out good on Avo CT160 -
I'd
forgotten how problematic , with high current valves, to get the
initial
zero on the meter before rotating the SET mA/V.
I always power up kit left idle for a long term with a variac +
current
meter + thermal trip.
Is it still perceived wisdom in such circumstances to power up valve
amps
with full speaker load on output with all valves in place and to
power
up
transistor amps intitially without load ?


That's my usual recommendation.

Arfa



On the slow variac power up, I usually power up to about 80 per cent
mains
with no valves in and then add the valves and then go low to 100
percent.


That's a good way of doing it, and avoiding the potential pitfalls that I
have pointed out regarding over-volting the smoothers. About the only
slight
difference that I would normally do with an amp of this type that had
been
standing unused for a long time, would be to first come up to about 30%
on
the variac, and leave it running like this for 10 minutes, then come up
to
50% for 20 minutes to half an hour, before going up to 80% to do the
checks.
This will ensure that there are no issues with the caps reforming.

Arfa



I've replaced the burnt stuff and its back working.
Unfortunately after about 10 minutes hum makes an appearance and after
half
an hour becomes excessive.
I stuck a piece of 20 to 35 degree C thermochromic paper to each of the 4
can electrolytics and one is heating up.
I assume its normal for each of the 4 EL34 to have a blue glow observable
through some of the holes in the internal metal-work as well as normal
orange heater glow ?


Yes, a gentle blue glow is entirely normal ( although a brighter ' hard '
glow can indicate that the vacuum is ' soft ' )

For some interesting stuff on this, see

http://www.jacmusic.com/html/article...w/blueglow.htm

and

http://members.aol.com/larrysb/blue_glow.html

Arfa

"Arfa Daily" bravely wrote to "All" (19 Jan 06 10:36:03)
--- on the heady topic of " Marshall 4140 Amp question"

AD From: "Arfa Daily"
AD Xref: core-easynews sci.electronics.repair:355768

AD In contrast, if the output stage is drawing anything like enough
AD current to damage a transformer - and, IME, these will happily drip
AD wax for at least 5 minutes without failure - then the HT fuse should
AD ideally blow, or the surge limiter / smoothing resistor smoke, glow,
AD and go open.
AD It's probably just a matter of opinion and experience, which you
AD obviously have. I'm just making sure that those who might be reading,
AD and have less experience than you and I, understand the possible
AD dangers and cosequences d:~}

AD Arfa


A typical primary uses wire about the diameter of a hair. It really
can not drip wax for long. In some cases they are damaged merely by
high voltage spikes. The insulation breaks down. This is especially
true for older transformers because if the enamel got hot over time it
tends to oxidize (turns to carbon) and the damage is cumulative.

The thing about audio transformers is that they are to an extent
responsible for the tonal character of the amplifier. So if you
replace it with something equivalent it might never sound exactly the
same. It is a matter of the stray capacitance and feedback in how it
was wound. So you understand my being more protective of an audio
transformer than of an electro.

Now we know one of the electros is bad. It is possible the voltage
stress would have uncovered the weak one right away in a shower of
sparks, just as you said. However, an esr meter might be better.

A*s*i*m*o*v

.... I remember the 6SN7...

"Asimov" wrote in message
...
"Arfa Daily" bravely wrote to "All" (19 Jan 06 10:36:03)
--- on the heady topic of " Marshall 4140 Amp question"

AD From: "Arfa Daily"
AD Xref: core-easynews sci.electronics.repair:355768

AD In contrast, if the output stage is drawing anything like enough
AD current to damage a transformer - and, IME, these will happily drip
AD wax for at least 5 minutes without failure - then the HT fuse should
AD ideally blow, or the surge limiter / smoothing resistor smoke, glow,
AD and go open.
AD It's probably just a matter of opinion and experience, which you
AD obviously have. I'm just making sure that those who might be reading,
AD and have less experience than you and I, understand the possible
AD dangers and cosequences d:~}

AD Arfa


A typical primary uses wire about the diameter of a hair. It really
can not drip wax for long. In some cases they are damaged merely by
high voltage spikes. The insulation breaks down. This is especially
true for older transformers because if the enamel got hot over time it
tends to oxidize (turns to carbon) and the damage is cumulative.

The thing about audio transformers is that they are to an extent
responsible for the tonal character of the amplifier. So if you
replace it with something equivalent it might never sound exactly the
same. It is a matter of the stray capacitance and feedback in how it
was wound. So you understand my being more protective of an audio
transformer than of an electro.

Now we know one of the electros is bad. It is possible the voltage
stress would have uncovered the weak one right away in a shower of
sparks, just as you said. However, an esr meter might be better.

A*s*i*m*o*v

.... I remember the 6SN7...

So do I ...

Are we talking output transformers or power transformers here ? If output
transformers, then I would agree that the design of the tranny is critical
to the audio performance of the amp. However, I cannot accept that for a
high power group amp, the primary is wound with wire as thin as a hair when
a) it has a resistance as low as a few ohms per winding half, and b) has to
stand peak currents approaching an amp. Even allowing for age, the windings
have to get very very hot, and stay so for a very considerable length of
time, in order to carbonise the winding insulation coating. When was the
last time you managed to burn this coating off successfully with just a
soldering iron tip at the better part of 300 degrees C ?

The situation is much the same with power transformers, which do not,
however affect the sound providing they can supply the required voltages at
the required currents. I can't remember in 30 odd years of repairing these
things, ever having to order a replacement power tranny, ( perhaps they're
more robust this side of the pond because they have to stand twice the
voltage ;-) ) and only one or two output trannies which had failed as a
result of clear problems such as a defective output tube which had
originally blown the HT fuse, which had then been bridged by silver paper to
finish the gig ...

I cannot agree that it would have been better to have found the defective
cap by letting it blow up in the OP's face as a result of overvolting. When
I was first in this business back in the early seventies, and working for a
large TV rental chain, there was a lad at another branch who suffered a
serious skin burn and partial loss of sight in one eye as a result of an
electrolytic can exploding in his face, so you see my reasons for advising
extreme caution with these things, and treating them with considered
respect. ESR meters are actually not very good with old can type
electrolytics, as their impedance was quite high in the first place, so any
readings you get are likely to be misleading if you are used to the readings
you get on modern caps, but yes, for all other cases, I would agree that an
ESR meter is an essential workshop tool for today's servicing.

Broadly, I am in agreement with you. We've both been around long enough to
know how to go about servicing this kit safely, and it sounds as if the OP
has too. There are many though, who will be reading this thread, who are not
experienced in this type of repair, and might one day decide to pull out
that old amp in the garage that uncle Willy used to play when he was in
Willy Wang and the CrapKickers, and run it up, and I'd hate for them to get
sprayed in exploding cap.

Arfa

Arfa Daily wrote in message
...



Yes, a gentle blue glow is entirely normal ( although a brighter ' hard '
glow can indicate that the vacuum is ' soft ' )

For some interesting stuff on this, see

http://www.jacmusic.com/html/article...w/blueglow.htm

and

http://members.aol.com/larrysb/blue_glow.html

Arfa



Replaced that dual cap and runs cool and no hum after 10 minutes.
But after an hour a hum develops, intrusive but not as bad as before and at
the same time a serious glow forms on the zinc coloured metalwork of one
pair of the EL34s.
All 4 EL34s tested good , otherwise just 2 off 1.5K resistors to the grids
of each EL34, correct values cold.
The push and the pull from the preamp are AC coupled, leaking cap here?
Swap pairs of EL34s to see if the core glowing ones are in the new position
or the original position ?

"n cook" wrote in message
...
Arfa Daily wrote in message
...



Yes, a gentle blue glow is entirely normal ( although a brighter ' hard '
glow can indicate that the vacuum is ' soft ' )

For some interesting stuff on this, see

http://www.jacmusic.com/html/article...w/blueglow.htm

and

http://members.aol.com/larrysb/blue_glow.html

Arfa



Replaced that dual cap and runs cool and no hum after 10 minutes.
But after an hour a hum develops, intrusive but not as bad as before and
at
the same time a serious glow forms on the zinc coloured metalwork of one
pair of the EL34s.
All 4 EL34s tested good , otherwise just 2 off 1.5K resistors to the grids
of each EL34, correct values cold.
The push and the pull from the preamp are AC coupled, leaking cap here?
Swap pairs of EL34s to see if the core glowing ones are in the new
position
or the original position ?


Yes, most likely problem, if it's not the EL34s themselves ( and if both on
one half are doing it, it's probably not ) is the coupling cap between the
phase splitter valve and output valves' grids. It is very common, and will
often result in hotspots on the anodes and even grid glow. I would just
replace both caps anyway. Check also that the bias is not drifting, and if
there is a balance pot for the two halves of the output, check that this is
set correctly. Check also, any resistors involved in splitting the bias
between the two sets of valves. As far as I recall, Marshall usually use a
single -50v bias supply, adjustable with a single preset, then distributed
to both grid circuits via a couple of fixed resistors. Check that someone
hasn't turned the bias down so that the valves are thrashing a bit anyway,
and the problem is then just being caused by ' normal ' thermal
characteristic drift of the valves, exacerbated by the increased dissipation
from low bias.

Arfa

"n cook" bravely wrote to "All" (20 Jan 06 16:37:24)
--- on the heady topic of " Marshall 4140 Amp question"

nc From: "n cook"
nc Xref: core-easynews sci.electronics.repair:355957

nc Replaced that dual cap and runs cool and no hum after 10 minutes.
nc But after an hour a hum develops, intrusive but not as bad as before
nc and at the same time a serious glow forms on the zinc coloured
nc metalwork of one pair of the EL34s.
nc All 4 EL34s tested good , otherwise just 2 off 1.5K resistors to the
nc grids of each EL34, correct values cold.
nc The push and the pull from the preamp are AC coupled, leaking cap
nc here? Swap pairs of EL34s to see if the core glowing ones are in the
nc new position or the original position ?


You should have checked the coupling caps for leakage first.
If then it still glows it could be a bias problem on that pair.
Seems like you are well on the way to restoring the amp.

A*s*i*m*o*v

--
..
Arfa Daily wrote in message
...


Yes, most likely problem, if it's not the EL34s themselves ( and if both
on
one half are doing it, it's probably not ) is the coupling cap between the
phase splitter valve and output valves' grids. It is very common, and will
often result in hotspots on the anodes and even grid glow. I would just
replace both caps anyway. Check also that the bias is not drifting, and if
there is a balance pot for the two halves of the output, check that this
is
set correctly. Check also, any resistors involved in splitting the bias
between the two sets of valves. As far as I recall, Marshall usually use a
single -50v bias supply, adjustable with a single preset, then distributed
to both grid circuits via a couple of fixed resistors. Check that someone
hasn't turned the bias down so that the valves are thrashing a bit anyway,
and the problem is then just being caused by ' normal ' thermal
characteristic drift of the valves, exacerbated by the increased
dissipation
from low bias.

Arfa



Not a problem with the DC blocking caps at final push-pull separator stage.
Replaced and still hum after 3/4 hour.
No DC on the ganged volume pots.
Monitoring the negative bias for the output EL34s.
10 minutes in, the -ps voltage at smoothing cap is -52V.
The voltage to the (schematic marked) B pair is -39.2V before going to 1.5K
then g1 of each EL34
For C pair -39.2V also
25 minutes in B = -37.8, C=-34
45 minutes in B= -36.9 , C=-32.5
hum is getting quite noticable and -ps rail is still -52
Increasing bias pot from its original of 8K to max of 22K
brings the -ps down to -53.5
now B=-39 and C=-33 less hum but still going more positive over time and hum
increasing.
I stopped before the B pair started glowing like before.
I was expecting from these voltage readings that it would be the C pair that
would start complaining.
Switching off the amp, not just to standby, for a couple of minutes ,
brought things back to original cold situation and another 3/4 hour
presumably before hum gets too much.

One thing that concerns me is each of these 1.5K g1 resistors has one end to
the valve base pin and the other floating in space , not soldered to an
insulated pin just the wire connecting through - would that be as original?
The schematic shows 1.5Ks but could someone have replaced 4 of a different
value? or is it EL34s leaking too much and need changing.

"n cook" wrote in message
...


--
.
Arfa Daily wrote in message
...


Yes, most likely problem, if it's not the EL34s themselves ( and if both
on
one half are doing it, it's probably not ) is the coupling cap between
the
phase splitter valve and output valves' grids. It is very common, and
will
often result in hotspots on the anodes and even grid glow. I would just
replace both caps anyway. Check also that the bias is not drifting, and
if
there is a balance pot for the two halves of the output, check that this
is
set correctly. Check also, any resistors involved in splitting the bias
between the two sets of valves. As far as I recall, Marshall usually use
a
single -50v bias supply, adjustable with a single preset, then
distributed
to both grid circuits via a couple of fixed resistors. Check that someone
hasn't turned the bias down so that the valves are thrashing a bit
anyway,
and the problem is then just being caused by ' normal ' thermal
characteristic drift of the valves, exacerbated by the increased
dissipation
from low bias.

Arfa



Not a problem with the DC blocking caps at final push-pull separator
stage.
Replaced and still hum after 3/4 hour.
No DC on the ganged volume pots.
Monitoring the negative bias for the output EL34s.
10 minutes in, the -ps voltage at smoothing cap is -52V.
The voltage to the (schematic marked) B pair is -39.2V before going to
1.5K
then g1 of each EL34
For C pair -39.2V also
25 minutes in B = -37.8, C=-34
45 minutes in B= -36.9 , C=-32.5
hum is getting quite noticable and -ps rail is still -52
Increasing bias pot from its original of 8K to max of 22K
brings the -ps down to -53.5
now B=-39 and C=-33 less hum but still going more positive over time and
hum
increasing.
I stopped before the B pair started glowing like before.
I was expecting from these voltage readings that it would be the C pair
that
would start complaining.
Switching off the amp, not just to standby, for a couple of minutes ,
brought things back to original cold situation and another 3/4 hour
presumably before hum gets too much.

One thing that concerns me is each of these 1.5K g1 resistors has one end
to
the valve base pin and the other floating in space , not soldered to an
insulated pin just the wire connecting through - would that be as
original?
The schematic shows 1.5Ks but could someone have replaced 4 of a
different
value? or is it EL34s leaking too much and need changing.




I think that it's getting to the point now where you've got to start trying
valve substitution. However, before rushing out and buying new ones, you can
start by removing both the C valves ( these are the ones that glow
ultimately - yes ? ), then removing one of the ( likely ) OK B valves and
putting it in the C side. The amp will run quite happily with just one valve
in each side ( it's a trick that I teach owners to allow them to finish a
gig if they have a serious valve failure ).

You will then be running it with two valves that have performed OK when they
were both in the B side, so if it now works ok, next put them into the two
unoccupied sockets instead. If it still continues to work ok, then the
chances are that it is a valve problem. If it doesn't, then it must be a
bias issue. This assumes of course, that it's nothing to do with the output
tranny, which could suffer a partial insulation breakdown, resulting in
shorted turns, when it has been running a while. Again, this could be
checked by swapping the winding ends betwen anode pairs, and seeing if the
bad behaviour swaps sides.

Arfa

Arfa Daily wrote in message
...

"n cook" wrote in message
...


--
.
Arfa Daily wrote in message
...


Yes, most likely problem, if it's not the EL34s themselves ( and if
both
on
one half are doing it, it's probably not ) is the coupling cap between
the
phase splitter valve and output valves' grids. It is very common, and
will
often result in hotspots on the anodes and even grid glow. I would just
replace both caps anyway. Check also that the bias is not drifting, and
if
there is a balance pot for the two halves of the output, check that
this
is
set correctly. Check also, any resistors involved in splitting the bias
between the two sets of valves. As far as I recall, Marshall usually
use
a
single -50v bias supply, adjustable with a single preset, then
distributed
to both grid circuits via a couple of fixed resistors. Check that
someone
hasn't turned the bias down so that the valves are thrashing a bit
anyway,
and the problem is then just being caused by ' normal ' thermal
characteristic drift of the valves, exacerbated by the increased
dissipation
from low bias.

Arfa



Not a problem with the DC blocking caps at final push-pull separator
stage.
Replaced and still hum after 3/4 hour.
No DC on the ganged volume pots.
Monitoring the negative bias for the output EL34s.
10 minutes in, the -ps voltage at smoothing cap is -52V.
The voltage to the (schematic marked) B pair is -39.2V before going to
1.5K
then g1 of each EL34
For C pair -39.2V also
25 minutes in B = -37.8, C=-34
45 minutes in B= -36.9 , C=-32.5
hum is getting quite noticable and -ps rail is still -52
Increasing bias pot from its original of 8K to max of 22K
brings the -ps down to -53.5
now B=-39 and C=-33 less hum but still going more positive over time and
hum
increasing.
I stopped before the B pair started glowing like before.
I was expecting from these voltage readings that it would be the C pair
that
would start complaining.
Switching off the amp, not just to standby, for a couple of minutes ,
brought things back to original cold situation and another 3/4 hour
presumably before hum gets too much.

One thing that concerns me is each of these 1.5K g1 resistors has one
end
to
the valve base pin and the other floating in space , not soldered to an
insulated pin just the wire connecting through - would that be as
original?
The schematic shows 1.5Ks but could someone have replaced 4 of a
different
value? or is it EL34s leaking too much and need changing.




I think that it's getting to the point now where you've got to start
trying
valve substitution. However, before rushing out and buying new ones, you
can
start by removing both the C valves ( these are the ones that glow
ultimately - yes ? ), then removing one of the ( likely ) OK B valves and
putting it in the C side. The amp will run quite happily with just one
valve
in each side ( it's a trick that I teach owners to allow them to finish a
gig if they have a serious valve failure ).

You will then be running it with two valves that have performed OK when
they
were both in the B side, so if it now works ok, next put them into the two
unoccupied sockets instead. If it still continues to work ok, then the
chances are that it is a valve problem. If it doesn't, then it must be a
bias issue. This assumes of course, that it's nothing to do with the
output
tranny, which could suffer a partial insulation breakdown, resulting in
shorted turns, when it has been running a while. Again, this could be
checked by swapping the winding ends betwen anode pairs, and seeing if the
bad behaviour swaps sides.

Arfa



Swapping each pair over, improved matters a lot.
The bias stayed about 6 volts more negative and much less noise for the same
time intervals from switch on. So to save buying new
bottles , a matter of switching off for 2 minutes every hour or so.
I can only assume it is due to over driven valves - well something had to
burn up those resistors and knock out the HT fuses.
Is there a name for this leakage? effect

"n cook" wrote in message
...
Arfa Daily wrote in message
...

"n cook" wrote in message
...


--
.
Arfa Daily wrote in message
...


Yes, most likely problem, if it's not the EL34s themselves ( and if
both
on
one half are doing it, it's probably not ) is the coupling cap between
the
phase splitter valve and output valves' grids. It is very common, and
will
often result in hotspots on the anodes and even grid glow. I would
just
replace both caps anyway. Check also that the bias is not drifting,
and
if
there is a balance pot for the two halves of the output, check that
this
is
set correctly. Check also, any resistors involved in splitting the
bias
between the two sets of valves. As far as I recall, Marshall usually
use
a
single -50v bias supply, adjustable with a single preset, then
distributed
to both grid circuits via a couple of fixed resistors. Check that
someone
hasn't turned the bias down so that the valves are thrashing a bit
anyway,
and the problem is then just being caused by ' normal ' thermal
characteristic drift of the valves, exacerbated by the increased
dissipation
from low bias.

Arfa



Not a problem with the DC blocking caps at final push-pull separator
stage.
Replaced and still hum after 3/4 hour.
No DC on the ganged volume pots.
Monitoring the negative bias for the output EL34s.
10 minutes in, the -ps voltage at smoothing cap is -52V.
The voltage to the (schematic marked) B pair is -39.2V before going to
1.5K
then g1 of each EL34
For C pair -39.2V also
25 minutes in B = -37.8, C=-34
45 minutes in B= -36.9 , C=-32.5
hum is getting quite noticable and -ps rail is still -52
Increasing bias pot from its original of 8K to max of 22K
brings the -ps down to -53.5
now B=-39 and C=-33 less hum but still going more positive over time
and
hum
increasing.
I stopped before the B pair started glowing like before.
I was expecting from these voltage readings that it would be the C pair
that
would start complaining.
Switching off the amp, not just to standby, for a couple of minutes ,
brought things back to original cold situation and another 3/4 hour
presumably before hum gets too much.

One thing that concerns me is each of these 1.5K g1 resistors has one
end
to
the valve base pin and the other floating in space , not soldered to an
insulated pin just the wire connecting through - would that be as
original?
The schematic shows 1.5Ks but could someone have replaced 4 of a
different
value? or is it EL34s leaking too much and need changing.




I think that it's getting to the point now where you've got to start
trying
valve substitution. However, before rushing out and buying new ones, you
can
start by removing both the C valves ( these are the ones that glow
ultimately - yes ? ), then removing one of the ( likely ) OK B valves and
putting it in the C side. The amp will run quite happily with just one
valve
in each side ( it's a trick that I teach owners to allow them to finish a
gig if they have a serious valve failure ).

You will then be running it with two valves that have performed OK when
they
were both in the B side, so if it now works ok, next put them into the
two
unoccupied sockets instead. If it still continues to work ok, then the
chances are that it is a valve problem. If it doesn't, then it must be a
bias issue. This assumes of course, that it's nothing to do with the
output
tranny, which could suffer a partial insulation breakdown, resulting in
shorted turns, when it has been running a while. Again, this could be
checked by swapping the winding ends betwen anode pairs, and seeing if
the
bad behaviour swaps sides.

Arfa



Swapping each pair over, improved matters a lot.
The bias stayed about 6 volts more negative and much less noise for the
same
time intervals from switch on. So to save buying new
bottles , a matter of switching off for 2 minutes every hour or so.
I can only assume it is due to over driven valves - well something had to
burn up those resistors and knock out the HT fuses.
Is there a name for this leakage? effect


Not that I'm aware of. I suppose that there probably is if you're a valve
manufacturer, though. Might be worth looking into the cost of some new
valves. Prices have come down a lot since the chinese have got involved.
Apparently, much of the old valve manufacturing equipment from UK plants,
was sold to Russian and far east countries. There's often some good deals to
be had on e-Bay. Also worth checking with your local music shop, if you've
got one. Valve PA amps are still as popular as ever with ' real '
guitarists, and my local shop buys in valves in quite large quantities, so
prices are not too bad.

Arfa

"n cook" bravely wrote to "All" (23 Jan 06 17:22:52)
--- on the heady topic of " Marshall 4140 Amp question"

nc From: "n cook"
nc Xref: core-easynews sci.electronics.repair:356259

nc Swapping each pair over, improved matters a lot.
nc The bias stayed about 6 volts more negative and much less noise for
nc the same time intervals from switch on. So to save buying new
nc bottles , a matter of switching off for 2 minutes every hour or so.
nc I can only assume it is due to over driven valves - well something had
nc to burn up those resistors and knock out the HT fuses.
nc Is there a name for this leakage? effect


The input grid on a tube can become "poisoned" by deposited material
coming from the plate and this diverts some cathode current. As it
starts to get hot it starts to emit its own electrons and loses some
control over the plate current and it increases. It is especially
noticeable in tubes that are driven very hard.

I saw an extreme of this effect in a 12AX7 phase splitter where 3K3
cathode and plate resistors were mistakenly installed at the factory,
instead of 33K. Must have been a colour blind worker. Anyway, the
12AX7 was practically a short. Just for fun I arced the input grid
with some very high voltage (5KV) and it cleared the junk on it enough
that after this it worked as a tube again.

A*s*i*m*o*v

.... You mean 15" sparks are SUPPOSED to come out of this thing?!?

Asimov wrote in message
...
"n cook" bravely wrote to "All" (23 Jan 06 17:22:52)
--- on the heady topic of " Marshall 4140 Amp question"

nc From: "n cook"
nc Xref: core-easynews sci.electronics.repair:356259

nc Swapping each pair over, improved matters a lot.
nc The bias stayed about 6 volts more negative and much less noise for
nc the same time intervals from switch on. So to save buying new
nc bottles , a matter of switching off for 2 minutes every hour or so.
nc I can only assume it is due to over driven valves - well something
had
nc to burn up those resistors and knock out the HT fuses.
nc Is there a name for this leakage? effect


The input grid on a tube can become "poisoned" by deposited material
coming from the plate and this diverts some cathode current. As it
starts to get hot it starts to emit its own electrons and loses some
control over the plate current and it increases. It is especially
noticeable in tubes that are driven very hard.

I saw an extreme of this effect in a 12AX7 phase splitter where 3K3
cathode and plate resistors were mistakenly installed at the factory,
instead of 33K. Must have been a colour blind worker. Anyway, the
12AX7 was practically a short. Just for fun I arced the input grid
with some very high voltage (5KV) and it cleared the junk on it enough
that after this it worked as a tube again.

A*s*i*m*o*v

... You mean 15" sparks are SUPPOSED to come out of this thing?!?


Sounds plausible,
well at least there is not the complication of minority carrier
recombination in thermionic devices.

"n cook" bravely wrote to "All" (24 Jan 06 08:40:28)
--- on the heady topic of " Marshall 4140 Amp question"

nc From: "n cook"
nc Xref: core-easynews sci.electronics.repair:356334

nc Sounds plausible,
nc well at least there is not the complication of minority carrier
nc recombination in thermionic devices.

There is still heavy ions to think about in heavy iron.

A*s*i*m*o*v

.... "Bother!" said Pooh, as he saw the sparks and smelled the smoke.