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

"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.

"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

"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...

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.

"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

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/

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