The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

"Neil" wrote in message
...
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

I had a VS150H and the power amp used TDA7293 that blew. The local store
wanted to sale them for me for 30$... I was able to get them online for a
few dollars. (ended up buying 10 at a think 1$ each)

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).

So first off make sure you have the speakers hooked up(correct ohms). Second
makes sure that whatever the darlington's are driving isn't somehow shorted.

If the darlingtons are driving the speakers then I'd bet you have a load
mismatch. If they are not then maybe whatever they are driving has changed
impedence. Check the resistors around the darlingtons to make sure they
have the right resistance too. Also sometimes the diodes can go out and end
up causing problems so you really need to look at everything around it.
(besides the fact that if one component goes out it can take out others with
it)

Obviously 100oC is pretty hot for such things so for some reason the
darlingtons are passing a lot of current. This is either because there
configuration circuit has changed or the load has changed or the
input(potentially) has changed.

Does the 4 darlington's look like a left and right pair? What stage are they
part of? (output, input, power, pre, loop, etc...)

Did it work fine when you replaced them when it was working? Your going to
have to do a little bit more work if you want some help. If you can take a
pic and post it then it might help too.

On Thu, 16 Oct 2008 15:44:46 -0700 (PDT), Neil
wrote:

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

Are you getting some parasitic oscillation? If any wires have moved or
capacitors are failing then some HF oscillation could cause this.



--
Regards, Paul Herber, Sandrila Ltd.
Electronics for Visio http://www.electronics.sandrila.co.uk/

In message . net, Paul
Herber writes
On Thu, 16 Oct 2008 15:44:46 -0700 (PDT), Neil
wrote:

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

Are you getting some parasitic oscillation? If any wires have moved or
capacitors are failing then some HF oscillation could cause this.

I was just about to suggest similar, could yo get hold of an
oscilloscope and look at the output and see if there is anything there
that shouldn't be?




--
Bill

"Neil"
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this?


** T6 ( MJF122) may be damaged - replace it.

It is the thermal sensor for the bias current.



..... Phil

On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).

Solid state amp: Ok with no load. Tube amp: not ok.

-DC

"Neil"

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,


** Where did you buy the BDVs ?????



...... Phil

On Oct 16, 7:36 pm, "Phil Allison" wrote:
"Neil"

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this?

** T6 ( MJF122) may be damaged - replace it.

It is the thermal sensor for the bias current.

.... Phil

And also, even if it's technically working fine, it might have
mechanically dislodged itself from the heat sink. Stranger things
have been known to happen.

Heck, he could just disconnect all 3 of it's leads out for testing
purposes, couldn't he? The amp would go into deep class B with
serious crossover distortion, but it might still work, and give the
added bonus that it would sound better when turned louder. So long as
it didn't go into a destructive oscillation, it might even sound cool
- built in noise gate. And his power transistors would run really,
really cool.

Ok, scratch that last post, I wasn't thinking straight. Disconnecting
it would give more bias current, definitely not less. Putting a diode
in, with anode where the collector of T6 used to be, and the cathode
where the emitter used to be, that would give a nice cold bias for
testing, wouldn't it?

"Dave Curtis" wrote in message
...
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).

Solid state amp: Ok with no load. Tube amp: not ok.


Thats not completely true. The TDA7293's are designed to work more
efficiently with a certain load. By not having a load you reduce the
efficiency and it gets warmer. In general it shoudln't be an issue but in
rare circumstances it can.

Even if that wasn't the case then the chips shouldn't have blow at all
because they were not in use. since I had no speaker cabs hooked up at
all(just headphone hooked up to the headphone jack). I was playing for a few
hours and it just stopped working. took it hope and opened up and saw one of
the TDA's slightly burnt... replaced it and everything worked fine.

So it might have been just a weak chip that got a little too hot(load or no
load) but in case it's the facts are the facts.

1. It blew with no load.
2. No other component failed, if it was from a different component then when
I replaced the TDA it would have blown again(I ran a stress test on it to
see and used it extensively afterwards).
3. There was no power issue's at the time it blew.

I understand the logic that if there is no load with a solid state, since it
being direclty hooked up to the load, that no current should flow... but
thats ideally and really depend on the circuit because there usually is
always some type of load.

So I agree that in 99% of cases it is true for solid state but I disagree
that there is no possible way for all cases to be load independent. (even if
it has to do with the chip malfunctioning)

Of course I suppose it probably was something else too but I all I can say
is that the load was the only thing that changed and there is no telling
with the design how bad it is.

If you even look at the manual it says not to do it:

http://www.marshallamps.com/download..._275%20hbk.pdf

(warning I)

Neil wrote:

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

It sounds like the 'bias current' is incorrectly set (high) and they
didn't bother with a thermal cut-out.

You'll need a tech to set it correctly unless you're competent with
electronics yourself.

Graham

Jon Slaughter wrote:

"Dave Curtis" wrote
"Jon Slaughter" wrote:

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).

Solid state amp: Ok with no load. Tube amp: not ok.

Thats not completely true. The TDA7293's are designed to work more
efficiently with a certain load. By not having a load you reduce the
efficiency and it gets warmer.

Uh ?

Would you care to explain your analysis of that ?

Graham

Solid state amp: Ok with no load. Tube amp: not ok.

Very few tube amps had this problem. Only some that weren't stable without a
load.


That's not completely true. The TDA7293's are designed to work more
efficiently with a certain load. By not having a load you reduce the
efficiency and it gets warmer.

"Efficiency" is meaningless unless the amp is actually driving a load.

Without a load, the amp is drawing only its bias current. Which, for most
amps, is not very much.

"William Sommer****** "


Solid state amp: Ok with no load. Tube amp: not ok.

Very few tube amps had this problem.


** Wrong - the majority of tube guitar amps ARE at risk when driven with
no load.


Only some that weren't stable without a load.


** It is not a "stability" issue in most cases.



"Efficiency" is meaningless unless the amp is actually driving a load.

Without a load, the amp is drawing only its bias current. Which, for most
amps, is not very much.


** I have seen examples of SS amps that would immediately self destruct if
driven to clipping level with no load. One used Germanium transistors and
another used Semelab lateral mosfets.

In both cases, Vcc was too high for safety.



..... Phil

"Phil Allison" wrote in message
...

"William Sommer****** "


Solid state amp: Ok with no load. Tube amp: not ok.

Very few tube amps had this problem.


** Wrong - the majority of tube guitar amps ARE at risk when driven with
no load.

This amp is designed for no load... there's a headphone
jack that changes the output, you don't do that unless
you're prepared for all kinds of load shenanigans.

Basically, too much bias current... best thing is to
test every component in the final circuit. If they
all check OK, then start shotgunning them. You'll
get there eventually... and it won't be that expensive.
__
Steve
..

On Oct 17, 7:02*am, "Stephen Cowell" wrote:
"Phil Allison" wrote in message

...



"William Sommer****** "

Solid state amp: Ok with no load. Tube amp: not ok.

Very few tube amps had this problem.

** Wrong - *the majority of tube guitar amps ARE *at risk when driven with
no load.

This amp is designed for no load... there's a headphone
jack that changes the output, you don't do that unless
you're prepared for all kinds of load shenanigans.

Basically, too much bias current... best thing is to
test every component in the final circuit. *If they
all check OK, then start shotgunning them. *You'll
get there eventually... and it won't be that expensive.
__
Steve
.

Firstly, I'd like to say a big thank you to everyone who posted a
reply, especially within minutes of the original posting.

In summary:

Speaker is connected and impedence was the first thing I checked when
it happened the first time.

Going to check the diodes later today when I get home from work.

It worked perfectly well immediately before it blew.

I have an 'scope that I used for my work, however I'm no electronics
expert and use it only for checking PSU noise & Lissajous signals on
counting devices.

I'll take the T6 MJF122 out of circuit later today to see if it looks
suspect later today

BDV's available at Farnell about £2.40 each + vat (you may have to pay
a minimum order value). Also on eBay about the same price.

All heat sink clips are firmly in place.

Fortunately I bought a couple of extra BDV's just in case. If I can
identify the MJF122 as the culprit and replace it I'll monitor the
heat sink temperature every few minutes and power down if it gets too
hot.

Does anybody know what the normal operating temperature of the
darlingtons is?

"Neil"


I'll take the T6 MJF122 out of circuit later today to see if it looks
suspect later today

BDV's available at Farnell about £2.40 each + vat (you may have to pay
a minimum order value). Also on eBay about the same price.



** So which ones did ** YOU ** buy ??????

Your evasiveness is SUSPICIOUS ....


Does anybody know what the normal operating temperature of the
darlingtons is?


** About 10 degrees C above room ambient.



...... Phil

Neil wrote:
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

Summats up with the claas B bias circuitry.

No direct experience of this design, but you may find a preset pot on
the board somewhere in that general area.

Assuming nothing else is blown, the off signal current draw should be in
the tens of mA range. You can check that by using a meter instead of the
fuse.

If it has a bias preset pot, adjusting that should adjust the queiscent
current smoothly. If it jumps around, replace the pot.

Dave Curtis wrote:
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).

Solid state amp: Ok with no load. Tube amp: not ok.

-DC
Correct.

Jon Slaughter wrote:
"Dave Curtis" wrote in message
...
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).
Solid state amp: Ok with no load. Tube amp: not ok.


Thats not completely true. The TDA7293's are designed to work more
efficiently with a certain load. By not having a load you reduce the
efficiency and it gets warmer. In general it shoudln't be an issue but in
rare circumstances it can.

No. You have it wrong. The only thing that can cause a poorly designed
amp to blow on no load, is either that its unstable and oscillating, or
that the actual voltage is too high off load, and blows the chip or
output stages

In a well designed amp this should never happen, however this IS a
Marshall, and thefore all bets I suppose are off.

FWIW I spent more than ten years designing power amps, and at lest 5
doing guitar amps.

I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design, with a
skimped on power supply and output transformer,and a lot of feedback
removed to give it more gain.

Pus a preamp that closely follows the mullard design, made by someone
who thought orange was red..and so got the valve bias wrong, thus
accidentally discovering the 'Marshall sound'

Don't get me wrong. I like the Marshall sound and Ive got one. But I
always laugh inwardly when peole talk about 'valve sound' and 'marshall
sound' as if it it was a stroke of genius, rather than the accidental
result of skimping on costs and a glorious mistake..

Paul Herber wrote:
On Thu, 16 Oct 2008 15:44:46 -0700 (PDT), Neil
wrote:

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

Are you getting some parasitic oscillation? If any wires have moved or
capacitors are failing then some HF oscillation could cause this.



Yes. That is the other possibility. There is normally something akin to
a resistor and inductor, in series with the output, and a resistr of
about 5-10 ohms in series with a capacitor of about 0.1uf across te
output. If that capacitor is bust, you may well get oscillation,
especially with no load.

"The Natural ****** "


I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,


** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the 5F6-A
schematic.

Subsequent models all followed a very similar pattern.


Pus a preamp that closely follows the mullard design,

** Pus is right.


Don't get me wrong. I like the Marshall sound and Ive got one.


** ROTFL ....




..... Phil

Phil Allison wrote:
"The Natural ****** "

I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,


** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the 5F6-A
schematic.

Subsequent models all followed a very similar pattern.



Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.



Pus a preamp that closely follows the mullard design,

** Pus is right.


Don't get me wrong. I like the Marshall sound and Ive got one.


** ROTFL ....




.... Phil

"The Natural ****** "

I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,


** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the 5F6-A
schematic.

Subsequent models all followed a very similar pattern.


Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.

** No relevance to Marshall whatever.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.


** All the early 60s Marshalls used beam tubes, 5881s and KT66s.

Marshall Majors used 4 x KT88s.

USA models used mainly 6550s.

EL34s were later substituted with a bias change and little else, as an a
economy measure.

Read the Wiki & check out the schems.

http://en.wikipedia.org/wiki/Marshall_Amplification

http://www.drtube.com/marshall.htm

You are totally WRONG.




....... Phil

Phil Allison wrote:
"The Natural ****** "
I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,

** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the 5F6-A
schematic.

Subsequent models all followed a very similar pattern.

Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.

** No relevance to Marshall whatever.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.


** All the early 60s Marshalls used beam tubes, 5881s and KT66s.

Marshall Majors used 4 x KT88s.


None of the marshalls I ever saw used KT88's

All EL34s.

Maybe that was a US export model you hade.


USA models used mainly 6550s.

EL34s were later substituted with a bias change and little else, as an a
economy measure.


They are not cheap, and never were.

Read the Wiki & check out the schems.

http://en.wikipedia.org/wiki/Marshall_Amplification

http://www.drtube.com/marshall.htm

You are totally WRONG.



Or teh Wiki is..




...... Phil

"The Natural ****** "


Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.

** No relevance to Marshall whatever.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.


** All the early 60s Marshalls used beam tubes, 5881s and KT66s.

Marshall Majors used 4 x KT88s.


None of the marshalls I ever saw used KT88's


** Then you have never seen a Marshall Major

Too damn lazy too look up the schem as well.



USA models used mainly 6550s.

EL34s were later substituted with a bias change and little else, as an a
economy measure.


They are not cheap, and never were.


** Philips /Mullard EL34s were much cheaper than either KT66s or US made
5881s.

You pig ignorance is showing.

Oink, oink.....



Read the Wiki & check out the schems.

http://en.wikipedia.org/wiki/Marshall_Amplification

http://www.drtube.com/marshall.htm

You are totally WRONG.



Or teh Wiki is..


** Fuuuuck ooofffff

- you know nothing **** HEAD.




........ Phil

The Natural Philosopher schrieb:
Phil Allison wrote:
"The Natural ****** "
I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,

** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the
5F6-A schematic.

Subsequent models all followed a very similar pattern.

Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.

** No relevance to Marshall whatever.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.


** All the early 60s Marshalls used beam tubes, 5881s and KT66s.

Marshall Majors used 4 x KT88s.


None of the marshalls I ever saw used KT88's

All EL34s.

Maybe that was a US export model you hade.


USA models used mainly 6550s.

EL34s were later substituted with a bias change and little else, as an
a economy measure.


They are not cheap, and never were.

Read the Wiki & check out the schems.

http://en.wikipedia.org/wiki/Marshall_Amplification

http://www.drtube.com/marshall.htm

You are totally WRONG.



Or teh Wiki is..




...... Phil







Hi Mr Philosopher,

it's very remarkable that you don't recognise one of the most polite
answers Phil.A ever made.

Let me assure you - his statement's correct, yours' not so...

There also were KT88 Marshalls, although they are a completely different
story. IIRC the indeed evolved from generic Mullard or the GEC designs.
But - the bread and butter Marshall began 1962 with the 5881/6L6/KT66
equipped JTM45 - an almost 100% copy of the 5F6-A - and they (Ken Bran)
have publicly admitted that they were clearly aware of that fact. The
evolvement to the usage of pentodes (EL34s) was mainly driven by
economical reasons. The similarities to the mullard designs may come
from the fact, that there ain't so many possibilities to connect a
pentode or a beam power tetrode to a circuit - and - the 5F6-A was also
based on a common available basical circuit (from WE or RCA IIRC).

The preamp of the marshall *was* the same as a 5F6-A and the future
evolvement was only incorporated by the addition/alteration of some
components to fit the sonic tastes of the upcoming rock musicians
(cathode Cs, mixer Rs, bypass Cs, different B+ filtering etc). Even the
MV Marshalls just rewired the existing preamp circuit to a cascaded
design. OK some Rs and Cs were added but that was all. Up to the generic
JCM800 the circuit still clearly showed it's origins.

regards

Jochen

"jh"

Hi Mr Philosopher,

it's very remarkable that you don't recognise one of the most polite
answers Phil.A ever made.

Let me assure you - his statement's correct, yours' not so...

There also were KT88 Marshalls, although they are a completely different
story. IIRC the indeed evolved from generic Mullard or the GEC designs.
But - the bread and butter Marshall began 1962 with the 5881/6L6/KT66
equipped JTM45 - an almost 100% copy of the 5F6-A - and they (Ken Bran)
have publicly admitted that they were clearly aware of that fact. The
evolvement to the usage of pentodes (EL34s) was mainly driven by
economical reasons. The similarities to the mullard designs may come from
the fact, that there ain't so many possibilities to connect a pentode or a
beam power tetrode to a circuit - and - the 5F6-A was also based on a
common available basical circuit (from WE or RCA IIRC).

The preamp of the marshall *was* the same as a 5F6-A and the future
evolvement was only incorporated by the addition/alteration of some
components to fit the sonic tastes of the upcoming rock musicians (cathode
Cs, mixer Rs, bypass Cs, different B+ filtering etc). Even the MV
Marshalls just rewired the existing preamp circuit to a cascaded design.
OK some Rs and Cs were added but that was all. Up to the generic JCM800
the circuit still clearly showed it's origins.


** The ECC83 phase splitter with 100k/82k plate loads and the use of 220k
bias feeds to the EL34's grids are standard across nearly all Marshalls,
plus the cause of problems with bias runaway. The direct coupled
ECC83cathode follower driving the tone stack is also a common feature across
nearly all models too, as is the presence control in the NFB path.

All come direct from the Fender 5F6-A schematic.

Fender wisely improved their basic design as time went by - ie changing to a
12AT7 phase splitter and 68k output tube bias feeds.

But Marshall never got the hint, even to this day.



...... Phil

Marshall Majors used 4 x KT88s.


None of the marshalls I ever saw used KT88's

Almost all UK Majors had KT88's. The first few
Major's (via Rose Morris n UniCord) had 88's,
then they switched to 6550's. This allowed the
amps to loft along, due to the less demanding
needs of 6550's. A Major with 6550's sounds very
little like one with KT88's. Of course, bias needs
to be tweeked. Alot of folks who dislike Major's
only heard them with 6550's. And today's KT88's
are not the same. Thank God I have a good supply
of real GEC's. Put aside for old age, ya know.

Besides, a Major on a slant 4-12 cab looks awesome.

Head is as deep as the cab top (11" I believe)

Last killer Major 1/2 stack I saw, because he paid me
danger $ for the KT88's ($200 each in 98) had a cab
full of old white cap'ed SRO speakers. He had to put
bricks behind the wheels, because the amp would roll
backwards when played. He liked the cab being off the
ground, but hated to chase it every other song.

Newest GP shows 'KRANK' amps using surface mount resistors
in the POWER AMP stage ?????????? Flocking ****** design.

Major's had chassis mounted round power resistors.

Transformers that weigh more then today's amps, cab n all.

Guess the times, they are a changing...

JJTj









FS: Old press/Fan Club kit for The GrandMothers,
(Mothers of Invention original members) by
'Panda Records' SIGNED by Band in 2000 !

#522 of about 700 made

About as RARE as it gets, boyz n girlz..


http://www.usaelectron.com/GM







She's 200 years old
So mean she couldn't grow no lips
She's 200 years old
So mean she couldn't grow no lips
(Boy, she'd be in trouble if she tried to grow a mustache . .)

She's 200 years old
Squatting down
And poppin' up
In front of the juke box
Like she had true religion

Boy

She's 200 years old
Squattin' down
Poppin' up
Front o' the juke box
Just like she'd had true religion, now,

Boy

Boy, boing,, it's 200 years
Half of this, none of that
Was 50 . . .
Oh squat, yeah, oh, now, yeah!

She got religion now, boy

Oh, she's 200 years old
Oh, she told me
She just, she just can't grow no lips

Squat

Down

So mean she
Can't grow no lips

200 years old
Whaddya mean she can't grow no lips . . .

Squattin' down
Poppin' up 'n down at the juke box

OWW!

She got the true religion, boy

Boing!!!!!!!!!!!!!!!

On Fri, 17 Oct 2008 01:31:01 -0700 (PDT), Neil
wrote:

On Oct 17, 7:02*am, "Stephen Cowell" wrote:
"Phil Allison" wrote in message

...



"William Sommer****** "

Solid state amp: Ok with no load. Tube amp: not ok.

Very few tube amps had this problem.

** Wrong - *the majority of tube guitar amps ARE *at risk when driven with
no load.

This amp is designed for no load... there's a headphone
jack that changes the output, you don't do that unless
you're prepared for all kinds of load shenanigans.

Basically, too much bias current... best thing is to
test every component in the final circuit. *If they
all check OK, then start shotgunning them. *You'll
get there eventually... and it won't be that expensive.
__
Steve
.

Firstly, I'd like to say a big thank you to everyone who posted a
reply, especially within minutes of the original posting.

In summary:

Speaker is connected and impedence was the first thing I checked when
it happened the first time.

Going to check the diodes later today when I get home from work.

It worked perfectly well immediately before it blew.

I have an 'scope that I used for my work, however I'm no electronics
expert and use it only for checking PSU noise & Lissajous signals on
counting devices.

I'll take the T6 MJF122 out of circuit later today to see if it looks
suspect later today

BDV's available at Farnell about £2.40 each + vat (you may have to pay
a minimum order value). Also on eBay about the same price.

All heat sink clips are firmly in place.

Fortunately I bought a couple of extra BDV's just in case. If I can
identify the MJF122 as the culprit and replace it I'll monitor the
heat sink temperature every few minutes and power down if it gets too
hot.

Does anybody know what the normal operating temperature of the
darlingtons is?






Dig up the data sheet...it should tell max operating temp

Jon Slaughter wrote:
"Dave Curtis" wrote in message
...
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe
it blew because I had no load hooked up(it was in a dorm and I
didn't have the cab and was running the line out to my comp).

Solid state amp: Ok with no load. Tube amp: not ok.


Thats not completely true.

Of course it is. The only way a transistor amp can blow up with no load, is
if it is designed incorrectly. Like, it oscillates off load, or the power
supply has such bad regulation that it raises its voltage etc....

Kevin Aylward

www.kevinaylward.co.uk

JP wrote:

Does anybody know what the normal operating temperature of the
darlingtons is?

It's not that simple. It depends on the dissipation and thermal resistance.

The *junction* or die temperature may be as high as 150C in plastic devices or 200C
in metal can and still be reliable but that's NOT the heatsink temp.

Google Motorola AN1040 (that's twice this week now I've mentioned that) !

Graham

Kevin Aylward wrote:
Jon Slaughter wrote:
"Dave Curtis" wrote in message
...
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe
it blew because I had no load hooked up(it was in a dorm and I
didn't have the cab and was running the line out to my comp).
Solid state amp: Ok with no load. Tube amp: not ok.

Thats not completely true.

Of course it is. The only way a transistor amp can blow up with no load, is
if it is designed incorrectly. Like, it oscillates off load, or the power
supply has such bad regulation that it raises its voltage etc....


Well that is what I SAID, and in the context of MARSHALL, its quite
possible.


Ive seen power amps blow from having transistors unable to handle the
rail to rail voltage they saw off load before.

First job in one company was to work that out and select transistors
that COULD.



Kevin Aylward

www.kevinaylward.co.uk

On Oct 17, 9:43*am, "Phil Allison" wrote:
"Neil"

I'll take the T6 MJF122 out of circuit later today to see if it looks
suspect later today

BDV's available at Farnell about £2.40 each + vat (you may have to pay
a minimum order value). Also on eBay about the same price.

** *So which ones did *** YOU ** * buy *??????

* * Your evasiveness is *SUSPICIOUS ....

((((( not evasive Phil, I just refered to them as BDV's as you did in
your question - and to be brief. BDV64 & 65 are discontinued at RS &
Farnell so I fitted BDV64a & 65a. Given that the failure is exactly
the same as when I carried out the FIRST repair using pucker BDV64 &
65's I assume suffix 'a' is compatible or adequate. Farnell p/nos
1208581 & 1208579 )))))

Does anybody know what the normal operating temperature of the
darlingtons is?

** *About 10 degrees C above room ambient.

((((( thanks Phil )))))


..... * Phil

Neil wrote:

"Phil Allison" wrote:
"Neil"

I'll take the T6 MJF122 out of circuit later today to see if it looks
suspect later today

BDV's available at Farnell about £2.40 each + vat (you may have to pay
a minimum order value). Also on eBay about the same price.

** So which ones did ** YOU ** buy ??????

Your evasiveness is SUSPICIOUS ....

((((( not evasive Phil, I just refered to them as BDV's as you did in
your question - and to be brief. BDV64 & 65 are discontinued at RS &
Farnell so I fitted BDV64a & 65a. Given that the failure is exactly
the same as when I carried out the FIRST repair using pucker BDV64 &
65's I assume suffix 'a' is compatible or adequate. Farnell p/nos
1208581 & 1208579 )))))

IIRC and I'm pretty sure I do, the letter suffix indicates the working
voltage, A being the lowest. That's certainly the case with some BDXs I
used. Then they deleted the BDXxxD and we had to have devices selected from
BDXxxCs. Not my choice of device I'm glad to say, so I could say "told you
so" !

Graham

Kevin Aylward wrote:

Jon Slaughter wrote:
"Dave Curtis" wrote

Solid state amp: Ok with no load. Tube amp: not ok.

Thats not completely true.

Of course it is. The only way a transistor amp can blow up with no load, is
if it is designed incorrectly. Like, it oscillates off load, or the power
supply has such bad regulation that it raises its voltage etc....

Wouldn't put it past the Indian factory that makes them these days (I've seen
the line) to achieve that. Usually by substitution of inferior components.

Graham

Eeyore wrote:
Kevin Aylward wrote:

Jon Slaughter wrote:
"Dave Curtis" wrote

Solid state amp: Ok with no load. Tube amp: not ok.

Thats not completely true.

Of course it is. The only way a transistor amp can blow up with no
load, is if it is designed incorrectly. Like, it oscillates off
load, or the power supply has such bad regulation that it raises its
voltage etc....

Wouldn't put it past the Indian factory that makes them these days
(I've seen the line) to achieve that. Usually by substitution of
inferior components.

Graham

Just watching BBC 4 right now. Its the story of les Paul and the 1st
electric...

Kevin Aylward

www.kevinaylward.co.uk

On Fri, 17 Oct 2008 15:04:19 +0200, jh
wrote:

the bread and butter Marshall began in 1962 with the 5881/6L6/KT66
equipped JTM45 - an almost 100% copy of the 5F6-A

Which is why the switches are (still?) upside-down, and the inputs are
on the opposite side.

On Oct 16, 6:44*pm, Neil wrote:
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

You likely have leaky drivers. Never just replace output devices
without checking upstream to see if other components were damaged when
the outputs shorted. It happens.....

"Neil"
"Phil Allison" "

I'll take the T6 MJF122 out of circuit later today to see if it looks
suspect later today

BDV's available at Farnell about £2.40 each + vat (you may have to pay
a minimum order value). Also on eBay about the same price.

** So which ones did ** YOU ** buy ??????

Your evasiveness is SUSPICIOUS ....

((((( not evasive Phil, I just refered to them as BDV's as you did in
your question - and to be brief. BDV64 & 65 are discontinued at RS &
Farnell so I fitted BDV64a & 65a. Given that the failure is exactly
the same as when I carried out the FIRST repair using pucker BDV64 &
65's I assume suffix 'a' is compatible or adequate. Farnell p/nos
1208581 & 1208579 )))))


** Just one more try:

Did YOU buy the BDVs from Farnell or off eBay ???????

I ask because there are so many **fake** devices offered on eBay.

Never buy semis from eBay.



...... Phil