Marshall Guitar Amp Transistors overheating - help

[Phil Allison]

"The Natural Wanker "

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

 

[The Natural Philosopher]

"The Natural Wanker "

** No relevance to Marshall whatever.




** 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 Allison]

"The Natural Wanker "

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.

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.....

Or teh Wiki is..

** Fuuuuck ooofffff

- you know nothing PISS HEAD.




........ Phil

 

[jh]

None of the marshalls I ever saw used KT88's

All EL34s.

Maybe that was a US export model you hade.



They are not cheap, and never were.


Or teh Wiki is..

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

 

[Phil Allison]

"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

 

[Kevin Aylward]

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
[email protected]
www.kevinaylward.co.uk

 

[Eeyore]

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

 

[The Natural Philosopher]

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.

 

[Neil]

"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 )))))

 

[Eeyore]

((((( 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

 

[Eeyore]

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

 

[Kevin Aylward]

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

 

[boardjunkie]

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.....

 

[Phil Allison]

"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

 

[Phil Allison]

"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?

** Have a very careful look at R29 ( 0.33 ohms 7 watt ) and make sure the
soldering of the legs is OK.

This resistor is CRITICAL as it conducts speaker current to ground and
supplies a take off point for the POSITIVE feedback loop.

If the soldering is cracked, the amp will oscillate at supersonic
frequency - probably intermittently.



...... Phil

 

[Eeyore]

You likely have leaky drivers.

Made of silicon ? !!!!!

Bwahahahahahhahahhaa !

Graham

 

[boardjunkie]

Made of silicon ? !!!!!

Bwahahahahahhahahhaa !

Graham

Umm....WTF are you talking about? You never heard of drivers taken out
as a result of output device shorting? Happens all the time.

 

[boardjunkie]

** 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

Fakes aren't just reserved for Ebay.......I've gotten fakes from MCM
recently. Last I checked they were still offered in spite of me
telling them they were junk and sending them back. Bean counters at
work.....

 

[Eeyore]

Umm....WTF are you talking about? You never heard of drivers taken out
as a result of output device shorting? Happens all the time.

Taken out =/= leaky.

Graham

 

[Eeyore]

Fakes aren't just reserved for Ebay.......I've gotten fakes from MCM
recently. Last I checked they were still offered in spite of me
telling them they were junk and sending them back. Bean counters at
work.....

No ! CROOKS at work.

Graham