Marshall Guitar Amp Transistors overheating - help

[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? The amp is a few years old but hasnt had a great deal of use. Thanks in anticipation.

[Jon Slaughter]

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.

[Paul Herber]

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

[Bill]

In message , Paul Herber writes

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?

[Phil Allison]

"Neil"

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

It is the thermal sensor for the bias current.

.... Phil

[Dave Curtis]

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

-DC

[Phil Allison]

"Neil"

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

..... Phil

[morris.slutsky]

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.

[morris.slutsky]

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?

[Jon Slaughter]

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:

I)

[The Natural Philosopher]

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.

[The Natural Philosopher]

Correct.

[The Natural Philosopher]

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..

[The Natural Philosopher]

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.

[Phil Allison]

"The Natural Wanker "

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

Subsequent models all followed a very similar pattern.

** Pus is right.

** ROTFL ....

.... Phil

[The Natural Philosopher]

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.

[Phil Allison]

"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.

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.

are totally WRONG.

...... Phil

[The Natural Philosopher]

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..

[Phil Allison]

"The Natural Wanker "

** Then you have never seen a Marshall Major

Too damn lazy too look up the schem as well.

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

You pig ignorance is showing.

Oink, oink.....

** Fuuuuck ooofffff

- you know nothing PISS HEAD.

....... Phil

[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.

regards

Jochen