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]

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.

 

[Paul Herber]

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.

 

[Bill]

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?

 

[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?

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

It is the thermal sensor for the bias current.



..... 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,

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



...... Phil

 

[Jon Slaughter]

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/downloads/files/AVT150_275 hbk.pdf

(warning I)

 

[Eeyore]

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

 

[Eeyore]

"Dave Curtis" wrote

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

 

[Phil Allison]

"William Sommerwanker "

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

 

[Stephen Cowell]

"William Sommerwanker "




** 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
..

 

[neon]

first thing to do is short the input to ground the output should go to ground regardless of loading. if it self destruct it means that something is active.MY bet oscillation and/or excessive power voltage.lookat your power supply see if it is stable dc.

 

[Neil]

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?

 

[Phil Allison]

"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

 

[The Natural Philosopher]

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.

 

[The Natural Philosopher]

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

-DC

Correct.

 

[The Natural Philosopher]

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..

 

[The Natural Philosopher]

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.

 

[Phil Allison]

"The Natural Wanker "

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

 

[The Natural Philosopher]

"The Natural Wanker "



** 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.