Marshall Guitar Amp Transistors overheating - help

As already been eluded to, "correctly" designed valve/tube guitar amps are liable to fail if their load is removed. This is because, the transformer acts a bit like a current transformer. That is, a current is being fed into the primary, and a current therefore wants to flow out of the secondary. Hence, the output voltage raises to try and force a current into an infinite load. There is usually a resister to try and mitigate this, say 500ohms, but it is not practical to include a resister low enough to solve this issue. In principle, one could design a protection circuit, but I have never seen this in a comercial product. Typically one gets the valve bases cracking...

Not since I switched over to using transistor guitar amplifiers.

Sorry its been a while since my last message!

I've just popped R29 out of circuit to test. Allowing for the error in my DVM (doesn't go to Zero) the resisitor value is fine (as are the other 0.33 ohm resistors).

All solder joints seem oaky and theres no sign of cracks or burning.

An earlier message suggested the MJF122 darlington so I popped this out too and there are no short circuits. With the dvm set to diode function on the ohm scale and the neg probe on the centre pin and pos probe on left pin the reading is 0.73 & 0.64 with the pos probe on the right pin.

Measure the bias currents into the 'base' leads... somehow, somewhere, there's too much, usually a leaky, perforated driver. __ Steve .