Smoke and the smell of burning

[JDA99]

I recently upgraded some resistors in my 300B PP (original) amp, and took the opportunity to adjust the components in the feedback line, to use the 4 ohm transformer tappings.

After some weeks of satisfactory use, upon switch on there was smoke around V1 and the smell of burning. It appears that both resistors, R10, in the feed back loop, have burnt out. There may be other damage, but so far I have resisted the urge to switch it back on.

I do not understand what circumstances would cause this to happen, any ideas please.


Thanks,

JDA

[Richard]

Hi

Attached is the circ for ref.

If both R10 are the only components visibly damaged I'd switch fb out and try the amp.

If it works ok I'd suggest you may have got the wrong value when replacing the R10s.

Playing at max, for 28W into 4R there will be 11v one side of R10 and nearly 0v on the other. With R10 at specified 1k5 it would be dissipating .075W so a small resistor would be fine.

If R10 was, say, 150R by mistake it would dissipate .75W and a small resistor, say 1/2W, would overheat.

With the fb switch open (fb off) you should be able to meter across R10 to check the actual value.