ampeg B-100R blows fuse when played loud

[dangin]

B-100R...got it, turned it on, loud current drawing hum sound..no signal output.
pull the output transistors and start from AC checking voltage supply, rectifier, filter caps..replace Qs 3-9
C2 C3, check diodes, resistors, replaced IC1 and 2...getting the 16v, 20v and 40v (within 10%), used a current limiter to re-power up and all is well...connect direct to outlet, play for a good hour and reasonable volume but turning upto about 6 on the MV and pop goes the fuse. OP transistors shorted. Speaker sounds fine..honestly I have not taken enough time to really learn transistor circuits as I have with tubes...measuring output current on tube amps often helps me find the problem...any ideas...thanks in advance.

[Enzo]

When outputs short, always check the ballast resistors - the "emitter resistors" - for opens.

If outputs are replaced, it is usually a god idea to replace the drivers as well, in this case the IC, and check any rsistors associated with them. Either of those 470s open?

Is there about 39v across D9?

Check Q3 Q4.

Is the bias cool eenough? Q9 is the bias transistor, should be against the heat sink, I'd expect. Note the circuit has a couple resistors in its base circuit marked optional. You may not have all of them. I don;t know which way to turn the trimmer for colder, but the direction that increases teh trimmer resistance is cooler.

[dangin]

Ah, 39v at q9, it's got a tad over 18v at each pin....isn't it about 4am where you are?! I'll shut up because it's awesome that you took time to reply, thanks! so there's no trim pot on this one, though the schem shows one...I can change the bias via the 2 resistors at the base (R40/45)?

[Enzo]

Don't confuse D9 and Q9. I mentioned both.

If Q9 has 18v all over it, then so will your output. Work with NO speaker load until the amp is OK and stable.

I don;t know that you need to adjust the bias, just wanted to check. One could probably mount a trimmer there, I bet ther is a empty space for it on the board. Might then have to remove the other resistor.

The IC drives the output, so it is probably telling the circuit to go to 18v. SO we need to make sure nothing is open and any feedbacks are intact.

Q3,4 are limiters, and can be disabled by lifting the diode associated with each.