Ampeg BA115 HPT help!!!!!!!!

[nickb]

Don't forget it's very imporatnt to match the FETs. I found if they have the same batch number they're OK.

[nanodocl]

Hello again and sorry for the wait.

I changed the resistance from 4K7 to 3K3. I checked that the sum of the voltages across R58 and R67 was less than 4V. Specifically 0.7V + 1.1V.
I removed the 1K resistor. I placed the MOSFETS and gradually increased the voltage while measuring the current.
When I have reached 220V the voltage at pin 7 of U3 is 4V and the BIAS at R75 and R72 is 250mV with the trimmer at minimum and it does not respond even if it varies.
The MOSFETS of course are very hot.
For these measurements, I have turned on the equipment 4 times for about 10-15 seconds. The fifth time the fuse has blown, Q4, and Q5.
This is already a great mystery.

[nickb]

If the current was so high why on earth would you keep increasing the voltage and then make it worse by repeatedly powering up the amplifier? The high current tells you right away something is wrong so STOP! Any more than a few mA is bad news.

With just 1.8V gate to source voltage total the only way the FETS would conduct is if there were bad, one or more of the 47 ohms gate resistors were open or the 1.8V measurement is in error.

I don't see any reason why you can't put in the FETs one at a time. Put in just Q4 and you should see 0V on the output. I'd expect it's gate to source voltage, should be the same as across R67. to be around 2V i.e. the threshold voltage of the FET. The current through R74 should be zero. If all OK install Q5 and slowly bring the voltage up. Stop if you get more than 5mV across R74 and find out why.

If I didn't say it before, MOSFETs are very easily destroyed by tiny amounts of static. Use anti-static procedures. I often temporarily short the FET pins out while installing if possible.