Prosonic problems

[nosaj]

I've got a prosonic I've been working on that's being real fun and was wondering if anyone had any advice.

Was working on the output with sine wave put into return jack got the signal looking clean to the load resistor 8 ohm. Well I moved the sinwave to the input jack and backed it off right before clipping and during this time I could hear the sine wave faintly and then it quit and d301 and d302 protection diodes on the output tubes are not any good now. Could something in the preamp section cause this ?

Thasnk,
Jason

[loudthud]

Were you driving the amplifier without a speaker or dummy load connected? That is the condition the diodes are ment to protect against. It is possible to create spikes on the output but it takes extreme drive into a square wave with a speaker load connected.

[nosaj]

was using an 8 ohm load resistor, I was using a 1khz sine wave.

[loudthud]

There shouldn't be anyting you can do in the preamp to kill those diodes. Sometimes stuff happends and the reason isn't clear... until you kill the fifth set of diodes.

[nosaj]

Well I just ordered a bundle of them, So when these diodes die they shut off the OT? Since I have no sound now.

Thasnk,
Jason

[loudthud]

Sometimes they take the output transformer to the grave with them. They will put lots of stress on the power supply so there may be a fault there. Check voltages around the output tubes and power supply. I would cut them out of the circuit until the amp starts working again.

[nosaj]

Sometimes they take the output transformer to the grave with them. They will put lots of stress on the power supply so there may be a fault there. Check voltages around the output tubes and power supply. I would cut them out of the circuit until the amp starts working again.

What do you mean cut them out of the circuit? And do you suggest replacing the diodes then checking the voltages?

jason

[Enzo]

They are not needed for the function of the amp, they are a protection device. You will want to replace them by the time we finish the whole job, but you can continue on the troubleshooting without them.

One of those goes shorted, and it grounds off the B+ through the output transformer, usually blowing the fuse. And that is stressful for the various parts involved as thud mentioned.