Troubleshooting SS amp...30vdc on OUTPUT!

[Jazz P Bass]

Did you ever check the original Tr51 diode for forward drop or a short with your meter.
I do not think it was ever verified if it was bad.
The cobbled together 1N4007 string "is" mounted to the heat sink, right?

[Prattacaster]

no o-scope here, I actually thought it could be the filter cap, and the Pin 6 connection, however the first time it made the aggrevated noise was when I was adjusting the Center Voltage. The noise level was climbing along with the bias voltage before I quicky shut it down. After that everytime I made physical contact with anything on the ouput board it went bonkers. Now it hums when i flip the switch.

did this include both drivers and both output devices?

Yes, I replaced all but ONE TO-3 transistor, the one I didnt replace was an NPN either right before the bias pot or a driver.

[R.G.]

As a first step, replace that 47pF capacitor between the collectors of Q5 and Q6 with 100pF. That may cause some trivial high frequency loss, but it will add more phase margin and might make it stable.

Sadly, the steadily-worse going bonkers when you touch anything may mean something is now dead again.

The basic circuit of an input diffamp followed by a differential voltage amplifier stage has known problems with stability.

[Prattacaster]

Did you ever check the original Tr51 diode for forward drop or a short with your meter.
I do not think it was ever verified if it was bad.
The cobbled together 1N4007 string "is" mounted to the heat sink, right?

My meter does not read a Fvd over 1v, it is NOT short, resistance as far as I can remember was around 10megs.

Yes, the cobbled together diode string is mounted on the heat sink.

As a first step, replace that 47pF capacitor between the collectors of Q5 and Q6 with 100pF. That may cause some trivial high frequency loss, but it will add more phase margin and might make it stable.

Can do!

Sadly, the steadily-worse going bonkers when you touch anything may mean something is now dead again.

I thought nothing different, with the existance of hum right from power up I can guarantee something failed, at least it isnt pulling enough current to make the bulb light up.

[Enzo]

In the common string of a few diodes, their job is to maintain a voltage difference between opposing power transistor bases. In that case, a shorted diode is actually not a problem. All it will do is increase crossover distortion a little bit. In fact on some power amps that are not adjustable, if they run too hot, a simple cure is to short across one of the diodes.

Well, not a problem in the sense that it won't cause blow-up.

[Prattacaster]

I thought I'd follow up on this thread.

I got the Yamaha G100 amp working on it sounds great. I was really getting tired of seeing this amp on my bench, and after trying a gizzilion different diode combinations I finially stuck the original back in. And it worked perfect. I wanted to exhaust all other options because the anode lead was broken right at the base of the diode, I had to scrape the diode body to get enough lead exposed to solder a new leg. I realy hate that it doesnt look "great" as far as the new lead goes...but it works and so I'm happy and the costumer is going to be elated.

I plugged a modded DS-1 into it and got some really nice tones....the 4x10 cab sounds phenomenal. Maybe I'm an SS guy after all. I'll stick with tubes.