Peavey Classic A Series Voltages Indicated on the Schematic

[The Dude]

Not your issue. My bad. I didn't realize they used the bias winding for something else until I investigated further. Now your 100 ohm resistor and 200UF cap make sense.

[DrGonz78]

I would have to think that is possible that the preamp could be dragging down the bias tap voltage. Well I would go that direction before thinking it was a transformer issue, but the truth will eventually be discovered. So lift a leg on that diode from the bias tap feed that leads to the preamp and then check to see if voltages return at the bias tap of transformer.

[Aerostoon]

I would have to think that is possible that the preamp could be dragging down the bias tap voltage. Well I would go that direction before thinking it was a transformer issue, but the truth will eventually be discovered. So lift a leg on that diode from the bias tap feed that leads to the preamp and then check to see if voltages return at the bias tap of transformer.

I tried lifting the diode on the page 1 bias winding circuit. Still no negative bias voltage. Then I connected my DVM leads to the bias winding of the power transformer and got 56VAC. I was scratching my head trying to figure out why the components next in line in the circuit didn't have any voltage. Then I decided to clip one end of the bias diode on page 2 and the one lead end was completely loose in the solder connection. I removed the circuit board and the lead area had a glob of solder and I couldn't see the loose lead. The fault was hidden. I had previously inspected every solder joint looking for a loose connection. What a find!! I removed the old diode and soldered in a new one, reinstalled the circuit board and fired it up.
-55 VDC on the bias circuit. I have been picking away at this amp for months refusing to give up. I am so happy and thankful for everyone's help with this!!

[Enzo]

Visual inspection is good at finding bad things, like burnt parts, cracks in the solder or the circuit board, etc.

What visual inspection cannot do is tell you if anything is good.

[DrGonz78]

I have been picking away at this amp for months refusing to give up.

Congrats!! Yeah those amps that drive you nuts for a long time will always be good learning lessons. I had a Roland Cube 60 (newer model) that drove me nuts for months. Kept getting really discouraged and would put it back on the shelf. At one point I even thought to just use it as a parts source. Then one night I decided that I am going to fix this damn thing and a few nights later it all fell into place.

[Enzo]

I had one of those not all that long ago, and amp I had been working on over a decade. every now and then I took it out and wasted some time on it for a while. I isolated the problem to one area of the circuit around an IC. I changed the IC, changed some other stuff, yet the one stage refused to pass more than a tiny signal. FInally one day when measuring resistance, I checked the gain control to see if it was functioning. I got a really low reading no matter what. Instead of 1 meg it was something like 2k. What? I knew the control was a feedback between two IC pins, so I changed the IC...again.

Finally took the control out, one of those with a pull switch on the back. Measured it, sure enough 2k, then I looked on it and saw clearly printed on it "2k". There was one other control with a pull switch, so I looked closely at it, it was supposed to be a 2k pot for EQ, but sure enough, it said 1 meg on it. The two pots had been installed in each others place when it was made. And somehow that detail escaped me for years. Swap places and now the amp sounds good.

Why did I not notice that in the first place? Who knows.

[DrGonz78]

Common theme it seems with many hair puller repairs, in reflection, is they can be very simple fixes once the problem is identified.

[Leo_Gnardo]

Why did I not notice that in the first place? Who knows.

We assume the manufacturer got everything right when an amp (or anything else) was built. And/or that the amp was working perfectly at some time in its past.

Heck, every now and then I find a solder connection somebody forgot to do in a 40-50 year old Fender. Even found a couple non soldered spots in highly vaunted McIntosh gear. Misteaks happen , even at the factory.

[Aerostoon]

I have a question regarding a dim bulb current limiter that is adequate to limit current should a fault occur and be able to supply enough current to power this amp. The back of the amp indicates 200 watts power consumption. I have a 300 watt bulb that I am using that supplies full power to the amp. When it had its faults before the bulb would light up. Now it isn't lighting up and my plate current is great. When I try a 200 watt bulb in the limiter the bulb filament faintly lights and the amp isn't getting full full wall supply voltage. To me it seems like the 300 watt bulb is the better fit to use as the current limiter protection bulb. Here's the back of the amp:

[Enzo]

We don't "operate" the amp on the bulb, the bulb is only used to prevent fuse blows and damage to the amp, it is used during service. A 60 or 100 watt bulb is more than sufficient. Once we can power up the amp and the bulb stays dim, we then get rid of the bulb. Using a 300w bulb would allow closer to normal operation while on the bulb, but the bulb that large offers a lot less protection.

[Aerostoon]

That makes perfect sense to me. The lower watt bulbs are dim and I put it all back together after setting the bias with wall current. The amp works and sounds great. This amp sounds really nice!