Peavey Classic A Series Voltages Indicated on the Schematic

[Aerostoon]

Peavey_Classic_(A_Series)_Schematics.pdf

I am working on a Peavey Classic 212 Series A amplifier. On the schematic there are high voltages indicated on the schematic. Should I be seeing voltages that are quite close to those? For example - 470 VDC at the 60 uf filter cap. Sorry If this is a silly question.

[DrGonz78]

What voltages are you measuring? They should be around the 470v and maybe a tad higher with modern wall voltages. Your schematic did not post correctly...

[Aerostoon]

Well this amp has some serious issues. I am trying to figure out why the plate current is so high. The plate current is going over 100 ma when measured with my Eurotubes bias probe. When I install the tubes the voltages drop very low to around 290 VDC. I am using a dim bulb current limiter with a 300 watt bulb which starts to glow fairly bright. Also I am not getting any negative bias readings at the bias pot. From the schematic it looks like it should be around -55 vdc and I am getting less that -1 vdc.

Without the power tubes installed I am measuring approx. 520 vdc at the main filter caps.

[DrGonz78]

Then focus completely on getting your bias voltage back up and running. There is a diode, resistor, 2 caps and the bias trim pot. Make sure the caps are good and check that diode with a your meter. Also make sure the resistor and bias pot are good too. Basically work backwards to see if there is a break in the connection of the bias voltage. I would use a variac if you have one but the light bulb limiter will help a bit. Just be careful running the B+ voltage higher than the filter cap voltage ratings allow.

Edit: Perhaps try putting in a 100 watt bulb to bring voltages down even more and keep the output tubes out for now.

[Enzo]

1v of bias instead of 55v explains the excess current draw neatly in one reading. Get the bias supply back and the rest should fall into place. The 470v is dropping like a stone because of all the current being drawn.

[Aerostoon]

Every component is checking out ok with the exception of this diode. I am getting a reading with the leads reversed.
Here is the reading where it tests ok:

[Aerostoon]

Here is a reading where it looks like the diode is showing a fault. Is it possible that this is a false reading. Every other diode nearby only shows a reading with the lead polarity correct:

[The Dude]

You'll have to lift one end of the diode out of circuit to get an accurate reading. You could be reading across a leaky cap or something else in that circuit.

[Aerostoon]

Thanks! I will give that a try. I have a new diode to try in there if it doesn't test serviceable.

[The Dude]

If you look at the schematic, there is a 10UF/150V filter cap after the diode. The positive side of the cap goes to ground as does the other side of the transformer winding feeding the diode. That's basically wire, or low resistance. So, effectively, the cap is across the diode as far as the meter is concerned. If the diode checks good out of circuit, I would next suspect the cap is leaky. Unsoldering one end of the diode will give you a direction.

Edit: I'm assuming you checked the voltage on the cap side of the diode first?

Edit 2: I don't know if it's been said already, but do leave the tubes out until you get the bias supply up and running. If not tubes or worse could be damaged.

[Aerostoon]

I did measure for voltage on the negative side of the 10uf 150 VDC cap. That is where I was measuring -1 VDC.
I lifted the diode and it is testing ok now.
I previously replaced both of the 10uf 150 VDC caps in my earlier troubleshooting but it didn't make any difference to the circuit.
The diode beside the one I lifted is on the 2nd page of the schematic and tests ok. The 100 ohm resistor tests ok. The 200 UF 75 VDC tests ok but I had to lift one side to measure the capacitance. With it fully connected it passed with my Anatek ESR meter.
I'll keep going down the circuit and checking components. Anything you think I might have missed so far?

[The Dude]

Did you put the caps in observing correct polarity? The bias supply caps need to have + to ground since it's a negative supply. Also, you mean 1k instead of 100 ohms?

[The Dude]

Ok, now I see where some of the confusion is coming from. On page 1 of the schematic, there is another supply that is derived from the bias winding of the transformer. This is separate from the bias supply. It is an independent supply that shares a transformer winding. We aren't worried about that right now, although it might tell us something about the transformer. Do you have +60V or so on that 100 ohm resistor? If that is also missing, we might have transformer issues.

[Aerostoon]

I did make sure the 10UF 150 VDC cap polarity is correct. I have before and after photos of the board to confirm that. I did check the 1K resistor and it is very close to 1k. The bias pot from the Center lug to each outer one measures a nice gradual 50 K each way. The 47 k and the 220 K resistors check out good as well.

[Aerostoon]

I was going into the bias winding circuit and checking things when I mentioned the 100 ohm resistor. Sorry for not being clear. I will check for the voltages you suggest and let you know tomorrow.