Peavey D Blues

[trem]

I imagine it applies to the C-30 as well.
Anyway...I feel like an idiot... (am i missing something obvious) but I cannot figure out a way to get my meter in series with the plate current anywhere.
I guess I could rig up something with that connector for the OT...and get my meter on the OT center and 2 of the power tubes.?
How do you bias a PV C-30 type of amp.?
Thank You

[Enzo]

Well, first, the bias is not adjustable anyway, so there is no setting it. Unless you have modified the amp for adjustment.

One crude way would be to remove the high voltage fuse and insert a current meter in its place. That would get you all the tubes. From that we subtract any preamp tube current. Pull one of the preamp tubes and they all stop conducting, so that part goes to zero. Now the remaining current is all going through the power tubes. That does include the screen currents, but those are small and don't upset the readings. Besides, the classic bias adaptor you plug into the sockets includes screen currents too. And if you just want estimates, my rule of thumb is that 12AX7 curent is typically more or less 1ma per triode. So three tubes is six triodes so very roughly 6ma.

You could extract the brown or blue OT wire from the molex and insert a current meter, or insert a 1 ohm resistor and measure voltage across it. Current for two tubes. Seems like a hassle to me. Or do the same for the red wire, and the current for all tubes appears.

Or the simplest way, and a way that does not include the screen current: power off, and measure the winding resistance of each half of the OT primary. From the red wire to the brown, and from the red to the blue. They will probably be uneven, like 90 ohms and 120 ohms, if I recall. Now power up and with the amp running, measure voltage drop across those windings. Use Ohm's Law to calculate current. So for example 3v across a 90 ohm winding gives us 33ma. And remember that is for two tubes, so 17ma per tube in that example. Due to the imbalance, the other side may be closer to 4v. SO split the difference. Adjust so the average between the two sides is your target current.

Me, I just plug new tubes into the stock amp and enjoy it.

[trem]

Hey Enzo -
This amp has been sitting in a barn for years. My friend said new tubes red plated right away.
I pulled the tubes...checked negative bias...and it was all the way up to -1.00. No wonder it was red plating. The 15 Ohm resistor was dropping almost all the voltage across it. It read something like 25k Ohms.
So I replaced ALL the Elyotcs while I was in there replacing that 15-5W.
Got it all back together...amp sounded great...real quiet. Just wanted to see where the bias was at. Will check the Ohm and voltage drop on each side like you suggested.
Was just getting ready to put the chassis back in the cab...and I lost all sound. Looks like the power tube filaments are not lighting. Just starting to see where I am loosing that voltage. I see where those two 3 Ohm resistors feed the power tube filaments...but I do not see where the current goes series... into and out of the preamp tubes...if you know what I mean.
I found where each side of the heater winding feeds Pin 4 and then Pin 5 of the 2 power tubes.
There must be a separate feed for the pre filaments...or are they just run parallel off of the feed for the power tube filaments.?
Not the easiest amp for me to work on.
BTW...those Two...3 Ohm resistors are reading 4.7 Ohms. Is that kind of bad for a 10% resistor.?
Thank You

[Jazz P Bass]

Do you have any spare output tubes that you can try?

[trem]

Do you have any spare output tubes that you can try?

Yeah I do...are you thinking I lost one of the heaters in one of the EL84.?
This PV and its series heaters are kind of new for me.....frustrating. So hard to get at anything in this amp.

[Jazz P Bass]

Yup.

[Enzo]

The yellow wires feed the heaters. The four power tubes are in series, plus the two 3 ohm resistors and the 10A fuse. At either end of the two resistors are bare wire jumpers to the other two boards.

I'd be surprised of the 3 ohm wirewound resistors are actually 4.7 ohms, unless someone installed 4.7 ohmers in a repair. Pull the 10A fuse and measure them again. I expect them to be either open or OK. Wirewounds don;t usually rise in value like that.

You ought to have about 24vAC between the yellow wires. The two 3 ohm resistors should have 24v from one to the other on the end nearest the fuse. And almost 24v between them on the other end. FRom that far end iof the resistors, go around the bend with th two jumpers and see if it makes it to the tube socket board.


The preamp heaters are totally separate from the power tube heaters. The three 12AX7 tubes are wired for 12v heater, and are all three in series across the -36vDC supply

[trem]

Yeah...the -36VDC supply. I just figured that out. Explains why the power heaters are dead and the pres are glowing...Thank You.
Will check those other numbers and get back to you.
Thank You So Much

[trem]

All voltages normal...heaters are lit...pick up the chassis and the heaters go out.
If I push down on the "top" board...where the Yellow wires from the tranny go to the fuse and then those 3 Ohm resistors... I can cause a short or an open...not sure which at this point. Maybe those jumper wires that carry current from the fuse to those two 3 Ohm resistors have a problem when I push down on the board.?
Anyway...that is where my problem seems to be. Guess I will have to pull all the knobs and yank the boards...check things out...take a look...check/solder those jumpers or maybe put new wire in their place.
Amp repair is Cheap...Fun...and Easy.

[Enzo]

I just did that exact repair a couple days ago. I don;t waste time crying, the boards come out in really just a few minutes. So there are a bunch of screws and nuts to come off.

See the two wire jumpers that leave the fuse board and connect to the ends of the two resistors? Grasp each one with some small pliers and tug up. Either come free? They do occasionally break from vibration. Otherwise, they probably just need to be resoldered.

[trem]

Yeah...one of the jumper wires had a bad/cold joint. It had pulled off the pad. This amp sat for a LONG time. I think this wire was bad right from the factory. It worked for me just by luck.
The wire did not want to take solder...even with flux added.
So I just pulled it out, and put in a new wire.
Those leads were also pretty long. One of them had made contact with the other board traces...where the rectifier diodes are. There was a small burn mark on the trace...but the diodes all tested 0.57 Volts in circuit.
I clipped all the leads a bit shorter and put a piece of black tape over that area.
All seems fine now...the amp plays.
Just want to check bias while I have it. I am sure it is fine...like you say...within an accepted range. Just curious...all new tubes...will just take a peek.
Thank You

[trem]

Hey Enzo -
I understand how to pull the connector from the OT ...and get the resistance from side to side.
How do I get the Voltage Drop...with power on...after I slide that connector back over the OT primary.?
Thanks

[Enzo]

How close was my 90/120 ohm guess?

I didn;t bother to pull the connector off the board. With power off, the plates of the tube are not connected to anything so they cannot interfere with readings. I find there is enough room to stick my probes down into the holes on the connector beside the wire until I touch the metal pin within. Same for taking voltage readings.

But if your probes are too fat for that or something, look between the connector and the edge of the board. You should see two bare wire jumpers over to the tube board. They go to the two end pins, I forget which is brown or blue, but they are on the same sides as their color wire. On the other side is a bare wire jumper on the board, next to two small resistors. That bare wire goes to the center pin and so is B+.

[trem]

Hey Enzo -
I was just investigating that now. Seems like my probes will fit in there.
The OT on this D Blues is at 98 and 73 Ohms.
Thanks for all your help (thanks to the Forum as well)

[Enzo]

So I was not very close, oh well. The imbalance is normal though.