Peavey Deuce series 240 amp Hum normal?

[steveh112@yahoo.com]

I haven't had much experience with tube amps so I'm just looking for a 2nd opinion here from someone who's worked with them. I'm working on a Peavey Deuce "series 240" guitar amp. Someone had busted off the flipper lever on the toggle switch so I replaced it. Double pole double throw 6 terminals with multiple connections on every one, so a bit time consuming but got it in without too much trouble. Also rewired the reverb tank because the plug was missing on the wires so they had individual terminals sticking into the plug in the cassis and they weren't even matching. So I replaced the connector on the amp with 2 RCA female connectors and ran a new dual RCA male to male cord from the tank to plug in there. (The tank has female RCA's for the input and output so it was plug and play ready.) I just got done testing it and it basically works but there's a very noticeable AC hum through the speakers even with all the volumes and gains and reverb level turned all the way down. Is this normal for a tube amp, or could I have some bad filter capacitors? Nothing looks bulged or vented and they all measure less than half an ohm on the ESR meter. Just wondering if this is an actual problem before I pursue troubleshooting it or normal for this model? It has 4 6L6 output tubes. I see online this is around a 1975 model so maybe it's so old it forgot the words and that's why it hums lol j/k but any input would be appreciated.
Thanks

[glebert]

Do you have a schematic that matches that amp? I don't know the 240 series, I know more about the VT series.

[Helmholtz]

Assuming this is a full tube amp:

Does it still hum with preamp tubes (not the PI) pulled?

If yes, what happens if you also pull the PI?
If still hum, what are power tube idle currents?

[TomCarlos]

The Peavey Deuce Series 240 is a hybrid amp. The Deuce has four 6L6 output tubes. The larger version "Mace" has six 6l6 tubes. Everything up to and including the Phase Inverter is solid state.

I would pull the tubes and run the preamp output to another amp. Is it quiet?

See my old post on titled Questions on the Peavey VT Series 240T Tube Amp. The two transistors in the phase inverter were "noisy." I also found some of the surrounding components to be way out of spec.

Schematic: Peavey_Mace-Deuce_VT_Schematic.pdf

[Helmholtz]

Anyway measure power tube idle currents. If not balanced (ripple) hum is likely.

[steveh112@yahoo.com]

Like I mentioned in my first post, I'm not at all experienced in tube amps, but I know a little of the theory. Can someone please tell me how/where to measure the tube idle current? If I remember correctly the idle current is also known as the bias. Does this amp have adjustment pots for the tube bias, or is it set with fixed value components? (mine is the 4 tube version btw not the 6) I have the chassis mounted back up in the cabinet and I'm trying to decide on dropping it back out or letting it go. The 6L6's do seem to throw quite a bit of heat so I guess it's possible they could be biased a little on the high side. The good thing is they have a good healthy orange glow, nothing red hot. If the tubes have individual bias adjustments then I may take it back apart to check and adjust. I assume it would be similar to setting the bias on a solid state amp by measuring the voltage across a low value power resistor similar to the emitter resistor but have never done it before.

[TomCarlos]

Steve....

If you are not experienced working with High Voltages, you need to be VERY careful here.

Helmholtz comment refers to the Bias current. This is the current that flows through each tube when the amp is turned on, Standby is turned off, power applied to the screens, but no signal applied.

You should go onto YouTube and look for videos that show you how to measure and set tube bias. There are multiple methods. The problem here is that you have a "set of tubes" ... two for the positive swing, two for the negative. So the only way to check the bias in each tube is to measure the bias for each tube. You can do that with a fancy bias probe or use the Cathode Resistor method..

BEING VERY CAREFUL and with the caps discharged, you would undo the connection between Pin 8, the Cathode, and ground. You then install a precision (1%) 1 ohm, 1/2 watt resistor between Pin 8 and ground. You do this on all four tubes. When the amp is turned on and Standby off (meaning you are running full boar), you measure the dc voltage at Pin 8. the negative probe of your voltage meter will be connected to ground. Set your meter to the 200mv scale. You will be reading millivolts across that resistor. For each millivolt you read, that translates to one milliamp. So you take a reading across all four resistors. This is how you can tell if you have close to matched tubes.

The Peavey circuit presets the Bias for you. Look at this part of the schematic - right in the middle. There you can see IR8, IR9, and IR10. Just to the right you see a negative voltage. The same voltage feeds all four power tubes. So that tells you that (1) Peavey has preset the DC bias voltage and (2) the only way to adjust the Bias (bias voltage and current) for all four tubes is to mess with those three resistors.

Typically, Peavey chooses a voltage so that the tubes are running "cool" - less than 70 percent power dissipation. Take it from me. I am one of those guys who experimented with resetting the bias voltage to a higher setting and I would NOT hear a difference. So best not to touch this - not unless those three resistors have gone way out of spec and the Bias voltage is way off base.

The other option here is that you install a set of "Matched" tubes. I have purchased several matched tubes over time and on average, the bias is within 10% of each other. I would call that good. Others might not agree.

I hope this helps. Corrections from Forum members welcomed !!

[glebert]

Tom C said most of what I was going to say, but would add one thing. Try grounding the input to the power amp by putting a guitar cable into the Power Amp In jack and short the cable plug tip to sleeve. Does that improve the hum? If not, the hum is totally in the power amp. If that is the case I think you are looking at either needing to recap or retube with better matched tubes and it is unclear without more information which could solve the issue. Caps are much cheaper than tubes, and with the age of this amp the factory caps are going to be somewhat suspect anyway.

[Helmholtz]

Tube current at idle (= powered but no signal) is often called the bias (current).
This should not be confused with the grid bias voltage which sets the idle current.

You could use the OT resistance method described here: https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

With all power tubes in this will give the sum of 2 tube currents per OT side.
We are looking for a gross imbalance between the 2 sides.

Ok, so this amp uses a SS phase inverter.
Measure base and collector voltages of the 2 transistors.