Peavey 5150 rising voltage on OT primary and slight hum

[circuits]

Guys, I hope you can help, I'd really appreciate it. I'm about tapped out of ideas.

A little background first:
I installed the adjustable bias (cermet 10K pot and 6.8K resistor) in parallel with C41, as documented on FJA's site. No issues there, I believe. I changed tubes to 5881's for a while (biased to about 30mA per tube) just to see if I liked those better. I still think the amp sounds terrible compared to my Marshall Vintage Modern. It's kind of muddy, lacks clarity, bass and treble. It's doesn't seem to be the 5150 that people rave about, metal-wise. This is the only one I've played.

Recently I installed some new JJ 6L6GC (already had fairly new JJ 12AX7's installed). I experimented with bias about 20-35mA per tube.
My biasing method is to:

Measure the resistance for each side of the OT primary, T2:
Blue to center => 37.5 ohms
Brown to center => 29.4 ohms

Then measure the DC voltage, power on, across each of those:
Blue to center (red) => -2.51 VDC
Brown to center => -1.97 VDC

So using I= E/R, taking the top example that would be
0.067 A = 2.51/37.5
Divide that by 2 (for each tube), that about 33mA bias to each. Is that the correct way to measure it?

Anyway, it may have been happening all along, but I noticed that the VDC measurement on the OT primaries keeps climbing!
I kept an eye on it and started lowering the pot value until it had to be lowered all way. It just keeps climbing and I figured I should turn if off before a tube glows red.

Also, there is a slight hum even when all volume is turned down and no guitar attached. It gets louder as time goes on and louder if I crank the Resonance.

I have already cleaned all the sockets and pots with some good quality contact cleaner.

Any ideas for the climbing VOLTAGE and the HUM? THANKS.

[Enzo]

First thought, failing power tube.

That is one way to measure current through the tube, yes.

Take the four power tubes out. Pick one socket, and install one tube. Monitor its current. Stable or not? (after a few minutes warm up) Now remove it and try a different tube in the same socket. Same question. Now do same with the remaining tubes. By testing one tube at a time we find if any act different from the rest.

You say the voltage across the primaries climbs? Both sides?

Tubes all in, flip it over and while it runs, monitor the bias voltage at pin 5 of each socket. Does it remain stable, or does it slowly drop as your currents rise?

[Alex R]

Never heard of your bias method before, usually one thinks of transformers as having impedance rather than resistance. I will let someone wiser speculate about that. The standard methos is to put an ammeter across the same nodes, thus bypassing the winding and measuring current flow directly. Just be a it careful because once one probe is on, the other will have the B+ on it looking keenly to make a big arc to ground! (though that's not such a disaster since it will generally only blow the normal 2A probe fuse in your meter, if it has one, or your meter, if it doesn't).

There could be two reaosns for the DC climbing, one is less current flowing throuh the tubes, the other is more potential through the power supply. The first is quite likely and the cause might be a fault in the tube of dropping bias voltage. However I think it's quite likely to be a normal artefact of the amp settling down and warming up.

Hum in push-pull power amps is likely caused by imbalances in current draw between the push-pull sides, as push-pull power amps are humbucking. So there might possibly be a connection between the hum and the voltage, you see? If the voltage thing isn't just something normal.

Does the voltage thing settle down after a while?

Maybe try rinning it on two power tubes instead of four, see if you can work out if there is a bad tube by swapping them in?

eta simulposted with Enzo. He knows best

[Enzo]

Ah, you read it differently than I did, Alex. I read it to mean the voltage dropped across his OT primary was climbing, indicating increasing current. You read it as the B+ voltage rising, indicating dropping current and B+ recovering. So we need to determine which is the case.


OTs have impedances, but only to the AC - the signal. But bias is idle DC curent through the tube, so the DC resistance of the winding is in series with the current, and so drops a corresponding voltage. If your transformer has a 3.4k impedance for example, you will never measure that with a meter. And in fact that impedance will only show up to the tube if the load on the secondary is as specified.

[J M Fahey]

Dear Enzo, you read it right (why doesn't it surprise me? ):

Measure the resistance for each side of the OT primary, T2:
Blue to center => 37.5 ohms
Brown to center => 29.4 ohms

Then measure the DC voltage, power on, across each of those:
Blue to center (red) => -2.51 VDC
Brown to center => -1.97 VDC

What's nice about this is that there is less possibility of getting that bright blue arc in your face.

[Alex R]

nice safe method then, I still have a scar on my left palm from brushing the second probe there somehow. Strangely I never did it again, I guess it's like cows and electric fences.

[J M Fahey]

Sorry to hear that.
And I agree, there's NO "safe method" when working with over 400VDC, in cramped quarters, on an unstable upside down chassis.
If anything, a "somewhat less dangerous way".
Oh well.

[Enzo]

I have worked on electric fencers, but over the years I have found cows to be less than efficious in the repair shop.

[circuits]

Thanks for all the replies. Appreciate it. The problem is SOLVED!

Enzo --- you were correct. The whole thing was a power tube. I didn't know you could test them one by one without blowing up the amp. There was ONE power tube that kept climbing in voltage while the others started lowering a bit and stabilizing after about 5 minutes. I had an extra JJ 6L6GC and I paired up the ones that seemed to match closest in the voltage measurement. So the matching ones went the two inside and two outside sockets, respectively. Not sure if that helps anything. ;-) All the measurements were still within 45 millivolts of each other.

There is still a slight hum, but a very minor issue. The tone is actually much better. It has more bass and treble and not as muddy (using a Mesa Recto cab). I do have some 12ax7 Tungsol's that I can try experimenting with in the preamp.

I still like the tone of my Marshall and greenback cab much better, so putting thought into selling the 5150 down the line. Thank you so much. A great learning experience from you guys.

First thought, failing power tube.

That is one way to measure current through the tube, yes.

Take the four power tubes out. Pick one socket, and install one tube. Monitor its current. Stable or not? (after a few minutes warm up) Now remove it and try a different tube in the same socket. Same question. Now do same with the remaining tubes. By testing one tube at a time we find if any act different from the rest.

You say the voltage across the primaries climbs? Both sides?

Tubes all in, flip it over and while it runs, monitor the bias voltage at pin 5 of each socket. Does it remain stable, or does it slowly drop as your currents rise?

[circuits]

New to forum, learning the posting ropes, my initial reply when above, just wanted to get it to the bottom.....

Thanks for all the replies. Appreciate it. The problem is SOLVED!

Enzo --- you were correct. The whole thing was a power tube. I didn't know you could test them one by one without blowing up the amp. There was ONE power tube that kept climbing in voltage while the others started lowering a bit and stabilizing after about 5 minutes. I had an extra JJ 6L6GC and I paired up the ones that seemed to match closest in the voltage measurement. So the matching ones went the two inside and two outside sockets, respectively. Not sure if that helps anything. ;-) All the measurements were still within 45 millivolts of each other.

There is still a slight hum, but a very minor issue. The tone is actually much better. It has more bass and treble and not as muddy (using a Mesa Recto cab). I do have some 12ax7 Tungsol's that I can try experimenting with in the preamp.

I still like the tone of my Marshall and greenback cab much better, so putting thought into selling the 5150 down the line. Thank you so much. A great learning experience from you guys.