Odd voltage reading...

[Bob M.]

I have a homebrew amp. It's a 2x6L6, 1 channel amp and it's fairly Fender-like with lots of extra bells and whistles. It is fixed bias amp with a bias pot for each power tube but also I have a switchable cathode biased mode. The PI is a typical long tailed pair.

I'm preparing for a long tour and I do a 'tech day' measuring every critical voltage and then some for (hopefully) trouble-free performance. I have an unusual problem that I've never seen before: In the power amp, on the plates of the power tubes, there is about a 30 volt difference, about 412 Vdc on Pin 3 of V7 and about 380 Vdc on Pin 3 of V6. I replaced the power tubes with a new matched set (same type) and this voltage difference at the plates remained exactly the same. Then, I reversed the new power tubes (plugged tube from V6 into V7 and vice-versa) to see if the voltage difference followed the power tubes or stayed at the same socket: it stayed at the same socket.

I re-checked the bias. Both tubes have -45.0 Vdc on the pin 5 grids, both tubes measured 29 mA via transformer shunt method. I removed the 6L6 power tubes and measured the resistance across both primaries of the output transformer - normal. Then I put back in the power tubes and put the amp into cathode bias mode. I then had 425 on both plates and 30mA current draw on both tubes thru the 510 ohm/10 watt cathode resistor - everything seemed perfectly normal in cathode bias mode with both plates reading normally.

I measured each part in the power amp and PI. The screen grids and grid stoppers all measured perfectly to spec (by the way, all the screen voltages were normal in both bias modes). The plate load resistors of the PI measured normal as did the cathode PI parts (470 ohm, 22k/ 1 watt resistors). All PI voltages seemed normal from previous measurements. I've used this amp continuously for about 20 years and have a ton of recorded data and voltage readings from benching it about once a year. It has been reliable and stable.

So my problem seems to be only in fixed bias mode at pin 3 of V6. What gets me is that the pin 5 grid voltages are equal on both power tubes and the current mAs are the same on both power tubes but I have a 30 volt difference at the plates of the power tubes - never seen this before. I'm thinking of replacing the tube socket at V6, where the low voltage is. What else could this be?

Thanks for your comments,

Bob M.

[imaradiostar]

What is the dc resistance of each half of the primary?

Transformer shunt has given me trouble in the past. Could you temporarily install 1 ohm current sense resistors between the cathodes and the fixed/cathode bias switch to verify your readings and tube matching?

JT

[Enzo]

Yes, what are both those OT resistance readings? Got no idea in the world what normal is on your homebrew amp.

But in cathode bias they even out. SO when in fixed bias, something must ground the cathodes of the power tubes. What does that? And in that mode, measure resistance to ground from each cathode. Maybe one shows some unwanted resistance. Are there 1 ohm resistors you failed to mentiion, and could one be not a 1 ohm by mistake or by damage? Or a set of switch contacts that have gotten dirty or oxidized.

You characterize this as a "problem," but is this a recent thing, or can we assume the amp has been this way and you just now noticed it? If this is a recent CHANGE, then we find out why. If the amp has apparently been this way, and it operates well and reliably and sounds good, then it is not really a problem. It is merely something you can't explain.

I can't imagine the PI involved in any way. One, it has no idea what tubes are in it, and two, there is no DC connection between the PI and the power tubes.

[Bob M.]

Thanks for the comments. I've been using this amp for about 20 years and every so often I bench it and measure all relevant voltages throughout the amp and record them in my amp log. This way, if there is any change in performance, it is easy to find the exact part of the circuit that has changed. In this way, I have defined 'normal', when the amp's voltages are the same or unchanged from service routine to service routine.

I seem to recall that the output transformer resistance readings from each side of the primary were very close to one another, about 240 ohms, within about 5 ohms of one another. In the past, I've seen shorted output transformers where each side varies from one another by 50 or 100 ohms.

I've been repairing amps for about 30 years and the transformer shunt method has always been the most accurate way of evaluating power tube bias, in my opinion. In fixed bias mode, Pin 8 on each power tube is connected to ground and both readings, from each power tube, are within 0.10 of an ohm. I don't normally use the current sensor resistors because it adds the screen current to the plate current and if the 1 ohm resistors are not exactly matched to one another, then the readings could have error introduced.

I think dirty switch contacts are a real possibility - one I will investigate. This is a new development in this amp. As the amp has seen lots of travel lately, I've been benching it at 6 month intervals and comparing all relevant voltages to data from the last 20 years. This is a recent change, without doubt.

I don't think the PI is involved either. I just look backward in the circuit as a matter of habit. All PI voltages are exactly the same as my last service/benching.

The amp does sound good and has been reliable but I would like to find the problem for this recent voltage change. I might have a bad/cold solder joint or a wire with an internal break of some of its strands. I think I'll replace the tube socket and see if the rewiring helps. It's a power amp problem, for sure.

Again, thanks for the comments.

Bob M.

[Enzo]

If your OT measures the same for both halves of the primary, so much the better. But there are many OTs not wound that way and the two halve are substantially different in resistance. The turns wound and turns ratio through the thing are fine, and in fact the transformers are fine, but the one half of hte primary is would around the other and so the same number of turns takes a much longer wire - so the resistance differs. SO a 50-100 ohm difference may be just the way it is wound.

When you are shunting the primary with a current meter, then that resistance become irrelevant, because it is then in parallel with the low resistance of the meter shunt.

When I install 1 ohm resistors, I use 1% resistors. 1% is close enough for me.

You may prefer the shunt method in the plate lead, I prefer that myself, but do you have a bias probe socket adaptor anyway? it might be illuminating to see if using it makes a difference in your mismatch reading. Plus with power tubes removed, measure resistance from the top of the adaptor pin 8 to ground. That way you are measruing not only the cathode switch resistance, but also you are measuring THROUGH the tube socket not just AT the tube socket. If you follow me.

[J M Fahey]

Please measure and post DC resistance of each transformer half and DC voltage measured, at idle, across same windings.
Thanks.

[Bob M.]

Unfortunately, I'm away from my bench and all my test equipment save my tech's Fluke DMM. We're in rehearsals now and I have to make do with what we have here. I don't have a bias probe but I did measure the resistance from pin 8 of the power tube socket to ground and get a reading of around .1 ohm for each power tube socket. I realize different types of transformer construction yields different numbers but as I've recorded these readings in the past in my log and they remain exactly the same - before the amp was exhibiting this problem - it seems safe to say the problem is probably elsewhere. I do have a duplicate output tranny at my house and I'll be stopping there in about three weeks and I'll take it with me, just to be sure.

It seems the evidence points to the bias select switch (it's a small-bodied switch) or the tube socket. I may unwire the biasing options - I'm more fond of the fixed bias sound for this particular amp and see if that corrects this low voltage situation. I need to inspect the tube socket with a magnifier to make sure there is no carbon tracking. As the amp seems to work and measure perfectly with cathode biasing, I could always use that for the next period of time but I'd prefer to get to the bottom of this situation. Again, thanks for the comments and bringing up things I hadn't thought of.

Once again, the primary resistance of the output transformer is around 240 ohms per side, within about 5 ohms of each other and the idle mAs are 29.0 per side exactly. The pin 5 grid voltage is -45.0 Vdc exactly for both power tubes, V6 and V7. BUT, the plate voltages are: V7 : 412 Vdc and V6 : 380 Vdc. The reason I can get grid voltages and the mAs exact is because I have a bias pot for each tube. Biasing all appears to be working fine, as intended, as it did before my low voltage problem appeared. Usually, I would suspect a transformer in this situation with these numbers but as everything works fine in cathode bias mode (the problem disappears), then I think the switch could be the culprit - or the tube socket. Tomorrow I will have time to do some advanced investigation and to replace the tube socket and the switch. And then I'll report back. Thanks.

Bob M.

[J M Fahey]

Once again, the primary resistance of the output transformer is around 240 ohms per side, within about 5 ohms of each other and the idle mAs are 29.0 per side exactly.

Once again,
Please measure and post DC resistance of each transformer half and DC voltage measured, at idle, across same windings.
Thanks.

[Bob M.]

I replaced the V6 power tube socket, re-soldered a few connections that looked possibly iffy, spayed contact cleaner into the bias select switch and now the amp is working, measuring and sounding perfect. I didn't really find the fault but I need this amp to be stable and it is now. Thanks for your suggestions and help.

Bob M.