Weird voltage reading Selmer

[EFK]

One of my old Selmers (TnB50) is acting a little funky. If I try to measure voltage on one of the power tube plates (3), I get a weird buzzy static-like sound through the speakers and it only reads about 35V. Doesn't matter if I wiggle the tube around, probe different parts of the tab etc. The other tube plate reads just fine, @ 430. Screens (4) on both are the same, @ 415, and both tubes read just fine for bias voltage (across 1 ohm resistor) within a few mA of each other. It doesn't matter what tubes I use, same thing on the one socket. I measured resistance from ct of OT to each side and it's within 5 ohms per side. If I pull the power tubes completely and power on, the voltage is obviously higher but then - and only then - it is the same on both sockets (pin 3) as it ought to be. Only when I stick in a tube - any tube, all known good tubes - does it act weird. The amp makes sound just fine, although it sounds a bit weak compared to others of the same amp I have. I don;t think it's making full volume. It's not making weird noises or anything, though. I've retensioned and cleaned the sockets more than once also.

Any ideas? I can't see any signs of arcing or anything being burnt etc, and no crackling when I wiggle the tube around.

[Chuck H]

Is the amp red plating that tube??? With the voltage down only with a tube in place that suggests extreme current draw. Though I would think this would be somewhat evident on the other side too. Maybe check for a leaky coupling cap from the PI.

[EFK]

Nope, not redplating at all. Heaters look fine also. All the other voltages everywhere are just perfect - it's only the pin 3 tab on V6 (V5/V6 are EL34), and only with a tube in it. Without power tubes, the voltage on pin 3 of both jumps up to 481, but it's the same on both. I removed the old solder and resoldered the wire connection as well.

????????

And what's with the weird static buzzing noise through the speaker when I probe it? The other plate pin doesn't do that, just the usual little tiny 'pop' with the volumes down.

[pdf64]

Maybe putting the probe onto that plate is sending the amp is going into oscillation, and the oscillation frequency is confusing your meter.
Try taking out the phase splitter tube (that should stop any oscillation) and see if the plates measure as you would expect.

[MWJB]

Ditto what PDF64 says, T'n'B is aseries of Selmer amps, rather than a specific model...some of the layouts were quite bizarre. You can see a similar effect in Fender Blues Jrs too.

[EFK]

Well hot damn it worked! Yank the PI and the plate readings are identical. So, someone feel like explaining to me what is happening here and why it is only one of the tubes doing this? Also - ok, so it oscillates when hit w/ a probe. Obviously I'm not playing it while measuring plate voltage. So, is this having any effect on the amp when I'm *not* checking that particular tube's plate voltage?

[MWJB]

If the amp sounds OK when you are not measuring/probing then it probably is, connecting up the meter could be setting it off. It's probably always been the case and it didn't bother you just because you weren't aware of it. Connect an AC RMS voltmeter accross the speaker, play through the amp at the highest volume that gives you a clean tone, determine an average voltage (it will fly around some), square it & divide by speaker ohms...what do you get?

[Enzo]

This is more common that you think. I don't keep track, but I encounter this on mainstream amps too.

Your meter leads act as antennas on the output tube plates. The tube on one side has opposite polarity from the tube on the other side. The tube that results in a positive feedback causes the oscillation. I think the average your meter can interpret this high amplitude and very high frequency signal on the DC scale is what you see. Stick another meter on the center tap and see what the B+ is really dragged down to.

Try scoping instead.