Unusual problem...

[Bob M.]

I'm working on a Crate Vintage Club 20 amp. It's a 2x EL-84 amp, push-pull, cathode bias, open looped - no negative feedback. It needed lots of new tubes. After installing a matched duet of EL-84 power tubes, the power tube current draw (mAs) of the two tubes was within .2 mAs of each other, a very good match. Screen and cathode voltages were normal and exactly matched between the two tubes but the plate voltages were 15 volts from one another, V3 = 320 Vdc, V4 = 305 Vdc. First thing, I reversed the two power tubes from socket to socket but the voltages stayed as is with the corresponding sockets. I then unhooked the output transformer and did an ohmage check on the primary which appeared to be normal. I then reversed transformer primaries going to the tube's plates, as there is no neg feedback in this amp, it seemed a valid test and the plate voltages remained the same. The only components on the power tubes, besides the cathode resistor and capacitor which are common to both tubes, are 2x 100 ohm screen grid resistors both of which measure perfectly, less than 1% off. And there are 2x 1N4007 diodes that go from plate to cathode. Initially, I suspected one of these - they both measured perfectly in circuit but decided to replace them anyway. After, there was no change from the condition I'm reporting; I'm still 15 volts down on the V4 plate. I closely inspected the socket with a magnifier and re-soldered all the pins to no avail.

What stumps me is that the mAs are perfect between the tubes and everything appears to be working normally with the exception of this 15 volt plate discrepancy. I have done everything short or replacing the V4 socket which appears to be in great, almost new condition. Any ideas?

Thanks in adv,

Bob M.

[MWJB]

Yes, it's either normal, or the difference is the result of oscillations caused by hooking up the meter (though this is often a bigger discrepancy that you have). If there are no audible symptoms, then you probably don't have a problem.

[jmaf]

The voltage should be constant for each OT plate connection. When you switch tubes the voltage should be the same on that socket, not on that tube. That is because the OT primary's two halves are asymmetric. About the current being the same, it's probably a coincidence the slight tube mismatch probably matches the OT's asymmetry...

[imaradiostar]

The voltage should be constant for each OT plate connection. When you switch tubes the voltage should be the same on that socket, not on that tube. That is because the OT primary's two halves are asymmetric. About the current being the same, it's probably a coincidence the slight tube mismatch probably matches the OT's asymmetry...

What he said- I'd probably measure the DCR resistance of each primary half just for a sanity check. If it sounds good I wouldn't sweat it but I can't help but wonder if it would lead to long term tube wear issues.

jamie

[Bob M.]

You're right and I did measure the resistance on the OT primaries are they were just as I would expect them to be - not exactly the same but pretty darn close. I also flipped the blue and brown end wires (going to the power tube plates) of the OT primary and all readings were the same as before, eliminating the output transformer as the source of this anomaly.

EL-84 tubes don't last long in these circuits, probably about a year with good use so I'm not concerned unless they burn out fairly quickly.

Yes, I considered that the asymmetrical windings on the OT primary could offset some small voltage and current discrepencies but 15 volts seems too large. Today, I'll try different sets of power tubes and just do an audio test and a burn in. I guess this is up to a piece of mind issue for me now.

Thanks for the replies,

Bob M.

[g1]

Like MWJB said, it is probably oscillation due to meter placement. Blues Juniors are notorious for this and I wonder if your amp has the same type of ribbon cable set up.
You have pretty much eliminated any other possibility. Two other things to check if you want to are the wiring/traces from cathodes to cathode resistor and you could swap the sockets.
Some people with this issue in Blues Junior's have found different types of tubes may not oscillate, but I doubt it is oscillating without the meter so it is probably normal.
A scope would be handy here.