Can't even think of a succinct search term to describe this, but in a nutshell . . .
Having fried all but two of my available EL34s, I'm trying to test my build with a mismatched pair -- 1 is an EH, the other a Mullard reissue that I had bought for a SE project (this is push-pull, cathode biased) If I have them in one orientation, say, mullard on the left, EH on the right, it sounds fine. B+ is 440V, For testing I'm trying to keep on the safe side, and measure around 22.7 mA from each. If I switch them, however, the Mullard is drawing 222mA, and the EH is drawing zero. I get very little volume, and B+ dives. I turn it off immediately, of course. Switch, back, and it sounds fine. I've attached the full schematic. I thought maybe something was funky in my balance/RMS configuration, so I bypassed all that, and hooked both cathodes up to a single 270-ohm resistor and a 220u cap -- same thing happens.
This may or may not be related, but that 250R resistor (R24) was added to keep the wattage down to where I wanted it (~36W total max), and seems to make the overall cathode resistances way higher than I would have thought correct.
Have I done something obviously stupid here?
Also, parenthetically, that 'Rx' 68k resistor in the preamp, shown in red, was also an ad-hoc addition. When in 'M'(arshall) configuration (12AX7->EF86->tone->PI), everything sounds fine at all volumes. In the 'F'(ender) position (EF86->tone->12AX7-PI), I was getting a high pitch squeal when the volume was turned up. Adding the resistor eliminates it, although the overall volume is decreased slightly compared to the other setting. (I tried a variety of values: 1K still squealed, as did 91k and 82k. 68k stops it, but I have a sense I'm addressing the symptom but not the cause.
Suggestions? Pointers?
(Oh, btw -- the (see inset) note refers to a removed diagram -- it just showed the layout where I mount the tone controls on a tiny perf board directly on a dual concentric pot.)
Thanks,
Joe