further testing...

OK, latest results.  Or really, mainly, lack of results.  Acquiring clues, I suppose.  Always need more of those.

So I went in and removed or disconnected most of the "extra" circuitry in the prototype.  Specifically, the variable-bias circuit.  And I replaced the 22uF/50V cap in the gain control, with the original 40uF/600V cap.  With the smaller 2.2k cathode resistance, the cathode voltage should never have been anywhere near 50V, but I just wanted to make sure that this cap wasn't in any way related to the "blue smoke" situation.  So now, the only place where the circuit really differs from a plain-jane tube gain stage, is the DC-coupled interstage connection, using a Zener diode where there would usually be a coupling cap.  How could that be releasing blue smoke?

Anyway, I fired it up, and again I found that there was very little gain from the first section; and the gain did not vary at all with the gain pot.  It's like it was always at minimum.  I left it powered up for a while, hoping to see some more blue smoke and isolate where it was coming from, at the cost of sacrificing the component(s) involved.  I didn't see smoke, but when I twiddled the gain knob, I again heard a bit of that scratchy, too-much-DC-on-a-pot sound, and then I actually witnessed visible arcing inside the pot.  The gain pot, not the bias pot.  And interestingly, the arcing that I saw was happening between the wiper and the track of the *second* stage (dual-gang pot).  I.e., not the section I've been making all the changes to; the section which actually seems to more-or-less work, producing reasonable gain, albeit with crappy tone quality.

This second section is still wired the original way, i.e., with equal cathode and plate resistances, so there is indeed a high voltage across the pot.  I had hoped that the 100k fixed resistor in parallel with the 250k pot would absorb more of the current flow and protect the pot, but maybe it's not enough.  Anyway, I'm no longer committed to that circuit design (although my new design works even worse!).  I guess I just need to assume this pot is toast and toss it, then figure out where I stand.  Maybe the side of the pot associated with the first section has been somehow completely destroyed during its days operating at high voltage, even though it seemed to behave normally and control the gain at that time, and so now that's why it has no effect on the gain.  That would be a nice explanation!  In that, maybe when I replace the pot, the circuit will suddenly be able to produce some gain.  We'll see...



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