tube fuzz

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 m...

Yesterday, testing the most recent circuit, I had a bit of a setback.  First off, the section I had modified seemed to have practically no gain.  As in, just slightly over 1; maybe 2?  Switching from bypass to enabled, I could get a little more volume if the level was really high, but there was very little difference.  And, importantly, the gain pot seemed to do absolutely nothing.  No change in gain, from min to max.  So this seems like maybe I had a circuit error, relative to my latest schematic.  Kicking in the second section (not yet modified from the older circuit), I had a fair bit of gain, just like it used to behave.  So that section was still operating "normally". But then, as I twiddled the gain and bias knobs, I seemed to hear a crackling sound, like too much DC on a pot (?), and then all of a sudden there was that dreaded, quiet little sound of something blowing up.  I cut the power.  There was a smell of too-hot, blown compo...

  ...Just keeps looking better and better. A few resistor and cap values changed.  But the biggest change is, now I'm going to use two 120V Zeners in series to drop the regulated 260V B+ rail down to 20V, so that the range of the bias control is more reasonable.  (I could, more plausibly, also use one 20V Zener to do this, but now that I have the 120V units in stock...) I guess I could put a capacitor on this newly-created voltage reference, and then feed it to both legs of the dual-gang bias pot; but I'll probably just duplicate it and keep everything separate, for (conceptual) simplicity.  I'm not initially building out the second section of this circuit anyway, until I can test the first and find out whether it actually behaves any better than the "take 2" circuit.