tube fuzz

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.

  How's that for a good-looking schematic.  I know it looks like I did it with a sophisticated CAD program, but believe it or not, it's actually hand-drawn.

Yes, I think the Zener diode level shifters may be an important piece to this puzzle.  Without the Zeners, i.e.  with the simple resistor dividers as level shifters, I'm up against a no-win situation as regards gain.  The tubes must be operated near voltage midpoint, i.e., with large cathode resistors.  This implies gain of 1.  In this configuration, all three resistances, i.e., the plate resistor, the effective resistance of the tube plate-cathode path, and the cathode resistor, are approximately equal.  To shift the plate voltage down to the proper grid voltage, requires at least a 1:1 resistor ratio, i.e., gain of 0.5. There seems to be an inevitable no-win voltage-scaling situation, in the absence of Zeners or other non-resistor level shifting.  Operating the tubes so that the target voltage at the grids is below midpoint to increase the DC gain above 1, means the plate output voltage must then be scaled down with a more-aggressive resistor ratio, ...

Thinking more about this DC gain issue.  If, indeed, what is needed to create the magical fuzz effect is relatively high DC gain, well, I sure don't have it. The innate DC gain of each of my tube stages is right around 1.  But then, I run through resistor dividers: one between the two sections, and the other (effectively) in my variable-bias circuit.  In each case, the divider is acting as a level-shifter, to bring the high DC level of the plate outputs down to the bias voltage required on the grids, which is considered to be roughly 4 volts below the cathode voltage.  That's why I'm running the tubes with large cathode resistors, to raise the target grid voltage up to closer to the plate voltage.  But nonetheless, the target voltage will always be well below the plate voltage, since the plate-cathode resistance of the tube is effectively somewhere around 60k.  So I need to shift the levels downwards, but the gain-reduction of the resistor ladders is an unf...

So as I continue to wait for the replacement of this digital oscilloscope to arrive (maybe by this Saturday, they're now saying), I'm running through thoughts and ideas in my mind about how to make the circuit work better. I've decided not to do any further work on the actual physical prototype, until I have a scope and can thus see what I'm doing.  This is something I've learned, in my experience of designing new circuits: it's very important to manage my own energy and emotions, and not let myself get overwhelmed by too much disappointment or failure at any one time.  Building circuits designed by someone else is a totally different endeavour.  In that case, there's a known standard of performance that I'm seeking to replicate.  If it doesn't work at first, it's always possible to keep trying and searching for the mistakes, and with enough effort, it's guaranteed to work eventually.  With a novel design, there are no guarantees, and so one ...