panel layout

 


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more changes to the schematic

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  ...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.
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blue smoke release

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...
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level shifting and DC gain

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, ...
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