first test results...

Well, I wired up basically what is seen in the schematic diagrams I've posted, plus-or-minus some different component values depending on what I had available.  I only ran one tube stage, because I'm still waiting for shipment of my dual-gang pots.  And initial results are, well, promising -- but not yet "home run" territory.

With one stage, into my Fender Champ, well, it definitely distorted.  Did it do anything different or more than just a regular tube overdrive stage?  It's hard to say, but I think maybe so.  Almost any kind of tube overdrive or boost into the Champ tends to sound pretty awesome.  It definitely produced some tasty distortion which was well beyond what a Champ can do on its own, but this was to be expected.  At the very max limits of the controls, it seemed like I could begin to detect a slight edge of that "fuzz thing", that shifting vocal quality that seems to ride the envelope of the incoming signal.  But I'll have to see how it does with two stages.

It certainly behaves like a transistor fuzz which is not turned up all the way; it makes me want to turn up the gain more, but it's already maxed.  This comports with the reality that the two triode sections have less gain than two transistors -- which is of course why I wanted to build a two-tube version of the circuit (four triodes).  The transistors used in typical variations of the Fuzz Face circuit tend to have "gains" in the 100 to 150 range, whereas the 12AX7 triodes are probably operating around 50.  So, 150 * 150 == 22500, and 50 * 50 == 2500.  There could be as much as a factor of 10 difference in the overall gain of one section.  (But, 2500 * 2500 == 6250000, so with two sections I doubt the problem will be lack of gain!)

There's tons of noise and oscillation, as well.  If I can get anything approximating the kind of fuzz tone that I'm seeking out of this circuit, then the next task will be to clean it up and try to tame it, perhaps with degenerative capacitors across plate to cathode, etc..

So, again, promising results so far.  I *think* I have something different/better than the usual no-feedback cascaded tube overdrive circuit.  More testing and experimentation remains to be done...


I'm using one of these very-convenient switching power supplies, designed specifically for tube circuits.  It produces 260VDC for B+, 6VDC at several amps for the heaters, and also has an auxiliary 5VDC output available for logic circuitry.  The B+ rail ramps up slowly after about a 30 second heat-up delay.  It's basically everything one would want in a tube-oriented switching supply.  It's aimed at tube preamps, but it produces enough current that it could even power small power amps.  These are about 30$ on the Internet (at least, prior to our Brave New World of punitive Tariffs...).

I've got my usual tube circuit pilot light in the upper right corner: an RGB LED, green unused.  Red is the 6V heater voltage (10k dropping resistor), and blue is the 260V B+ (1M dropping resistor).  So red comes on immediately at power-up, then after a delay the blue fades in, resulting in a purple colour for "ready to play".  After disconnecting the power, the red goes off immediately, but the blue slowly fades out, indicating the discharge of the high-voltage caps.


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