the McGrath Mini amplifier

    I've redrawn the v-mid circuit more cleanly, to help myself avoid confusion (hopefully).  I'm calling this circuit "v1-b", because it is similar to the old "v1", but has some differences. I made the changes needed to take the v2 circuit back to v1-b, in the prototype amp.  I did this all from the front side of the board, as I had discussed.  But unfortunately, it doesn't work!  Specifically, the level and frequency knobs seem to have no audible effect on the sound at all; however, there is a slight increase in volume when the circuit is switched in, consistent with its approximate gain of 2 (actually probably a little less, but more than 1).  So this seems to indicate that at least the second triode stage is functional.  But there's never any audible boost or cut of any frequency band, it seems stuck on "flat". It is possible that my soldering heat caused something on the underside to short together or come disconnected; but after much ...

  When I discovered that I couldn't use my variable-frequency midrange circuit with the Baxandall topology, this also threw into doubt my 3-band EQ for the McGrath Stereo amp.  I had planned to use a conventional Baxandall bass & treble EQ, and add my v-mid circuit in parallel with the other two, in the same feedback loop.  Now that I know that's not possible, I was left in a quandry.  Run a separate v-mid circuit in series with the Baxandall?  (I've seen this done in some other amps like Ampeg which have some degree of variable mid frequency.)  But I couldn't see a way to do this without adding at least one triode stage, so it would no longer fit in a single 12AX7. But last night it occurred to me, I could just ditch Baxandall entirely, and implement the bass and treble with the same "cathodyne" circuit that I'm using for the v-mid: all three bands in parallel (just like I wanted to do originally with the Baxandall).  I have drawn this up, above (...

Before undertaking the substantial effort of changing my v-mid circuit back to "v1" as I discussed previously, I took some time to try to understand the Baxandall tone control design a little better, and I think I made some headway.  In short, I don't think there's a practical way I can use it for my v-mid circuit: i.e., there's no way to save the "v2" circuit, it simply has to go back to "v1". This shows the bass & treble active tone control from Baxandall's original paper.  The fact that the treble pot is connected directly between the input and output lines, was what misled me in my attempt to adapt this circuit to the variable-frequency midrange control.  I basically have my level pot directly across input and output in the same way, and then I substituted my variable-frequency bandpass circuit for the capacitor C3 in the circuit above.  But this is not really valid (as the failure of my circuit to operate properly, demonstrates!). ...

  This shows the changes needed to take the v2 v-mid circuit back to v1 (more or less).  I'm hoping I can make these changes entirely from the top side of the board, because getting to the bottom side would entail a lot more disassembly.

    So I'm considering rolling the circuit back, from v2 to v1 above.  The v1 circuit I have built and verified in the past: it was in the first prototype of this amp.  The v2 circuit is what I have now, which is not working.  That circuit is purely theoretical, my own design but closely based on known-good existing circuits.  There's probably a wiring error -- but so far I can't find it.  By the time I convert the circuitry to v1, I'll probably find and correct the problem, but at least I can be viscerally certain that if I build the v1 circuit, it will definitely work.  If it doesn't work, there's an error somewhere.  With v2, I just can't feel as certain of that.

Well, I've transported the McGrath Mini down to San Diego, where I'm continuing the testing and burn-in process.  Also, I hope to make some playing videos, because for guitars I have not only my Squier Mini-Strat for the single-coil sounds, but also my lovely Carvin DC-200K, a humbucker guitar with Gibson scale length, i.e., about the closest approximation to a Les Paul that I'm likely to get any time soon (actually the Carvin is much better than a Les Paul IMO). Anyway.  In the testing arena, I continue to be pleased with how much the 10pF "degenerator" caps have improved stability.  It is still possible to get some pretty nasty self-oscillation out of her by turning up all the gains and maxing the mid and treble in the M-section, but it's at least less likely to happen at "reasonable" settings.  I still have some more shielding work to do, so maybe this will help. But the main outstanding problem I have noticed is that the v-mid EQ doesn't seem...

  I determined that the lower-than-expected fx loop gain was due to a wiring error: effectively, I was only running through one of the two gain stages on the return, so instead of a gain of 100, I had a gain of 10; given the send attenuation of 20:1, this led to an overall loop gain of 0.5, which was commensurate with the lower level I was hearing when the loop was engaged (-6dB). After fixing the wiring error so that both stages were operational, the gain (as expected) was too high.  Theoretically, the overall loop gain would have been 5, but my ears told me it wasn't quite that high (for complicated math reasons, the gain of these negative-feedback triode stages isn't quite as high as the simple resistor ratios would suggest; akin to how cathode followers have a true gain slightly less than 1).  Nonetheless, I first tried substituting a 1M resistor for the 4.7M feedback resistor in the second stage.  This should have theoretically resulted in a perfect gain of 20 o...