the McGrath Mini amplifier

  Changes to the circuit since the previous schematics: * input section: voltage divider (attenuator) ahead of boost circuit, to moderate the level of boost when activated. * vmeq section: redesigned back to my older "cathodyne" circuit (v1-b). * M-drive section: cold switch has three settings: 10k, 39k, 2k. * fx loop section: feedback resistor in second return stage reduced to 2M, to trim overall gain close to unity. * power amp: 13 kHz LPF between vol pot and 5k6 grid leak resistor, to suppress parasitic oscillation. * relay control section: LED resistors increased to 56k on green (K3) and blue (K5) LEDs, to normalize brightness. * power supply: initial 1k resistor ahead of first electrolytic ("sag resistor") removed, other resistors changed from 10k to 1k. * all 12AX7 tubes: 10pF snubber caps across pins 1,3 and 6,8. * EL84 tube: 10pF snubber cap across pins 3,7.

https://youtu.be/gHR9m0GR7P0 So I applied both of the "technological" solutions discussed in the previous entry: i.e., snubber cap on the EL84, and 13 kHz LPF after the master volume pot.  The latter seems to have fixed the oscillation issue. Next work is to fit the replacement v-mid eq circuit board into the box, and then test all the amp features and functions before buttoning it up.  E.g., I have never fully tested the fx loop with an actual effects unit, I have merely adjusted its gain and fixed the oscillation issue it was having.  Also, most of the non-relay switches (the four on the upper back panel) have never been tested, or only minimally (voice, scoop, bright, and coldness).  I just need to comprehensively run the amp through all its modes and extremes of operation.  Because, cramming it all back into the chassis box is such a nightmare, that I'll hope to never need to repeat it again!  After shipping this prototype, I will implement the final ci...

  https://youtu.be/irM8_pmS-yQ So I noticed, last time I tested the amp (prior to all my recent work with the v-mid EQ), that there was a problem with the effects loop.  At low to moderate volumes, it seemed to operate normally, but with the master volume near max, the amp would seem to break into ultrasonic oscilllations. This was before I got my new digital scope, so it was just my auditory assessment, based on past experience: everything would be operating as expected, getting louder as the volume was turned up, but then suddenly it would hit a point where the volume dropped precipitously, and the tone became sort of ragged and muffled. Now that I have my scope, as you can see in the linked video, I can confirm my diagnosis.  The amp breaks into oscillations at around 45.6 kHz. So obviously, I am going to examine my layout -- a lot of wires and components are necessarily very close together, with my compact design.  I don't have shield covers on the preamp tubes....

So I built a copy of the "v1" v-mid EQ circuit on a separate board, as discussed earlier.  But when I patched this into the amp, no joy.  There was no sound passing through at all.  Initial circuit tracing didn't find the problem.  So I took some time out to step up my troubleshooting game.  I built a 555-based tone generator for signal tracing, I ordered a cheap Chinese digital oscilloscope online, and I built a general-purpose tube circuit test fixture.  Running the v-mid board standalone in the test fixture, I was able to trace it out with the scope and discover the wiring error which was leading to no sound.  So now I have it roughly operational, which is a big boost to my sanity!  However, some more work is needed before I can try to fit this circuit board back into the amp -- which will be a challenge in itself. Next task when this is done, will be sorting out the effects loop.  It's been several months of setback and delayed progress n...

Issues continue, with the v-mid circuit and also with the fx loop.  Seems like stuff is breaking, right and left!  It had seemed originally that the fx loop was working properly, after I straightened out some circuit errors.  But I hadn't actually tested it comprehensively, beyond setting the gain level to be near unity.  Just now, when I was trying to use it as part of the regime for testing the new v-mid circuit, it malfunctioned badly.  When volume was up near max, it broke into obnoxious squealing.  Was it doing that before, and I just hadn't tested it?  Or is this due to the chassis being out of the box, with a bunch of test wires going every which way?  I need to track down the problem, obviously.  I'm not certain that I actually have the chassis securely tied to ground, so that'll be one thing to examine early on. But anyway.  I've been feeling increasingly hampered by not having an oscilloscope, for this kind of debugging.  ...

    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 ...