v-mid issues continue...

 

 

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 careful probing with the DMM, I can't find any problem: it seems that everything is connected as it should be.  But I can't be 100% sure, without seeing the underside of the board.  However, I am loathe to disassemble the amp to the extent needed to get to the bottom side, there is substantial risk of stressing other connections or causing other problems if I do this.

Instead, I am now contemplating building a standalone copy of the v1-b circuit, which I can test by running it through the amp's fx loop.  I need to see this circuit actually work as designed, before I make more changes to the amp itself.  Possibly, if my standalone circuit works properly, I may just wire it in, piggyback on top of the amp main board.  Space is super-tight, but I'll find a way to make it fit, I'm sure.

I wish I had my oscilloscope!  It's in San Diego.  I should've brought it with me up to Seattle on this last trip.  Sigh...