Initial Testing and Tweaks

I've been playing it a lot. It's nice and quiet and behaves pretty much as expected. I made a few alterations/fixes:

- I had done the start-up and first week or so of playing with 5881 tubes (these came stock in the '59 Bassman) in the power section of the amp. They are/were a 23 watt version of the common 30 watt 6L6 tube. I changed over to KT66 tubes last week, and when I tried to adjust the bias for them, found I could not dial in a low enough voltage to get them into their optimal operating range. So, I reduced the bias circuit resistor value from 220k to 150k, and now the adjustment pot should cover a good range for both sets of tubes... I'll try some 6L6s tubes next, so hopefully the adjustment will work for them as well. In the process of doing that work, I made a mistake and blew one of the two 10uF capacitors in that part of the circuit. Luckily I had ordered extra of everything and had another cap to replace it. Live and learn - When I was organizing my resistor warehouse, I had mislabeled a resistor as 100k Ohms when it was just 100 Ohms. Eeeek! 

- added a 130pF capacitor bypass to the resistor on the V2GL switch (controlling grid leak and voltage divider just before the grid of the first V2 triode) the in the hopes of bleeding off some high frequencies when the switch is engaged, mainly for high gain configurations.

- reduced the 56k negative feedback (NFB) resistor value to 22k to increase the impact of the NFB when that switch is thrown. I left the 75k side of the switch as is, so now it provides selectable NFB of 22k, 75k, and none, which prevents any feedback at all from reentering the phase inverter. 

- reduced the value of the mode switch resistor from 470k to 100k to increase the signal strength from V1A to V1B in cascade mode (also a high gain improvement modification). It turns out, an un-bypassed 20k cathode resistor increases the clean input headroom on a 12AX7 (on the V1B triode) by quite a lot, and the V1A channel at full gain was just barely overdriving it. The lower resistor value has helped, but boosting the input signal strength is really the thing that pushes it to the right (sounding) place.  I'll play with lower cathode resistor values to address this, so that an input boost is not needed. 

- realized I had mis-wired the presence control, which, besides rendering the presence adjustment practically useless, resulted in a lower total resistance to ground (10.5k) for the phase inverter cathodes than designed (15.5k).  Changing this improved the functioning of the presence control and increased the overall gain of phase inverter. 

I also mocked up a paper faceplate so I can see where I am setting the controls as I play around with things. I now realize that any lettering below a knob is hard to see from above, so I'll have to reset the layout to improve the visibility and usability of it.

Here's a link to a deeper dive into analysis of the pre-amp section as built.