As I mentioned I might do, I looked into lowering the resistance on the "cold clipper" function, did some modeling and testing, and decided to make the change down to something more like 10k, which is typical for the Marshall JCM-800 and many others. Now the total Rk (cathode resistance) is 11.7k when the switch is engaged (the sum of the regular resistor and the 10k addition), the output is a bit hotter, and it produces a lovely smooth distorted tone when driven by the channel I input at any setting over about 50% (7 or so on the dial). The updated pre-amp circuit now looks like this:
Before anything else, let's look at the switches that impact tone ahead of the second tube and the tone stack.
The input impedance switch (the “Z” switch) was included to give some options for signal management in front of the channel II triode. The 5F6-A and JTM-45 both include options for reducing the instrument signal here, and it seemed like a good idea to add a voltage divider in that part of the circuit for managing the high gain signal that would be coming from the channel I when in cascade mode, not unlike second stage circuit designs of higher gain amps from the 70’s. For similar reasons, I added a grid leak resistor on a switch just prior to the grid of V2 (the “V2GL” switch), which would be the 3rd gain stage when in cascade mode. I later added by-pass cap to the resistor on the V2GL switch in the hopes of also draining off some of the upper frequencies from the signal at this point, if the switch is engaged.
Below are the relative frequency response curves at the V2 input grid, for these two cases. The purple curve shows the Z switch set to Lo (200k resistor to ground), with and without the V2GL switch engaged, and the teal trace shows the Z switch set to Hi, with and without V2GL engaged.
(Cascade mode, channel I @ 80%, channel II @ 50%)
Other than simply cutting signal strength, there are some frequency response differences particularly in the low end, although they seem minor. However, what this plot can't show is the effect of the "grid leak" resistors provided by both switches, in preventing blocking distortion at high gain settings (tube behavior is a complicated thing, but Rob Robinette's explanation of distortion and grid current dynamics is about the most understandable I've come across). The by-pass cap on the V2GL didn’t quite work as intended, and modeling various different values doesn’t change the response in a way that I think makes sense to bother with, at least until I learn more. I’m glad to have both switches at this point, as they do provide a convenient way to manage breakup and some high end (via the channel I gain bypass cap) at V1B and V2 somewhat independently. They probably also help prevent blocking distortion, but I've never heard that from this amp, so it's hard to say.
Construction of the amp cabinet is underway....
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