Boost input gain on Carvin X100b

[Helmholtz]

The values of R63/64 and C41 may have been chosen to give some compression effect when V2B asymmetrically clips and supply current increases.

[Helmholtz]

[QUOTE=glebert;n918223]

I'm wondering about the effect of that 220k R62 and if it is going to load down the circuit and reduce the gain as well.

Maybe this helps:

The gain of a fully bypassed common cathode stage is G = gm * (rp//Rp//RL), where rp is the internal plate resistance, Rp is the plate resistor and RL is the external AC load.
"//" means parallel with and the appropriate values of rp and gm for given plate voltage and current have to be taken from the tube charts.

The formula shows that gain is proportional to the resultant resistance of rp, Rp and RL in parallel.
Gain increases with larger Rp and/or RL. Larger Rp values increase sensitivity to loading by RL

https://www.allaboutcircuits.com/too...ce-calculator/

[Chuck H]

I'm wondering about the effect of that 220k R62 and if it is going to load down the circuit and reduce the gain as well.

The load from that resistor isn't that significant. Removing it would buy you less than 1dB of gain and almost no change to the EQ of the circuit (there IS an increase of more than 1dB at something like 40Hz).

As I said, I am trying to use load line concepts to understand the circuit behavior, mostly using Rob Robinette's tutorioal. I tried plugging the AC load into LT spice and it starts at 55k ohm and drops as frequency goes up. If I run a Hot Rod Deluxe first gain stage the AC load starts at 84k doesn't drop as fast. At 1kHz the HRDlx AC load is 2x the Carvin.

Yes, the additional load does make impedance rise more with frequency. This is neither good or bad, just a difference. The Trainwreck Express and Liverpool amps, for example, take great advantage of the HF impedance characteristics of a smaller load with a 150k load following the coupling cap. Different circuit ideals will be more or less useful in the context of an overall design. Like Helmholtz said, considering load lines here isn't practical (paraphrasing). It's fine to want to know more about how a bias condition and load affect a triodes operation and the resulting signal. And you seem to be learning that with your simulations. I would encourage you to experiment with that too so you can hear the real world differences.

Speaking of AC load, I don't get what is going on with the V1a load at all. I need to check the board and see what is there for the "C3 6.8k Resistor" (???) thing.

Yes, that's confusing and certainly represented inadequately (and possibly inaccurately). I looked up some other version schematics for that model and found a couple that have that circuit drawn more clearly. It looks to be a simultaneous increase of the V2a cathode bypass cap and a low pass filter (in the form of a resistor and capacitor in series to 0V) added to the output of V1a.