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At the risk of being an a$$ and answering for the expert on his own amp, if you look at V1 on the Spit, you will see it is running in parallel. So, it has a shared plate, a shared grid, and a shared cathode. That R/C pair is what he is referring to.
I know it's running in parallel and that the plate, cathode and grid resistors are all shared. Thanks for the news flash.
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Both V1 and V2 have triodes that share the same cathode risistor. 1.5 K on V1 and 1.2K on V2. Raising the resistor on V1 to 2.2K increases the voltage and increases the clean headroom a bit. The PI on the Spitfire uses values that are the same as vintage Plexis except the Plexis use a 470 Ohm Shared Cathode resistor instead of the 1.2K of the Spitfire. Putting the 820 Ohm resistor in parallel with the 1.2K on V2 brings it down to around 480 ohms and less voltage. Less voltage = Mo Dirt. Putting that 820 Ohm on a Switch back to the cathode's ground (before the tail resistor) allows standard Spit vibe and a very satisfying Plexi-type Overdrive when engaged. Not as much grind as a Marshall 18 Watt TMBMV but, that's not what I was going for.
Ok, now I follow what you were suggesting. I was looking at my layout and saw that pair of shared cathode resistors (V1 & V2) but I didn't follow where the placement of the 820 ohm resistor would be, but it all makes sense now...