trinityamps.com

Hello all! I'm new to the forum and somewhat new to building amps for the most part. I've been building for about 6 months and I'm looking to build what will be my fourth amp and I'm trying to make up my mind between a few of the trinity kits. I've been studying the layouts/schematics provided in the resource section for the 18 watt models and I've read enough of the threads to understand the evolution and differences between both amps but I'm a bit stumped on one thing. On the TMB there is a single 470k "mixing" resistor on the grid of V2A which I understand it's purpose but on the Plexi MkII, there are 2-470k resistors that go to the grid of v2a. One of which grounds at the same spot the cathode of v2a is grounded. What is the purpose the one that goes to ground? I also noticed that on an earlier layout of the Mk II there used to be only one 470k mixing resistor and it also had a 500pf cap in parallel as found in a typical plexi layout. Why the change? Any help would be greatly appreciated! I've learned SO MUCH since joining this forum and look forward to talking with you guys!

The two 470k resistors (R8 and R42) on the MkII form what is called a "voltage divider." It is a circuit that reduces the amount of voltage passing through it. In this case, where the two resistors are of equal value, it divides the voltage in half. The result in this application is that there would be less gain headed into V2A because this voltage divider cuts it in half.

If you want a really good series of videos on everything resistors do in a tube amp (including voltage dividers) then I highly recommend Uncle Doug's five-part video series starting with part one here....

https://www.youtube.com/watch?v=LBtoarc6Nls

It starts pretty basic but gets very in-depth and tube-circuit specific before too long.

Thank you very much for the clarification. Do the values used in the voltage divider have an effect on what specific frequencies are attenuated?

You're welcome.

No, you don't choose them for frequency attenuation. The voltage divider will treat them all equally. The values used need to add-up to your chosen grid-leak value for the triode they are feeding. Since the "typical" grid-leak value for a 12AX7 triode is 1M, you would want to make sure your two voltage divider's resistors add-up to 1M... or very close to it anyways.... 470k+470k=940k... close enough to 1M for tubes

Ahhhh gotcha! That makes sense now. Thanks for the further clarification!