Single-channel 6G6B: mixing resistors

[passaloutre]

I'm planning for a single-channel (Normal channel only) version of the Fender 6G6B (schematic below). The original amp mixes the two channels directly from the plates, with no coupling caps between the plates and the mixing resistors. I assume I can treat this mixer as a voltage divider with one 470k in series, and one 470k shunt to ground, but I haven't done this with a DC coupled mixer, so I want some reassurance.

My intuition is these resistors are big enough that I don't need to worry about dumping DC to ground. Would it be better to include an additional coupling cap between the plate and the mixer?

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_6g6b_schem.pdf

[sluckey]

You don't need any mixing resistors in a single channel amp. Don't need any additional caps either. The 500pF cap at the PI input is all you need.

Don't think of those 470Ks as a voltage divider. Think of them as isolation/mixing resistors.

[passaloutre]

In general I agree, but in practice people seem to keep the mixing resistors as a voltage divider when removing a channel as the mixing resistors affect the sound of the original amp. Case in point: your own AB763 Lite (https://sluckeyamps.com/misc/AB_Lite_II.pdf). In that case, there is a coupling capacitor between the preamp plate and the mixing resistors; in the 6G6B there is no coupling cap between the plate and the mixing resistors. I'm mostly wondering if it's safe to have that divider in DC land rather than decoupling it from the plate.

[sluckey]

In general I agree, but in practice people seem to keep the mixing resistors as a voltage divider when removing a channel as the mixing resistors affect the sound of the original amp.

That is not a general statement. It is very dependent on the particular amp being discussed. I don't think it will have much if any effect on the 6G6B.

I'm mostly wondering if it's safe to have that divider in DC land rather than decoupling it from the plate.

Absolutely. The best and easiest way to do that is to use a split load plate resistor, i.e., instead of a single 100K plate load, use two 51K resistors in series and take the output from the junction of the two resistors rather than directly from the plate. The ratio of the two resistors sets the ratio of the voltage divider, in this case, the ratio is 2:1. If you're not familiar with a split load plate resistor, look at the 6G15 Reverb unit. The input triode uses a split load plate resistor.

[passaloutre]

Thanks Sluckey. The split load does seem like a cleaner way to do it. I will try with and without.

I know you were just using 100k vs 51k/51k as an example, but for the record this stage has a 220k plate load so i will try 100k/100k (just so I don't get confused when I come back and read this later).

[sluckey]

The split load does seem like a cleaner way to do it. I will try with and without.

Once you split the load you can easily switch between full plate signal or 1/2 signal by connecting directly to the plate or to the junction of the resistors. Might be useful to use a SPDT switch.