18w Lite - 1M and 100k resistors on volume and tone pots????

[yello]

I have an 18w Lite amp that I am working on and it has a very peculiar (to me) volume and tone pot set-up. It has 1M resistors spanning each pot, and a 100k resistor connecting them. I think they are 1M pots. Only thing I can think is that someone was trying to get their 1M pots to be 500k with those 1M resistors. I'm not sure what the 100k is supposed to do, seems to be in lieu of a 500pf cap, and the other cap is wired wrong.

Amp works but sounds muffled, lacks dynamics, and overly gained and

Here are pictures of the pots and preamp area, as well as quick layout drawings of what is there. Circuit is built into a Hammond Ao-35 chassis with stock transformers, cap can, and running the 5Y3.

I plan to bring it up to the latest Lite IIB'ish spec, including correct value pots and a few other resistor values, and new filter caps, but want to learn if anyone has some insight into those resistors.

[Bieworm]

Hey Dayn

I think you are correct about the pots being brought from 1M to 500k by parallelling a 1M resistor. While that’s ok when the pot is set to 100%, it stinks everywhere in between. The 1M resistor will be parallelling every resistance on the pot’s sweep. Useless IMHO

The 100k resistor in the tone circuit was done on the early 18W amp normal channel. It’s effective, but the modern version is more desirable for most players.

[yello]

Hey Dayn

I think you are correct about the pots being brought from 1M to 500k by parallelling a 1M resistor. While that’s ok when the pot is set to 100%, it stinks everywhere in between. The 1M resistor will be parallelling every resistance on the pot’s sweep. Useless IMHO

The 100k resistor in the tone circuit was done on the early 18W amp normal channel. It’s effective, but the modern version is more desirable for most players.

Thanks Bieworm, I learned something new! And now I see an earlier question about the same thing that provides a bit more info:

viewtopic.php?t=25284

I will say that I don't like how it sounds - mine looks to be wired differently than those early Marshall examples in how the cap is incorporated, even though the 100k resistor is there. I'll wire it up the current Lite IIB way and see if it improves the sound and dynamics, or if it is another aspect of the amp I don't prefer. At a minimum I would think using 500k pots without the resistors will help.

[Bieworm]

Or experiment some. Put a 220k plate resistor on V1, double the cathode resistor of V1. Leave the 1M pots and wire the tone stack like the lite2b. You’ll get the 6G3 brown deluxe preamp doing that. That is a nice one

[yello]

Or experiment some. Put a 220k plate resistor on V1, double the cathode resistor of V1. Leave the 1M pots and wire the tone stack like the lite2b. You’ll get the 6G3 brown deluxe preamp doing that. That is a nice one

I certainly love the idea of leaving 1M pots in place - then tweaking to taste.

Looks like I also remove one capacitor from that old 18w wiring, the current .01 cap from v1 plates to tone pot. Part of rewiring it to lite2b.

Thanks for the suggestions!