Preamp gain pot mod

[sand]

In a typical Marshall, what does a resistor from the preamp gain wiper to ground do for the sound/tone? Does it dump some of the signal to ground, reducing the voltage to the next stage and thus the gain of that next stage? Thanks.

[Enzo]

Go over to Geofex.com and look up the secret life of pots.

What it will do is alter the taper of the pot action.

[sand]

Go over to Geofex.com and look up the secret life of pots.

Thanks for the tip.

I see that it can improve the taper of a linear pot, but why would someone use it with an audio pot Make an audio pot even more "audio-er" It looks like it would serve no purpose...

[pdf64]

If you want a gain control to cover from clean to super distorted, then to get good resolution over that range, an 'even more audio-er' pot might be just what's needed.
Also, in conjunction with a preceding coupling cap, as the gain is increased, more bottom end will be rolled off, which can be useful to avoid the sound 'farting out'. Pete.

[Enzo]

There are any number of audio tapers, many are expressed as a ercentage at mid point. So I can see if an OEM wanted a different curve but alreay had a ton of these controls, the addition of a resistor costs almost nothing and serves their purpose.

There is no right and wrong in pot tapers, there is only what you want in any given application.

[sand]

Thanks a lot, guys. Learned something new again

[redelephant]

And I always thought the resistor was there b/c pots are known to go bad and the wiper may lose contact, which will float the input of the stage after the pot. The resistor acts as a safety in case such an event.

[Chuck H]

I'm thinking on the same lines. The resistor could serve many functions. If it follows a cap and is right behind the target grid it's most likely a bias resistor and must be there for the circuit to operate correctly. In which case it's a bias resistor and possibly an attenuator. If there is a cap ahead of it (signal on it's way to the next grid) then it could do a few things. It could be a taper adjuster and or gain attenuator and in some cases part of a phase correction circuit.

In your amp it's most likely a gain attenuator/taper adjustment. But since there's almost no such thing as a "typical Marshall", especially with a "preamp gain" control (unless you just mean the "volume" control) it's hard to say for sure.

[sand]

In your amp it's most likely a gain attenuator/taper adjustment. But since there's almost no such thing as a "typical Marshall", especially with a "preamp gain" control (unless you just mean the "volume" control) it's hard to say for sure.

Sorry, you're right, I haven't mentioned which pot that was. I was talking about the preamp gain pot in a JCM 800 2203/2204.

[redelephant]

Hmmmm, I see no resistor to gnd from the wiper in those circuits...???
Schematic:http://web.archive.org/web/200609191...l_50w_2204.pdf

[sand]

Hmmmm, I see no resistor to gnd from the wiper in those circuits...???
Schematic:http://web.archive.org/web/200609191...l_50w_2204.pdf

This is a part of a mod I read about.