Distortion please...Amp Distortion

[Xtrchessreal]

I have a DIY amp built to Marshall 2204 circuit specs that is about 15 years old now. I use it at my gigs. I want to experiment a bit.

A couple ideas I have barely researched but have in mind:

Add a post PI volume pot - Without seeing a schematic I believe this would allow clipping in the PI while attenuating the AC signal from the output tubes without affecting the negative bias. The result being a nice control over clipping distortion without having to go deaf. There may be more examples but I think Triode Electronics has a Mod like this for sale.

Attenuating the NFB - Simply adding a pot to do this seems easy but I know there are risks with oscillations. Any suggested pot values to use safely? What about caps to help filter RF oscillations?

I'm curious what known issues there may be that could damage my amp, assuming the mods are done properly. I have an O-scope and DMM to test with, what should I be looking for and where?

I wonder what other mods I could play around with to achieve some amp distortion? I have room to add more preamp valves.

Anyway I ain't got money but I do have time - quarantine time.

Thanks for any direction to help make this possible.

X

[Phil Allison]

Xtrchessreal wrote:

--------------------

> I have a DIY amp built to Marshall 2204 circuit specs that is about 15 years old now. I use it at my gigs. I want to experiment a bit.
>
> A couple ideas I have barely researched but have in mind:
>
> Add a post PI volume pot - Without seeing a schematic I believe this would allow clipping in the PI while attenuating the AC signal from the output tubes without affecting the negative bias.

** Easy way to do this is with a single, log pot wired between the EL34 grids.

The pot should be about 500kohms, log taper and wired as a simple rheostat.

>
> Attenuating the NFB - Simply adding a pot to do this seems easy but I know there are risks with oscillations.

** I wouldn't bother, Marshalls have low enough NFB as is.

Reducing NFB does not usually cause oscillations - increasing it does.

>
> I wonder what other mods I could play around with to achieve some amp distortion? I have room to add more preamp valves.

** Beware of that idea.

Pedals are your friends.



.... Phil

[Alan Marcy]

sy...@metasonix.com