Plexi amp channel mixing…

[Mastershon]

Hello question about Marshall ‘plexi’ style amps and similar amps that have 4 inputs and 2 volume controls…

I have 2 stock Marshall 90’s reissue 1959’s. I want to run them all controls on 10 including both volumes 1 and 2 with guitar going straight into input 1 high and no jumper. However, when channel 2 is raised above about 7 or 8, the amps compress out killing the highs and dynamics and turning into a mushy mess. I know that others successfully run all on 10 so some are more prone to this phenomenon than others.

I have seen this general topic discussed before, and member Roe gave a great explanation that because of the mix resistors, when the unused volume 2 is raised it acts as a voltage divider and changes the signals resistance to ground going into V2, and that max gain is achieved at 10 on volume 1 and roughly 7 on volume 2. If mine was max gain 7 and DROPPED in gain after that it would be fine (there’s plenty of gain at that point), but what seems to happen is that the signal or gain continues to increase and overloads V2.

So the question is; What can be done to the circuit so that when volume 2 is on 10 it sounds and reacts like it does at 7? Can I change the mix resistors or place a voltage divider before V2? Or perhaps a better question would be; Why do some amps exhibit this signal ‘over compression’ phenomenon and some don’t?

[bepone]

and? where is the problem? that you want to run all on 10?

[Mastershon]

Yes. It’s really something that has driven me nuts for a LONG time.

[bepone]

ok, then change V1 tube for 12AT7 and there is no problem

[Mastershon]

Thanks for the advice. And yes you’re correct. The original post was already long so I didn’t mention that tube strength (and type) along with pickup output level affects the ‘over compression/flabby’ effect.

But the point of my frustration is that some plexi amps can do it just fine with average 12AX7’s and low to medium output humbuckers, and some can’t.

So I’m looking for the circuit reason why and a circuit tweak to circumvent it.

[bepone]

it is impossible to answer shortly to your reply. you can't put an extreme amount of gain into a stage, and expect a good sound ? this is not going in that way. it is better to shape gain throung one more stage in the way of cutting, formin, shaping more than what is possible to do with one stage.
you are approaching to saturation and there is not more after that. thats why you have gain pots in the front plate , to adjust them, not to put them on 100%, to find level which tastes best to your needs.

volume pot position is not important at all, you are bothering with the angle, and not with the tone

if you want to tweak your amp to work on 100%.. then you can cut some cupling cap values to half, cut a lot of gain (12AY7, 12AT7 in firs two stages) or add resistance for gain cutting, add some pF capacitance over added resistance, cut some highs too (extreme gain is developing more bass and more highs and it is un'listenable etc.. )

[Mastershon]

Thank you so much for your response and suggestions! You are correct to say I’m bothering with it. Yes indeed it’s an annoyance rather than a serious issue, but I’d like to know why some stock Marshall 1959’s are able to run all on 10 and some aren’t.

There must be a logical circuit reason as to why it happens.

[brewdude]

It seems that you are aware that this is an expectable characteristic of this amp as you’ve cited an explanation by Roe.

I wish I had some insight to offer, but I think it’s a fools quest to expect an amp to sound good with everything dimed. It’s useful to add or subtract one attribute so that another can be expressed and tuned to sound good with a given speaker in a given room with a given instrument played by a given player.

You could always adjust the other volume with resistors added around the potentiometer such that they limit the maximum (or minimum) signal that bleeds through when the pot is at max.

As for why some similar amps don’t suffer this… who knows?

[Mastershon]

All excellent input (no pun intended) thank you.

[Mastershon]

It’s fascinating to to me that some are ‘ok’ all on 10, but mine needs to be all on 10, except one of them at 7, for optimal tone. Again the search continues.

[bepone]

on strat sigles standard output voltage is cca 50mVpp, and for the humbucker LP output is more than 500mVpp it is 10 times difference in output from the guitar. amp cant be setted in the same way for this 2 guitars.

with the strat single coils marshall can be dimed, gain to 10 and still sound good, but is the same amount of gain with the LP whereis gain pot on 4 or 5/10..

[roberto]

This is why so many treble booster, including the TS808 et alii came out: raise the gain level keeping the sound tight and focused.
Indeed you could do something similar with your amp too.

[pdf64]

Hello question about Marshall ‘plexi’ style amps and similar amps that have 4 inputs and 2 volume controls…

I have 2 stock Marshall 90’s reissue 1959’s. I want to run them all controls on 10 including both volumes 1 and 2 with guitar going straight into input 1 high and no jumper. However, when channel 2 is raised above about 7 or 8, the amps compress out killing the highs and dynamics and turning into a mushy mess. …

Maybe the amp is oscillating?

[Mastershon]

Oscillating? Can you tell me more about this?

[Mastershon]