Marshall Schematic question

[Fred G.]

Hi All,

Looking at the schematic here:

http://www.drtube.com/schematics/marshall/4010prem.gif

Can somebody explain to me what's going on with the extra cap (C13) from the mid control ?

Thanks,

Fred G.

[Arthur B.]

My first impression was that it is used to scoop out the high mids.

[mooreamps]

Looks like a mid scoop to me as well.

-g

[cbarrow7625]

It alters the tone control interaction

Fred,

Exactly what that cap does depends somewhat on the tone control setting.

One thing it does is make the midrange control act more like a true bandpass filter instead of just a hi-cut in the bass/mid sub-circuit. Imagine the Mid control turned up to 10. Without the cap all of the high frequencies coming throught the .022 mid cap pass unaltered. In that schematic, the extra cap will roll off the highs that would otherwise sneak through the Mid cap.

Alos imagine this: The bass turned down to '0' and the treble turned down below 5. What happens here is the extra cap is hanging off the 'Bass' side of the 'treble' pot actually cutting any highs sneaking through the treble pot. Without the cap, the treble control would have less effect on the sound.

Even with the bass pot turned up, this cap still has an effect on both the treble and midrange interaction.

I suspect that Marshall thought the amps where this was used were too bright and/or the treble control did not make enough difference to the tone so they added the cap.

I have tried this arrangement numerous time and I am not a fan of it. IMHO it make the tone sound too constricted & 'cleaned up'. I'll take the stock set-up anytime.

However it might be the ticket for some designs and it is stupid-easy to try.

-Chris Barrow