2 stage FET Overdrive mod help

[fretzburner]

This is the schematic of the overdrive section of my hybrid amp and the sound is very bright.need help to modify it to tame the brightness.The clean and OD channel shares the same tone controls so i cannot adjust the high pot without compromising the clean channel tone.

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thanks

[jafo]

Just send a cap to ground -- that creates a low-pass filter, which cuts the high frequencies by a bit. If you add a pot before, it's the same as the tone control on your guitar. There are formulae you can apply to decide exactly how big of a cap, but honestly, you'll get better result if you experiment with values and placement. I'd suggest starting with maybe a 0.022 mF cap after the gain stage. Just twist some leads until you get a result you like.

[Davelectro]

Small cap from drain to gate on the second transistor should tame the highs a bit. 22pF is a common value for this.

BTW, that's an interesting diode arrangement. I guess they are providing compression, aren't they?

[CynicalMan]

I'd try a 2.2nF from drain to ground on the second stage.

[fretzburner]

Thanks for the hints,i got a modded schematic from the net of this drive stage please have a look


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what should be done to this,what purpose changing R3 and R4 and what is the result for the diode clipping change?

[Davelectro]

Diodes: Asymmetric clipping and HF reduction.

R3/R4: less gain.

[fretzburner]

So maybe the diode with cap mod will do the job for my too bright sound problem,or i will just add a cap as what you guys thinking.

[PRR]

The 47p on the VR5(?) is a "bright cap". You probably want to omit it.

A cap either across the diodes or on the output pot will trim highs.

[PRR]

> that's an interesting diode arrangement

Later versions of Ampeg VT-40 had a similar ploy (with BJT) on the "Distortion" channel.

Most users thought it was nasty.

[Johan]

Later versions of Ampeg VT-40 had a similar ploy (with BJT) on the "Distortion" channel.

interesting..
also looks like a fet version of Westbury W20 tube overdrive
compare the schematic from GEOFEX   http://www.geofex.com/FX_images/westbury.gif

J