OT - too much top end?

[Chuck H]

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Let's see a schematic. NFB circuits in guitar amps seem to be misunderstood even by builders with some experience. On this forum I see a lot of builders do things like use the "Marshall" type presence control on a "Fender" type build and end up with whacky FB ratios.

[GainFreak]

I tried the other OT (MOP50) and there was some improvement but not much.

From your measurements, yours is pretty bad.

Did you mean the OT or the peak?

Removing the FB entirely makes the amp much brighter. When it's on it helps decreasing the HF.

Schematic is Marshall 9100 rack amp.

[Chuck H]

I tried a 8k2/2n2 shunt filter at low volumes and it took care of the problem but I don't like the idea of a 100W resistor heating up in my amp. I tried also a 470p cap across PI plates and it had almost the same effect so maybe I'll stick with it as a more simple solution. Have to try the other OT yet.

If you locate it with even a little care the shunt resistor shouldn't cause any problems. Also, when you squelch high end at the PI you lose high end harmonics that would otherwise be part of the OD in the power amp power amp feel/response/character. If you never push the power tubes into OD it makes little difference though.

[Steve Conner]

I meant that the bandwidth of the OT looks bad. Probably OK by guitar amp standards though.

If you find removing the NFB makes it brighter, that says to me that you are used to the sound of solid-state and don't like the presence boost you get from a tube amp's low damping factor. So, try more NFB. Maybe move the NFB to the 16 ohm tap? But be careful, too much and it may oscillate.

Or again, the conjunctive filter does much the same as the extra NFB. It would also help to quench oscillations caused by too much NFB, I guess.

[GainFreak]

If you find removing the NFB makes it brighter, that says to me that you are used to the sound of solid-state and don't like the presence boost you get from a tube amp's low damping factor.

I never owned a SS amp I'm comparing it to other tube amps I made. Removing the FB makes the amp louder and with more HF content. The problem is I'm playing at low volumes close to the cab. At higher volumes things are not so bad but the HF I'm talking about is above the usual presence boost frequency.

[GainFreak]

So, try more NFB. Maybe move the NFB to the 16 ohm tap?

I tried the 8Ohm tap and it helped. Currently I ended up using a 220pF cap after the caps feeding power tubes. I tried the Chieftan style presence circuit but I didn't like how it sounded.
Thanks to all for the feedback and your comments.