Need more sizzle on Super-Sonic 22 burn channel

[Tubis]

Put a resistor in series with c25. That will dampen the roll-off of the higher freq but maintain the roll-off of the lower ones. Let's say about 100k. Rolling off hard after the distortion stages makes the amp lose its "sizzle" for sure.

[Chuck H]

In other words... It's a semi complex amp in that it's a high gain channel switcher and that means attention and effort went into the design regarding grounding, layout and lead dress where stability is concerned. Very often with such amps changing anything is a crap shoot for stable results. Though I think adding a presence circuit is probably a safe bet. Be sure to route any NFB control well away from the preamp board circuitry and leads.

[electronhole]

I don't generally agree that "the NFB is the best place".

Amp instability is often related to the global NFB in the power amp section. So changes can be critical. Not saying that it can't be done especially as you would reduce NFB at high frequencies.

Thanks!

[Helmholtz]

...but maybe it makes most sense in the NFB if that's safest. I don't want oscillations, and it seems like the NFB is best place to avoid that since there's no more tubes besides the power section.

I don't generally agree that "the NFB is the best place".

Amp instability is often related to the global NFB in the power amp section. So changes can be critical. Not saying that it can't be done especially as you would reduce NFB at high frequencies.

[electronhole]

Juust a thought. When you advance the 8k control, the Q of the circuit is not large, so when you add +12db at 8k, you are also boosting 4k and further down, though to a lesser extent.

Borrow a 31 band EQ where you can lop off the very top end and explore what just the 3-5k area does,

I hear ya'. That's why I chose a cap value of 120pF, since it will force f(cutoff) at +3dB to be near 6 kHz, which allows the full 6dB boost by 8kHz. So yes, you're right: I am desiring lower harmonics as well, and I did account for it. I boosted the 16 kHz slider as well on the MXR 10-band. It also gives the sound a nice lift since it's a shelving EQ.

[electronhole]

In your initial post, you said you wanted more highs in the burn channel only. If you make any mods post preamp, it will affect both channels. So, which do you want?

No doubt--that's why I said maybe. Having it for the burn channel is good, but maybe it makes most sense in the NFB if that's safest. I don't want oscillations, and it seems like the NFB is best place to avoid that since there's no more tubes besides the power section. But if I can be reasonably certain that the preamp mods are sufficient, I'll go that route. Just don't want it going nuts in some environment that drives the amp crazy.

[The Dude]

The EQ was in the loop. Boosted 8k and 16k. Amp running low enough not to generate distortion post-loop. The change in tone boosting 8kHz & 16kHz +6dB in the loop is dramatic. The resulting tone is lively and present without being harsh. Carvin tube amps had Presence controls at 10 kHz. It adds a tremendous amount of glass, sparkle, and sizzle. A boost in that range was also what gave old C12R Jensens their amazing crunch.

In other words, I'd like it later in the chain, but not necessarily before the phase inverter. But maybe it doesn't hurt to have it in the NFB loop.

In your initial post, you said you wanted more highs in the burn channel only. If you make any mods post preamp, it will affect both channels. So, which do you want?

[Enzo]

Juust a thought. When you advance the 8k control, the Q of the circuit is not large, so when you add +12db at 8k, you are also boosting 4k and further down, though to a lesser extent.

Borrow a 31 band EQ where you can lop off the very top end and explore what just the 3-5k area does,

[electronhole]

EDIT: We were posting simultaneously. Maybe try a 100p cap across R42. Don't be surprised if the added HF induces instability under some settings and playing circumstances.

Thanks! I'll try there if my method doesn't work out.

And yes, your concerns are the kind of thing I'm worried about. I know the physics, but I don't know the risk of oscillations as well, or if I'll end up getting RF pouring in somehow.

[Chuck H]

R42 is the earliest place in the circuit to boost highs without affecting the clean channel. A 100p cap parallel with R42 should boost 8k about 8dB. That would be a lot, so refinement may be needed.?.

EDIT: We were posting simultaneously. Maybe try a 100p cap across R42. Don't be surprised if the added HF induces instability under some settings and playing circumstances.

[electronhole]

Well then you're probably best off to attempt your boost as early in the signal chain as possible. Helmholtz is right that there isn't a lot to be gained by boosting 8k in the final tone. It seems like what you're after is something that changes the character of the overdrive and harmonics generated in the preamp clipping.?.

I haven't looked at the schem yet. Doing that now...

The EQ was in the loop. Boosted 8k and 16k. Amp running low enough not to generate distortion post-loop. The change in tone boosting 8kHz & 16kHz +6dB in the loop is dramatic. The resulting tone is lively and present without being harsh. Carvin tube amps had Presence controls at 10 kHz. It adds a tremendous amount of glass, sparkle, and sizzle. A boost in that range was also what gave old C12R Jensens their amazing crunch.

In other words, I'd like it later in the chain, but not necessarily before the phase inverter. But maybe it doesn't hurt to have it in the NFB loop.

[Chuck H]

Well, if you ever use an MXR 10 band EQ on a darker amp with high gain, you'll notice that its 8 kHz slider and 16 kHz slider (the latter is a shelving filter) affects the tone quite a bit—more than just "air". It changes how forward the sound is. It's like having a Presence control with a little bit of harsh lower treble content removed.

Well then you're probably best off to attempt your boost as early in the signal chain as possible. Helmholtz is right that there isn't a lot to be gained by boosting 8k in the final tone. It seems like what you're after is something that changes the character of the overdrive and harmonics generated in the preamp clipping.?.

I haven't looked at the schem yet. Doing that now...

[electronhole]

8kHz? The SPL of most guitar speakers is down by at least 15dB at 8kHz. Otherwise nothing wrong with trying your idea.

Well, if you ever use an MXR 10 band EQ on a darker amp with high gain, you'll notice that its 8 kHz slider and 16 kHz slider (the latter is a shelving filter) affects the tone quite a bit—more than just "air". It changes how forward the sound is. It's like having a Presence control with a little bit of harsh lower treble content removed.

[Helmholtz]

8kHz? The SPL of most guitar speakers is down by at least 15dB at 8kHz. Otherwise nothing wrong with trying your idea.

[pdf64]

R50 is effectively in series with the source impedance from the tone stack, and in parallel with R51. So the LPF freq will be around 2kHz.
How does it sound if C25 is lifted?