More Headroom For A Blues Deluxe Reissue

[Malcolm Irving]

Probably not relevant to the original posted question in this thread, but I've found that many guitarists are perfectly happy to get distortion at lower sound levels by setting the 'vol.' or 'gain' high with the 'master' low. However, when they want clean high sound levels, they seem reluctant to put the 'master' at max and control the level with the pre-amp control.

The definition of 'headroom' that I like is: the input signal swing (in volts) that a stage or series of stages can tolerate before the output gets clipped. (Strictly speaking the positive-going headroom and negative-going headroom can be different.)

The average guitarist's concept of 'headroom' is therefore not far off the mark, but the problem is that the maximum headroom of the entire amp (assuming it is properly designed) is limited only by the output stage and can't be increased by changing pre-amp tubes.

[Malcolm Irving]

... However, when they want clean high sound levels, they seem reluctant to put the 'master' at max and control the level with the pre-amp control.
…

One reason for this could be, for some high gain amps, when you try this you are limited between about '0' and '1' out of '10' on the pre-amp control, which makes setting the level difficult or impossible. - So in this case a lower gain tube in V1 might be good.

[Guitarist]

You can:
parallel the Feedback resistor R93 with another, say a 10K or less, to clean up the power section
use a 12at7 in the PI
swap the cathode caps for 4.7 uF (reduce phat-farting)
perhaps up the cathode resistor to 3.3K or so - colder biasing
and a lower gain tube in V1, V2 such as another 12at7 or y7 as recommended
empty neg fb on a preamp stage

Also on the other end of the gain story, I offer another thing I tested and it works well. Add diodes for the dirty channel. This only applies to relayed Ch 2. If you're using a diode equipped stomp box anyways for crunch... As a tube purist I didn't expect this to work so well!

[J M Fahey]

Thanks

Pity the OP left 4 Months ago never to be seen again but anyway your contribution stays here to benefit others with a similar problem.

[Leo_Gnardo]

Add diodes for the dirty channel. This only applies to relayed Ch 2. If you're using a diode equipped stomp box anyways for crunch... As a tube purist I didn't expect this to work so well!

Germanium at that, for a 60's "soft fuzz" effect. I've subbed Ge for Si as clippers in solid-snake Peaveys to good effect. Gotta find more Ge diodes now, I'm running short. Fzzzzzzshhhhh !

[Guitarist]

Coo thanks.
Actually the diodes are bit strong and it could use some that kick out sooner or that ride that knee well for those harder picking dynamics.

[R.G.]

One of the simplest ways to get more apparent "headroom" is to lower the gain of the first tube gain stage or so. A lower plate resistor or a lower loading resistor after the cap off the plate will cut the signal level down, and allow the rest of the amp to work clean up to some apparent volume knob setting. It's an easy way to tune an amp down to cleaner, but keep the same overall output. The volume control on the amp may or may not be able to achieve the same result if the earlier stages start distorting before the volume control.

[Guitarist]

Good point R.G.
I was just looking, IIRC, at a VHT schematic and the clean channel was similar with three gain stages. The last stage had R+Cap negative feedback. But whenever I tried this, the tones was lossy and I didn't prefer it.

As I've now discovered dropping the whole B+ rail going to the preamp dropping so as we get "Ultimate Marshall" voltages of 100V on the plate, the opposite could work too. Maybe, maybe it was carefully graduated soap the signal got bigger without compressing the next grid which had available headroom - sort of thing

Ya, what you said.