cut control in preamp?

[M Fowler]

High cut or bass boost.

Every thing I have seen does not have the cut control pre PI.

[iknowjohnny]

A capacitor and resistor in series going to ground. The resistor is a pot. The values of each will depend on where it is in the circuit. Think a simple tone control for a guitar--that's all they do is cut the highs.

I've tried that many times in many places, but it never cuts highs the same way a cut control does it.

[funkmeblue]

then it's a phase thing......

[gearhead]

then it's a phase thing......

And it would be useful if you would answer the question of what the project is. Availaible pieces of the preamp to be reworked would be helpful. Does it have an extra, unused triode, ala a Trainwreck?

Since the cut control characterisitc you desire appears to be tied to phase cancellaition (and not cutoff at a corner freq in a low pass filter), that is what needs to be replicated. Which means mixing a normal signal with an inverted signal.

Just thinking out loud, but if you do have an extra triode, maybe you could set that up as an inverting triode. Bypass some signal to it, run thru inverting triode, and mix it back in with a -high pass- filter. The more you turn it up, the more inverted (phase cancelling) high value frequencies are mixed into the signal.

[Jana]

Ah, phase stuff. Beam me up Scotty.

How about feeding some of the signal from the plate to the cathode? No bypass cap on the cathode for it to work. A small cap coming from the plate then a pot and then to the cathode. The plate is 180 out of phase with the cathode (and the grid too but that might get a little dicey).

[iknowjohnny]

I was going to ask about that Jana because you had suggested a cap there to cut fiz and i wondered if it would work the same as a cut by using a much larger cap the way i do on the power tube grids. i use a .022 there because it rolls off over a larger spectrum of frequencies than if you use a small cap. Sorta like taking a graphic EQ and starting at say 1k and moving each slider 1Db more then the next as you go up. Whereas with a 250pf you're only cutting a very narrow area and thats the kind of high cut that doesn't work for me. So i wondered about doing the same at V1 but i haven't tried yet. the main thing i am trying to accomplish is to be able to tweak the tone after the master gets to stage volume to try and preserve the smooth rich tone i get just under stage volume. But when the master gets to that certain point that tone gets lost to a degree.

gearhead...it's a high gain preamp with cathode follower and tonestack similar to a jcm with a fairly typical EL34x2 output.