Baxandall tone stack for guitar amp?

[bluefoxicy]

Hey folks I'm building a Baxandall Tone Stack for my guitar amp, re:

http://www.novotone.be/_site/projets...ndall%20WW.pdf

(Figure 6 is completed)

i'm putting it between the 12AX7 gain stages on a single preamp single ended; but not sure what values to use. I seem to have accidentally mixed up the Baxandall and the James (so has everyone else on the net), and would rather use the ungrounded Baxandall with NFB than a James, for a cleaner signal.

I'm considering isolating the two grounds of the James from ground and connecting them to themselves, and then adding negative feedback to this junction from the following gain stage. Thoughts?

[Steve A.]

Hey folks I'm building a Baxandall Tone Stack for my guitar amp, re:

http://www.novotone.be/_site/projets...ndall%20WW.pdf

(Figure 6 is completed)

i'm putting it between the 12AX7 gain stages on a single preamp single ended; but not sure what values to use. I seem to have accidentally mixed up the Baxandall and the James (so has everyone else on the net), and would rather use the ungrounded Baxandall with NFB than a James, for a cleaner signal.

I'm considering isolating the two grounds of the James from ground and connecting them to themselves, and then adding negative feedback to this junction from the following gain stage. Thoughts?

You might want to look around the internet for drawings of the R-o--u---te 6--6 amp by D---r -Z--- (I don't want this post showing up in a Google search... )

A lot of these hi-fi tone controls sound like sh*t in a guitar amp, but this one is nice.

Steve Ahola

[bluefoxicy]

http://www.triodeguy.com/other_projects.htm

Found one

Couldn't find schematics on the R66 but...

[Steve Conner]

Here is my implementation of the Baxandall tone stack. Mine is the real deal, not the James! It uses negative feedback around both triodes: the first one uses cathode degeneration for an adjustable gain control, the second has the Bax tone stack wrapped round it.

I built it years ago, but still use it. I recently changed the bass capacitor to 4.7nF and thought that improved it.

http://scopeboy.com/baxeq2.gif
http://scopeboy.com/baxeq.html

[loudthud]

There are a couple of things you need to take into account. First, the James uses log pots and the Bax uses linear pots. At the center pot position the James gives you about 20dB of attenuation. The Bax gives you a gain of minus one at the pot center position. Now if you just stuck the James components in a feedback loop like the Bax you could get a gain of 20dB at the pots center position but you would want the input to be the formerly grounded side of the James and connect the feedback to the former input. The problem in a tube circuit (as opposed to using op-amps) would be that to get 20dB of boost at the pot's end you would need to have a gain stage of at least 100 with low output impedance. Not your simple 1/2 12AX7 gain stage.

Steve Conner's circuit is rather clever in that it uses a voltage divider (R13/R14) in the feedback loop so it gets the (roughly) 20dB of gain but uses the Bax's linear pots. But, it's a little short of gain at the end of the pot. If having the capability to get 20dB of boost is important, you would need to dial back the gain at the pot's center position to 10dB to 12dB (voltage gain between 3 and 4).

One problem with the Bax is that the input impedance always gets lower when you try to dial in some boost. But the James does this too. An advantage of the form of Bax that Steve Conner used is that you only have to change one cap to shift the frequency of each control. There is also a mid control circuit that uses two caps, one across the pot and one in series with the wiper back to the summing node.

[loudthud]

There was something bugging me about what I wrote in the above post. I think you would need to use reverse log pots if you wanted to try to make a circuit with a gain of 10 (20dB) when the pots were centered.

[Steve Conner]

When I designed that circuit, I was trying to strike a balance. To get the textbook response curves, you need a loop gain of almost infinity, as an op-amp would give.

But on the other hand, a lot of NFB in a tube circuit will spoil the pleasant harmonic distortion from the tubes, as well as lowering the gain and needing an extra tube stage to make it up.

So, the circuit is a compromise, and yes, R13 and R14 adjust the compromise as well as setting the drive level to the following stages. With the values I chose, it is indeed running out of gain, since the 12AX7 only has a gain of around 50. You might notice that I left out one of the resistors from the boost end of the treble pot, to try and get more treble boost. (there was plenty enough bass boost for my taste)

[CarlZ]

I know active Baxandall vs passive James circuits have been pretty well discussed at this point but I just thought I'd stick my two cents in and suggest you check out the Carvin Legacy and MTS series amp schematics. They both use a James type passive circuit and may give you a few ideas as to how you might want to implement them and what values to start out with. They seem to sound fairly decent and haven't heard many if any complaints about the tone controls on these amps.

Carl Z