Tone control placement

[Cartman]

I’m converting a Stromberg-Carlson AU-33 for use with guitar. The circuit is mostly stock but with new e-caps and new input jack and grid stoppers on the preamp tubes. Sounds decent as is but the tone controls don’t do a lot. As shown in the attached schematic the Bass pot comes before the Mixer tube and the Treble pot comes after it. I’m planning to change it to a James tone stack. Would it be better to put it before or after the mixer tube? Thanks.

[maxkracht]

Before would make more sense to me. I would check to see if some coupling/ tone caps are failing. I don’t think those tone controls are supposed to be that subtle. Paper oil and some other kinds of film don’t tend to last as long as you might think.

[solderhead]

You might want to experiment a little to confirm that the frequencies that are shaped by the control lie in the range that you want them to be in for guitar. Having f3 at the right frequency can make all the difference. (in case anyone doesn't know, f3 = 1/(2Pi * C13 * R), where R is the total effective resistance in series with C13).

One option to consider would be installing a rotary switch that allows you to change the values of the coupling cap in the tone control. This type of solution was featured in the old Orange 'Graphic' amps. Or you could just use a decade box to dial in the right cap value and then keep a one value control.

The 'James' stack is actually a combination of a "one knob" bass and a "one knob" treble control. It's OK to split the bass and treble sides of the James circuit rather than "stacking" them. In some cases it would be better to do that -- for example if you're designing a high gain cascaded preamp you might want to filter out the bass early on (pre-distortion) and to filter the treble later on (post-distortion).

[Stevem]

What are the plate voltages like in the preamp section?

[Cartman]

Thanks for the replies. I should have been more specific and mentioned that I have changed all of the caps, e-caps and coupling caps. They all looked like they were shot. I used the same values as shown in the stock schematic.

Preamp plates are around 145 VDC, and the mixer tube plate is 166 VDC. The line voltage was 117 VAC.

[Stevem]

Since you likely will not use the 3rd 6SJ7 maybe wire it up as Tridoe and use it to drive a real tone stack.

[solderhead]

I think it's fair to say that most of us would look at that original schematic and end up with different visions of the amp as a mod platform. One person might think about adding tubes and building a 3 channel amp while someone else might want to just cascade the gain stages to make a single channel amp.

Steve, what are you thinking about when you say 'real' tone stack? Are you thinking FMV?