James stack in Fender

[Tonewood]

I was about to copy a James stack from a book I have into a Champ & then noticed many different versions of the stack online. (very different R/C combos) Does anyone have a tested version that they have used with a small Fender style preamp?

Thanks

[teemuk]

The R/C combos might be different but don't let that fool you. Instead of actual component values, look at the interrelating ratios of the component values. They are usually more or less the same, which means the various circuits don't actually behave all that differently.

[JHow]

I believe the Duncan Tone Stack calculator (link) has the James tone stack in it. You can play around with values before you build:

TSC

[afm_380]

I have just recently completed a sctratch build using the Top King Royale schematic as a base. I chose to build it with a James tone stack as referenced in the Duncan model and on Adam's Amps website. The stack works well but quite different than a expected.

I am REALLY pleased with how this amp responds to touch ... but I'm not quite sure of the tone stack yet ... I'm thinking that part of my issue with the tone stack is because I'm driving it off the cathode instead of the plate (like most examples that I can find using a Baxandall/James circuit). The Duncan TSC has a Zsrc of 38k for the input of the simulation. Is this value a typical anode/plate value? What would be a typical cathode value? Here's a copy of my build schematic ...

[teemuk]

What would be a typical cathode value?

Someone else correct me if I'm wrong but isn't it approximately 1/gm (gm being the transconductance of the specific tube). A 12AX7 as a cathode follower should therefore have output Z of about 600 ohms.

[Tonewood]

afm_380

I have only been looking at small Fender type amps so far. I see you have .0047mf & 470pf coupling caps. These are way smaller than what I have been dealing with. Is this typical of larger amps? I was thinking of a large cap for my Champ- .047mf to pass everything then cut with the tone controls.

Is the big cap a bad idea?

thanks

[afm_380]

Those values were stolen directly from the TopHat design which I believe was influenced by the Vox AC30 circuit. This build is a definite 'keeper'. I deviated from the tophat design by using basically a Fender 5E3 power supply circuit (it was already built from a previos build), octal family power tubes (6v6, 6L6, EL34), and the tone stack. My previous builds have been mostly Fender Tweed based and I wanted this build to be a little different. It's REALLY SWEET. The coupling caps are an easy section to tweek.

[GibsonLover]

James Vs. Fender

Use the Duncan tone stack calculator. If you set the controls flat, the Fender has ~5dB more loss. You won't use it that way, but realize the mid frequencies will always be scooped 5dB more, which is substantial when it comes to signal voltage driving/overloading your next stage. Remember, 6dB is approximately a 2X voltage difference.
The big thing the James control has to offer is (+) and (-) gain relative to the center frequencies. With Fender, you can never get a mid hump. With James, you can hump or dip the mids.

[Tonewood]

TSC on Mac?

Thanks GL-
Just borrowed (last night) my son's Windows laptop and played with the TSC. It was very useful. Anyone use TSC on a Mac with a Windows emulator?