enough gain for James tone stack

[PRNDL]

Originally posted by d1camero
I have used a lot of James tone circuits in my guitar amps and like them very much for the wide variability versus the typical Fender TMB stack.

Do you use the values suggested by Adam Alpern?

[d1camero]

My values are similar. Here is what I have had great luck with:

R1 - 68k
Bass pot - 500k LIN
R3 - 100k switch to ground for mid boost
C2 - 500pF
C3 - .0047uF


R2 - 200k
C1 - 680pF
C4 - .001uF
Treb Pot - 500k LIN

There has been a lot of experimentation from a few of the members at www.el34world.com Search their archives for "James" or "Baxandall" (many threads were misnamed). They have also posted a number of schematics under their schematics board.

d1

[kevinkr]

Depending on inflection points and boost/cut required you might even want to try a baxandall based tone control circuit. It's quite tweakable even for mi applications.

[PRNDL]

I'm in the process of ordering parts for a James (Baxandall) tone stack, but am stuck on getting the right values and being able to order only one.

Adam's components can be purchased from Antique, but the caps come in bags of five.

I've checked a few other places (Hoffman, Torres) and a few other stacks (diCamero's and TubeNit's), but am finding that they don't sell all the right cap values.

On the other hand, I did a bunch of improvements:

• put NFB into the cathodyne (ala 5e3 with 100K resistors instead of 56K ... will order the right ones)
• replaced the PI 12AX7 with a 12AU7 (plan on ordering an AT7),
• put in a preamp (Mic1) and master volume (post tone stack)

The amp sounds really nice!

[PRNDL]

I'm wondering what the best tube and resistor values would be for V2, the last preamp stage and cathodyne phase inverter. Currently I have a 12AU7, which seems to work better than the 12AX7. There's more than enough gain already.

The first thing you'll notice is that the resistor values are unusual: 100K for the phase inverter and 360K/4.7K for the gain stage. I plan on changing to 56K for the PI (the values used in a 5e3).

From my limited knowledge, 12AU7 has lower resistance and can supply more current that the AT or AX. The AT has the same resistance as the AX and can also supply more current, which may make it the best choice for the combined role.

The 12AT7 would also be better off with 220K/2.2K or 100K/1.5K for the preamp gain stage resistors.

Does this sound right?

I'm seeing the need for an advanced book on tube amp design, although I do have several from the web, including Norman Crowhurst's articles. Any suggestions would be greatly appreciated.

Thanks!!

[ray_moth]

12AU7 would probably do a good job of driving the tone stack. I didn't recommend it because it's a non-linear tube but, for a guitar amp, it may be acceptable - it's up to you whether you like it or not. A better choice with similar plate resistance and gain IMHO would be either 6CG7 or 6FQ7 but you would need to rewire the socket because the pinout is different.

If you want a good reference book, 'Valve Amplifiers' by Morgan Jones (3rd edition) is my personal favourite. He's written it to be user friendly but, like any book, it doesn't cover every topic.

Web sites I've found useful include Aiken Amps, Duncan Amps, The James-Baxandall Passive Tone-Control Network and Max Robinson's Fun with Tubes.

[kevinkr]

Originally posted by PRNDL

I'm seeing the need for an advanced book on tube amp design, although I do have several from the web, including Norman Crowhurst's articles. Any suggestions would be greatly appreciated.

Thanks!!

Kevin O'Connor amongst others has written a book on designing guitar pre-amplifier circuitry - do a search on amazon and you will find it. Lots of guitar amplifier books there actually.

As much as I admire Crowhurst, his writings are oriented entirely towards hifi and are not really a source of good information for anything other than tube based acoustic guitar amplifiers. Much better to avail yourself of the very specialized knowledge accrued specifically in MI amplifier design. The original Fender sound was an "accident," what grew from that was not.

[PRNDL]

I've named this amp the Jazz Tank, because the name fits.

The steel case is covered in tweed, and it has a very thick steel chassis, which means it is extremely sturdy (i.e. built like a tank).

There are four teflon and one paper-in-oil coupling capacitors. Those HiFi caps and the 7868 output tubes gives this amp a unique sound.



I'm waiting for parts to make the James tone stack suitable for guitar.
I also have to add a handle and feet. I could put in a leather handle, but it is a bit heavy.

Here's the Jazz Tank web page
http://www.naturdoctor.com/Chapters/Amps/JazzTank.html

PS
I wonder if this is the first tweed metal amp (i.e. a metal case covered in tweed).