enough gain for James tone stack

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I'm considering using a James tone stack for a guitar amp, but am concerned about how much gain is needed.

The James circuit appears to have much more insertion loss than those used on guitar amps. I see about a ÷100 with the James and ÷30 with a Fender tone stack.

Several people have recommended the Ampeg Gemini as having the ultimate tone control, which is a James plus a 270K resistor to ground on the bass pot.
http://www.drtube.com/schematics/ampeg/g15-jp.gif

The Ampeg has the following topology:
gain - volume - gain - James tone stack - phase inverter - push-pull output
The phase inverter is somewhat unusual, but appears to be low gain.

In my amp, I used the same topology, but used a long-tail-pair phase inverter.

The 7868 output tubes like to see about 12 volts driving (similar to an EL84).

Is an additional gain stage needed?

I have an unused 7-pin socket - perhaps an EF86 could go before the phase inverter. (I'd like to use tubes commonly available from a guitar tech).

Thanks!!
 

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Dude,

The gain of a LTP is approx. mu/4. The mu of a 12AU7 is 20, with a stage gain of about 5. Changing the LTP to a 12AT7, with its mu of 60, gets you a stage gain of about 15. If the 3X gain increase in the LTP is enough, follow that path.

I know you have a 7 pin mini socket available. If changing the LTP tube does not yield the gain you need, switch to a Mullard circuit with a 6AU6 as the voltage gain tube and keep the 'U7 LTP. IIRC, you already have a 6AU6 in hand. :D

BTW, you want to bias the 7868 control grids to something somewhat shy of -25 VDC, to obtain Class "AB" operation.
 
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A James tone stack gives an attenuation of 12dB @ 1kHz.

Your schematic shows a 12AX7 without a cathode bypass cap preceding the tone stack. That stage will have a high OP impedance of about 270k. Problem is, a tone stack needs to be driven by a low impedance source.

You might find it useful to get the free download of Dave Cigna's tone stack calculator from the Duncan Amps site
 
ray_moth said:
A James tone stack gives an attenuation of 12dB @ 1kHz.

Your schematic shows a 12AX7 without a cathode bypass cap preceding the tone stack. That stage will have a high OP impedance of about 270k. Problem is, a tone stack needs to be driven by a low impedance source.

You might find it useful to get the free download of Dave Cigna's tone stack calculator from the Duncan Amps site


Ray,

Adding a DC coupled ZVN0545A source follower between the 'X7 section and the tone stack should solve the impedance mismatch. MOSFET Follies describes the technique. Thankfully, even the wimpy 12AX7 can drive the tiny reverse transfer capacitance of the ZVN0545A.
 
Thanks for the help!

Ray, do you recommend adding the bypass caps decrease the output resistance?
That will also increase gain.

I've attached updated schematics.
The amp went into distortion with the previous one.

Also, please ignore the tone stack, which now shows a Blackface Pro. I'll probably switch to that.

PS
Duncan's TSC is very cool, but only for PC.
 

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IMHO a 12AX7, with its high plate resistance of 62k or more, depending on the operating point, is not a good choice to drive a James tone-stack, even if the cathode resistor is bypassed.

A double triode with lower plate resistance (e.g. 12AT7, 6CG7), or Eli's suggestion of a source follower, would be more suitable for your needs. If you do try a different 9-pin double triode, make sure you wire up the pins properly, because they're not all plug-compatible.

It's easy to see the effect of source impedance using TSC, which is why I suggested trying it. But, from your comment, I guess you have a Mac? I have no experience of those.
 
new design works

I've made some changes to the amp, which now looks like the Ampeg Portaflex.
http://members.aol.com/portaflex/schems/sb127868.gif

I figured out that someone had tried to modify the James stack by adding a 4 meg resistor in front plus a 100pF cap to the output. Removing them yielded a functional tone stack.

It now sounds great, especially with teflon coupling caps. It's loud, like it should be.

It does distort at about 3, which I'd like to fix. I'm pretty certain the phase inverter is not right, especially since I changed the resistors to 100K (from 68K) and left out the 1.5 and 5.6M resistors to the grid. I also took out the NFB, since it was out of phase.

I'm a bit pooped after all the work, and thought some helpful guidance might be nice.

I'm wondering whether the Portaflex schematic is missing the 1M feedback resistor, or if I should modify it to look more like 5E3 (for legendary tone).

Thanks everyone!!
This amp has been a real challenge.
 

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Eli Duttman said:
Put the NFB back in.

Done.
Someone's on the phone, so I couldn't turn it up, but it still seems to ramp up right at about 3. It may be that there's one too many gain stages for guitar, especially since this one's for jazz.

The tone stack needs work. It's got one of those all-in-one chips, which makes it difficult to mod.
I'll most likely replace it with a Brownface stack, which sounds great for guitar.
 
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.

Yes you have too much gain. To reduce:

- lower you plate resistors, try even down to 100k
- put in voltage divider on the V1b and V2a grids to knock down the signal before each stage.

d1
 
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
 
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!
 
12AU7, AT7 or AX7 for PI and gain?

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!!
 

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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.
 
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Re: 12AU7, AT7 or AX7 for PI and gain?

PRNDL said:

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.
 
The Jazz Tank is born

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.

JazzTank1.JPG


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).
 
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