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Old 16th December 2007, 02:01 PM   #1
PRNDL is offline PRNDL  United States
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Default enough gain for James tone stack

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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Old 16th December 2007, 04:28 PM   #2
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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.

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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Old 16th December 2007, 04:53 PM   #3
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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
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Old 16th December 2007, 05:34 PM   #4
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Quote:
Originally posted by ray_moth
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.
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Old 16th December 2007, 05:48 PM   #5
PRNDL is offline PRNDL  United States
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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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Old 17th December 2007, 05:47 AM   #6
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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.
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Old 18th December 2007, 12:04 AM   #7
PRNDL is offline PRNDL  United States
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Default 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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Old 18th December 2007, 12:25 AM   #8
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Dude,

Put the NFB back in. To get the phasing right, criss-cross the O/P trafo to plate connections.
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Old 18th December 2007, 01:29 AM   #9
PRNDL is offline PRNDL  United States
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Quote:
Originally posted by Eli Duttman
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.
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Old 18th December 2007, 11:41 AM   #10
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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
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