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Old 4th September 2008, 11:05 PM   #1
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Default Differential Amplfiers - current sources under/over the triodes

Considering a differential stage in a push-pull amp consisting of two triodes, is there anything problematic about using current sources as plate loads? I realize that there is huge gain by using one current sink under the cathodes, but I'm running out of room for a negative voltage supply, so I figured this is about the best I can get. I plan to implement something close to Pete Millet's generic driver schematic, but have a regulated supply for each stage with a current source on all the plates (250v regulated for the first stage, 350 for the second).

This setup will be driving basically the equivalent of four EL84's in DPP.

TIA,
-Paul
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Old 4th September 2008, 11:52 PM   #2
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Paul,

If you use Mullard style topology, you can put an IXYS 10M45S under the LTP's tail, without having to use a negative rail.

EL84s don't need much, in the way of drive voltage. So, a 5687 as the LTP and a 7AF7 section as the voltage gain device rate to be satisfactory. High gm small signal types provide resistance against loop NFB induced slew limiting.
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Old 5th September 2008, 12:08 AM   #3
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As I undestand it you won't convert a push pull stage to a differential stage by using plate load CCS's. You need to have the cathodes tied to one current source to force the tubes to run at the same current.

You do know that you can't run other than full Class A1 like this? I suspect you may get fairly hard clipping if using a differential output stage. That said Allen Wright's differential power amp designs get unanimously great press.

Regards, Rob.
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Old 5th September 2008, 12:38 AM   #4
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Quote:
Originally posted by Eli Duttman
Paul,

If you use Mullard style topology, you can put an IXYS 10M45S under the LTP's tail, without having to use a negative rail.

I should have mentioned that I am using an input transformer to split phase, so I'm not interested in bumping up the grids.
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Old 5th September 2008, 12:39 AM   #5
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Quote:
Originally posted by Robert F
As I undestand it you won't convert a push pull stage to a differential stage by using plate load CCS's. You need to have the cathodes tied to one current source to force the tubes to run at the same current.

Regards, Rob.

This is in the driver, not the output. My bad for not mentioning it.
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Old 5th September 2008, 12:51 AM   #6
exeric is offline exeric  United States
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Paul,
I was investigating the same thing you are. I recently got Morgan Jones book, which is a great reference, and the only way to have a "practical" CCS in a differential pair plate circuit is to use a current mirror CCS. If you don't use this type of CCS you will be constantly fiddling and matching the currents between the two individual plate circuit CCS.

I'm in the thinking stage of a PP amp with a differential pair driving 4 EL84s and 2 6V6s. I pretty much decided decided against putting the current mirror CCS in the plate stage of the differential pair and instead going for just the single cathode CCS. The previous advice about not needing much drive voltage for EL84 is right and if you need more than what you are currently getting just go for a "single" voltage amplification stage that uses a tube with an MU of 30 or better. It is better design to get the right tube for the job and eliminate needless stages.

If you do decide to add a separate stage it should be a cathode follower and not a needless additional voltage gain stage for an EL84. A cathode follower will eliminate capacitor blocking distortion between driver/voltage gain stage and EL84s, ala Tubelab dot com.

By the way, the reason I'm adding a 6V6 to mine is because I had an amp that had that topology that sounded like a little slice of heaven. This will be an upgraded version of that one with Mosfet cathode followers for each power tube. I also plan on having selectable pentode/triode for each tube pair for ultimate fine tuning of the sound.
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Old 5th September 2008, 12:59 AM   #7
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This may or may not have some information you are looking for.

LTP w/CCS tail and plate loads
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Old 5th September 2008, 01:17 AM   #8
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Quote:
Originally posted by SpreadSpectrum
This may or may not have some information you are looking for.

LTP w/CCS tail and plate loads

I think that answers it. The crude vs. strong current source discussion lends itself to what I'm doing. If I set up a common cathode 12AU7 diff stage with a resistor under the cathodes and current sources on each plate, the current sources should be happy enough. I suppose I could always split the cathodes apart too...
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Old 5th September 2008, 01:30 AM   #9
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Quote:
Originally posted by audiowize

I should have mentioned that I am using an input transformer to split phase, so I'm not interested in bumping up the grids.

Paul,

If you use a Schmitt circuit with both CG1 and CG2 connected to the I/P trafo, replacing RK2 by a CCS achieves your purpose. Select the value of RK1 to yield the appropriate grid bias voltage.
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Old 5th September 2008, 01:33 AM   #10
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Quote:
Originally posted by Eli Duttman



Paul,

If you use a Schmitt circuit with both CG1 and CG2 connected to the I/P trafo, replacing RK2 by a CCS achieves your purpose. Select the value of RK1 to yield the appropriate grid bias voltage.

This is pretty close to the Mullard circuit idea... With 350v available, is this sensible? I'm starting to think I should just run independent VA stages for each phase and ditch the common cathode idea.
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