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Old 20th October 2010, 04:25 AM   #1
athos56 is offline athos56  United States
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Default Mixed bias for balancing unmatched tubes in PPP?

So some might know I'm designing a 6B4G PPP amp. The bias scheme I have now can balance the bias between the both sets of output tubes (on either side of the opt) but not individual tubes (on one side of the opt). So would a resistor and pot from each tube cathode to ground allow me to fine tune each tube in concert with the fixed bias?

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Old 20th October 2010, 06:05 AM   #2
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Whats wrong with a pot for each grid to the fixed bias supply?
You won't need any more than an unbypassed ohm under each
cathode, just to take balancing measurements...

Last edited by kenpeter; 20th October 2010 at 06:08 AM.
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Old 20th October 2010, 06:24 AM   #3
athos56 is offline athos56  United States
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I have the bias supply feeding the grid of a cathode follower driver for each pair of tubes. Does that make sense? This is my first try at at fixed bias amp. Here is the schematic. Post #17 6B4G PPP idea

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Old 20th October 2010, 10:47 AM   #4
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2 grid direct coupled to 1 cathode. I see now why this is no easy solution...
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Old 20th October 2010, 11:22 AM   #5
Ian444 is offline Ian444  Australia
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Is it possible, within the four tubes, or eight if its a stereo amp, you can put reasonably matched pairs together?

I can suggest another way around it, but you may not like it. Ditch the 6SN7 for four mosfet source followers, each with its own bias pot. I'm sure it will sound just as good.
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Old 20th October 2010, 02:26 PM   #6
Brit01 is offline Brit01  United Kingdom
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Quote:
Ditch the 6SN7 for four mosfet source followers, each with its own bias pot
I've been experimenting with IRFU310 mosfets but still struggling. Had 4 to play with, burnt one of them, and the other 3 seemed to be so unmatched that I need to apply 9/10 more volts on 1 rail to get a balanced/no DC output.

Thought it was unmatched tubes or something else in the chain/driver side.

I started swapping the FETs and the issue followed the FETs.

But as Ian said, try the Fets as described in Tubelabs Powergrid section.

I need to work on my issue more.
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Old 20th October 2010, 04:20 PM   #7
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LED, Zenier, MOSFET diode between 6SN7 and CCS?
Span a pot per 6B4G grid across that voltage drop?
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Old 20th October 2010, 04:40 PM   #8
athos56 is offline athos56  United States
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Well this may seem weird, considering I have a 10m45 CCS under most elements of my amp, but I was looking for a low tech solution. Originally this was an all tube project, but 6 pentode CCSs was unreasonable and the 10M45 is pretty bullet proof.

What I was thinking was a 10 ohm resistor and a 100 ohm pot. Or, I could double up on the 6SN7s and run one to each 6B4G , but that would be more work I think.

I could get matched tubes but I'm seriously thinking of getting the Soviet 6S4S (6C4C).

Athos

Last edited by athos56; 20th October 2010 at 04:46 PM.
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Old 20th October 2010, 05:01 PM   #9
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You can have many pots (and parallel output tubes)
across the diode mentioned above. Still only needing
just one 6SN7, Voltage drop diode, and CCS per side.
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Old 20th October 2010, 05:09 PM   #10
athos56 is offline athos56  United States
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Quote:
Originally Posted by kenpeter View Post
You can have many pots (and parallel output tubes)
across the diode mentioned above. Still only needing
just one 6SN7, Voltage drop diode, and CCS per side.
hehe, yeah, complexity no matter which way I go . Thanks for your contribution, I'll see if I can puzzle it out.

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