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Old 28th December 2005, 05:41 PM   #1
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Default Compound bias

I am trying to get the best of both worlds using a fixed and a cathode bias at the same time,; I know it's has been done before but I need to know how to calculate it.

The output stage is a push-pull of 6L6 GC (shared cathode resistor) 310V on the plates, bias: 80ma/tube.

If using a compound bias ( may the academy forgives me if it's not the right word), which value can I use for the grid leak since the max for fixed bias is 100k and 500k for cathode bias?

Thanks
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Old 29th December 2005, 12:45 AM   #2
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Dennis Boyle is an advocate of combination bias. I think Dennis makes a good case.

While it may be overly conservative, treat the assembly as being fixed bias. You're sure to be on SAFE ground. 150 KOhms MIGHT be an OK value for the grid leak resistors. Is there a good reason to run the risk?

The implementation of combination bias is pretty straight forward. Stand the 6L6 cathodes on an unbypassed 100 Ohm resistor. Start with the C- supply set to the max. voltage. While measuring the drop across the 100 Ohm part, lower the adjustable bias voltage until the drop is 16 V.
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Old 29th December 2005, 12:59 AM   #3
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Thanks Eli!
Is there any further benefit using two cathodes resistors in lieu of a shared resistor and one pot on each grid for the negative bias voltage?
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Old 29th December 2005, 01:09 AM   #4
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Quote:
Originally posted by kooltubes
Thanks Eli!
Is there any further benefit using two cathodes resistors in lieu of a shared resistor and one pot on each grid for the negative bias voltage?
Yes, there is! While a pair of tubes may start out as being tightly matched, they will drift apart with use over time. Individual bias adjustments allow you to compensate for the drift. Also, how well matched are the tubes you get to begin with? Separate bias adjustments are good for dealing with poorly matched tubes.
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Old 29th December 2005, 02:19 AM   #5
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I've used a split-bias setup in a EL34 amp i'm working on.
So far I've experimented a little with cathode R values of 10-50ohms.
Think right now i've settled on 33ohms for each Rk.

My though was that a bit of added cathode bias might help get rid of some of the crossover distortion,etc.
A little local NFB can't hurt,right?
Plus it's a nice simple way to measure the bias.
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