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Old 29th September 2008, 12:29 AM   #1
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Default Cathode bias 6LU8 UL/Triode

I have done a lot of searching in order to understand how to choose Rk for desired operating point and I think I can manage it if I have either reasonably accurate mu and rp or plate curves. The problem comes with trying to figure it out without the availability of either one. Specifically WRT tubes like the 6LU8 and 6LR8 compactrons for which I can find no triode or UL curves or even mu/rp data in these modes.

So, has anyone come across the curves for these tubes in triode and UL modes? Is there another method? If one had triode and pentode curves could one extrapolate a reasonable Rk by averaging the Rk results for triode and pentode to get a good UL choice?
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Old 29th September 2008, 02:07 AM   #2
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For DC, UL is the same as triode (or pentode with plate and screen at the same voltage...)

I think the 6EZ5 or 6HE5 is about the same as the pentode in the 6LU8 or 6LR8.
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Old 29th September 2008, 11:37 AM   #3
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I wondered if triode wouldn't be the same for bias purposes. Thanks. I will check out those data sheets. So far just by eyeballing the pentode curves for 250V g3 I suspect that I would need about 25V at 300ish V B+ to get around 50mA idle current. Something in the 500ohm range.
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