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In Reply to: RE: Type 10 grid resistance values posted by vetmedrobert@gmail.com on October 23, 2016 at 06:57:43
Correct me if I'm wrong, but I think the original question was about the grid-to-ground resistance. The question arises because the Type 50 has such limitations - in the specs I have, Sylvania says nothing, Cunningham says "not recommended for RC coupling," and RCA says "... any conventional type of input coupling may be used provided the resistance added to the grid circuit by this device does not exceed 10000 ohms."
I can't find any Type 10 data sheet with a similar condition, but the 10 is an older tube than the 50, and the necessity of this caution seems to have appeared with the 50 (hence the inconsistent data sheet advice). I speculate that in older designs transformer coupling was much more common.
I will note that the requirement is there to limit bias drift through grid leakage current. In that case, the requirement is most severe with fixed bias. For example, the 300B specs from 1950 specify 50K maximum for fixed bias, or 250K for self bias.
Follow Ups:
Yes, see my second post. I am using a 10 to drive a 10 with an IT. The input 10 is using filament bias so would likely be ok with 100 to 250 kilohm on the grid. The negative bias supply probably would be ok with a resistance of 25 to 50 kilohms. Please let me know if these would be appropriate values, and thanks for your help.
It doesn't matter whether a negative voltage is applied to the grid or a positive voltage to the cathode, anything other then self-bias with a cathode bias resistor is fixed bias. This is the "most severe" scenario Paul described. If you believe RCA, no more than 10K should be used at the grid.
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Yes
He gave you his best estimate. IMO 100K is fine.
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