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I've looked at all the data sheets on Frank's web site and no mention of resistance values for fixed or cathode bias. If anyone knows please let me know. TIA
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Hi!
You can go with the recommended values of the 801A which is very similar. For the 801A 500k for cathode bias is set as max and 100k for fixed bias. Go with 100k or less and you are safe.
I use 100k on the 10 in preamps both with cathode and filament bias and it works reliably
BR
Thomas
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.
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.
I guess I did not express myself very well, I would like to know the maximum recommended resistance values for self or cathode bias, or grid bias supply. These are often in the order of 500 kilohm for cathode resistance and 50 kilohm to 100 kilohm for grid supply. I do not seem to see any such values in any of the 10 data sheets I have looked at.
This is one use of tube curves.Say you want 250 volts on the plate and 10mA of current. The sheet has both a little table for this and the tube curves. The tube curves have the information to construct a table of what ever operating conditions you care to explore. Then you can look at the curve to make sure it is in its linear operation.
So, 250 volt plate and 10mA says -23.5 Volts on the grid (Grid to cathode voltage)
Since Ohm tells us that I = V/R , you solve for R with the simple R = V/I transformation (algebra) so R = 23.5 / 0.01 This gives R = 2,350
Another setting is 450 18mA and -40 R = 40 /0.018 or 2,222 ohms
Edits: 10/23/16
This sheet shows three different self-bias resistor values. If you want to calculate your own the formula is simply, desired bias divided by plate current. For example, using the 425V sample on the sheet 40V/.018mA = 2222,22 ohm which you would round off to the nearest available resistor value.
Cal
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