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Another grid resistor question

Carbsandcaps said:
... in order to get the bias correct I would have to use a 250k ohm resistor on a 6l6 that calls for 100k for fixed bias
Click to expand...
The grid resistor should not be connected in such a way that it determines negative grid voltage. And the 100K maximum recommended by tube manufacturers for the 6L6 should not be exceeded. The maximum value is based on being able to securely hold the grid at the intended DC voltage. Here's an explanation that discusses things such as positive ion current within the tube:

Grid Resistor Value (from Morgan Jones Valve Amplifiers 4th Ed)

Jack
 
TriodeLuvr said:
The grid resistor should not be connected in such a way that it determines negative grid voltage. And the 100K maximum recommended by tube manufacturers for the 6L6 should not be exceeded. The maximum value is based on being able to securely hold the grid at the intended DC voltage. Here's an explanation that discusses things such as positive ion current within the tube:

Grid Resistor Value (from Morgan Jones Valve Amplifiers 4th Ed)

Jack
Click to expand...

It’s not affecting the negative voltage, the resistance I need to lower the negative voltage is affecting the final resistance number for the grid resistor. Using a 6l6, it only allows 100k grid resistance for fixed bias. I’m using I’m using a negative voltage doubler with a 47k pot, and a 10k resistor to ground. With the pot set to where I need it, it reads about 3kohm so that plus the 10k resistor gives me 13k or 87k left to play with. Now I have balance pots between each set of output tubes. I have 75kohm grid resistors going to the grids giving me a total of 88kohm.

Im trying to figure out a compromise for the balance pots. I’m thinking 50kohm. Problem with that is that if the sweep is all the way to one side to set the bias than one grid resistor will be 63kohms and the other will be 113kohms. Obviously matched tube would fix this problem but this also just has me thinking if anyone has ran into this before. Most bigger power tubes are 500kohm with fixed bias making this a whole lot easier. The 6l6 seem to be the odd man out.
 

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The problem is the limited current available from the bias supply. It's not critical though; I would use the circuit as-is and use 100K grid resistors. Also, a 220K from the top of the pot to the wiper will prevent a catastrophic failure if the wiper loses contact.

Jack
 
TriodeLuvr said:
The problem is the limited current available from the bias supply. It's not critical though; I would use the circuit as-is and use 100K grid resistors. Also, a 220K from the top of the pot to the wiper will prevent a catastrophic failure if the wiper loses contact.

Jack
Click to expand...

Sounds good to me. I tried 50k dc balance pits and they didn’t change the voltage at all either way. I’m assuming because there’s not enough current flow to the grids any way. I think you’re right and I’ll do the 100k resistors and just deal with the way it is and work on some balanced tubes.
 
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