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cb wrote:

Yes, it is imposable to have + 0.4V on the grid, 0.0V at input, with R5 resistor in place and the gas tubes outputting +150V and -150V. My apologies, I meant to say I have  minus 4.4v on the grid with the inputs shorted. Then I unshort the input to measure the dc voltage at the input and to set it to zero volts via VR1. Here is where I get +0.4v, this is with VR1 at max resistance (20kohm). If I lower the resistance of VR1 the voltage goes up. But if I increase R9 from 5k to 10k, I manage to set the input voltage to zero volts. But R9 should be a little smaller as the R9 + VR1 in the middle of its range should be approximately equal to R10.

If you are around minus 4.4v on grid then you about to be right on money. Your positive bias has too much voltage and it pushes the positive potential over the R5 resistor. To increase R9, or to increase R4 or to flip the gas tubes with a positive tube having less voltage is an easy solution.

cb wrote:

And here my mistake, R9 was supposed to be R5. To get around minus 3.4v on the 6e5p grid and to set 0.0mVdc on the input, I tried 510kohm resistors in R4 and R6 and increased R9 from 5k to 10kohm. Increasing R9 to 10k allows the input to be adjusted to zero volts. The plate on the 6e5p now drops from around 195v to 185volts.

Sorry, I am little confused. Let keep the R5 as is and adjust the positive bias with the resistor of positive chain. If you have 0.000V at input and sub 200V on plate then do not even measure anything else – you are fine.

cb wrote:

15) To get around minus 3.4v on the 6e5p bias and 0.0mVdc on the input, I tried 510kohm resistors in R4 and R6 and increased R9 to 10kohm. This seems to work, plate is 186v.

Which is perfectly fine. Do not worry to get minus 3.4v on the 6e5p grid, worry to get 180-200V on 6e5p’s anode and to get tune to have ~32-35 times of gain. I do not know exactly why you need to drop more voltage on both R4 and R6 but most likely it is because you have too high voltage on the gas tubes. My gas tubes regulate at 145V-146V yours at 149 and 151.  That is approximately 4-5V difference that is approximately the difference between your minus 4.4v and +0.4v. In this case to change R4 and R6 is a perfect solution. I say “perfect” because the higher R4 and R6 is better as higher impedance of those resistors will better separate grid form the noises of gas tube.

What OPT you use? Care to post some pictures? What acoustic system you will be using? When you finish the project and if you feel to do so then post in an appropriately thread your feeling how do you feel the amplifier Sounds.

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