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Post Subject: Bias helpPosted by cb on: 10/9/2007

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.

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.

Here going through the list;

1) ok

2) Gas tubes are about 149 and 151 volts. bias lines at 192v, 194v

3) current flow to the gas tubes was at 9mA. Changed R11 and R12 to 3.3k to get 12mA.

4) R9 is 5k,  R10 is 10k, VR1 is 20k

5) R4 and R6 is 390k

6) R5 is 12.1k caddock mk132

7) Shorted the inputs, I get minus 4.4v on grid.

8) ok

9) around 195v on plate

10) This is later on: to get around minus 3.4v on the 6e5p bias I tried 510kohm resistors in R4 and R6, and plate voltage reduces to 186volts.

11) ok

12) ok

13) can't get zero volts. 0.4v is the min i can get, VR1 set at 20k. Why I cant zero it, I dont know...

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.

Thanks

Rgs
Charles

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