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cb wrote:

The bias voltage I get on the 6e5p is around minus 4.4v on both the circuits. This is with the inputs shorted. With the inputs connected to adjust voltage to 0.0mvdc, I get a reading of plus 0.4vdc. This is with VR1 set at maximum (20kohm). This is the “lowest” reading I can get without increasing R9, VR1 or R4. I can’t figure out why i am getting these readings. 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 R5 to 10kohm. This seems to work.

OK, you made some mistakes and I am sure that 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.

Let do it step by step:

1)    Disconnect R4 and R6 and put the C4 and C5 in the right polarity

 2)    Confirm that your gas tubes are 150V (144-151 is fine, depending of the brand of the tube). If you pull them out they should have ~200V (>190V) in bias lines.

 3)    Measure current that fallow to the gas tubes. I should be between 10mA and 15mA. The lover current the longer the gas tube will live but still stay above 10mA and below 20mA. Use R11 and R12 to set the current for the gas tubes. It wills all depends from how many volts your PS outputs. You just burn the spare voltage on those power resistors.

 4)    Check the R9 and R10. The R9 should be a little smaller as the R9 + VR1 in the middle of its range should be approximately equal to R10.

 5)    Put identical R4 and R6, it does not matter the values, let keep it ~400K

 6)    Confirm the value of your R5 and connect the R4 and R6. You do not need to apply B+.

 7)    Short the input and set minus from minus 3.2V to minus 4.2V on grids. If you have different voltage then modifies your R6 to set the correct voltage on the grid.

8)    Apply B+ on the first tube.

9)    Measure the plate voltage in a point “A”

10) Now you need to adjust the amount if bias on the tube in order to have 200V on plate. You understand that amount of bios is irrelevant and it would depend from the given tube of yours.  Averagely the minus 3.4V-3.5V will give you 200V and 15-17mA of plate current. Do not be too finicky about it – the current in the driver tube in THIS application is not really critical. Set plate voltage between 170V and 200V with whatever bias voltage it will be (let presume -3.4V+/.5V)

11) Turn down B+ and un-short input.

12) Make sure that R4 is the same as R6

13) Set with VR1 null volts at input and to confirm that the negative voltage at grid is the same as it was when the input was shorted.

14) You are all done now. Still, I would like you to understand the basic operation that you will be able to find out what is wrong:

Higher bias voltage = Less plate voltages
Lovers R6 values = Higher bias voltage
Lovers R5 values = Lower bias voltage

15) So, if you wish to go for 10K in R5 and 500K in R4 and R6 then it is perfectly fine, in fact I had one amp running this way and the first revision of Milq had R5 as 10K and  R4/R6 as 499K. I made many experiments trying to determine what would be better: to keep R5 lover or not. I run the R5 from 5K to 14K. Theoretically it is a large resistor right in grid and to have it too high value should kill some HF (low path filter with tube capacitance).  In practice the filter kicks in above 100KHz and I recognized that there was no effect. The last revision I changed 10K to 12.1K but it is absolutely irrelevant. Use R5 as any best quality resistor you have and juts just R6 to maintain 200V on the plate. In here is not the value are important but the concept the you block DC not with capacitor but with an “active” resistor that backed up with positive voltage. BTW, eventually if you wish to go “kinky Milq” and have a very low impedance front end then you might do not use positive bias at all letting the excessive DC voltage on your Milq input to bias your cables (it might be a different subject and it has own “danger”)

cb wrote:

On the melquiades circuit revision 3.6 the C5 (10uF 250v) is wired up +ve to ground and in revision 4.0 C5 is wired –ve to ground. Revision 3.6 works for me, with revision 4.0, I get around -130v on OA2 and -0.75v on the 6e5p bias. Which is the right way?

Charles, the last revision (Ver4) while I was pained the circuit in colors and made it easier to understand/talk about it, I made a mistake when I moved the C5 cap around making it sexier and forgot to move the polarity identifier. Surely, it is the cap in the negative supply and it should have it negation lead on the negative line and the positive lead on ground. My apologues, though It might be self-explanatory how to hook up the cap in a negative supply. I will make the correction tonight. If you did try this cap in a wrong way then you might partials re-polarized it and if I were you I would get another cap. Thanks God it was a very small caps and it did not blow up…

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