Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on January 19, 2013, 04:48:37 pm
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Hi guy's, This part of the build always gets me buggered :dontknow: I have a fixed/cathode switch on a pair of JJ 6L6GC and want to get the bias setting just right or defind the lowest to the highest setting for both modes.
OT hammomd 1750k - 4K@ 2 ohms but using 4 ohm speakers
At the moment the cathode bias, if i am correct is pumping out a great 32W :-
Plate to cathode volts - 347v
Plate volts - 385v
Across 1ohm resistor - 34.4v
Cathode resistor - 360 ohms
Fixed bias set at -45v
Plate to cathode - 387v
Plate voltage - 380v
Across 1ohm resistor 27mv
I always have a problem with the how much i can go as the lowest and hightest setting in these modes.
Please explain again(i'm sure that its been done a thousand times) for a math noob.Thanks
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Your numbers don't jive together. Post a schematic and recheck your voltages.
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I always have a problem with the how much i can go as the lowest and hightest setting ...
Highest: Tubes don't turn red, but if you pass more idle current they turn red
Lowest: 0mA
Really, anything between those extremes will work. You might decide 0mA doesn't sound too good.
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Rechecked volts and added to schem.
Did notice that with the bias pot set @-46.3v and the ppimv @ max the bias reading on pin5 was -36.2v and when ppimv turned down pin5 would read -46.3v, is this right??
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TIMBO,
Just use the Weber Bias calculator;
http://www.webervst.com/tubes1/calcbias.htm (http://www.webervst.com/tubes1/calcbias.htm)
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Did notice that with the bias pot set @-46.3v and the ppimv @ max the bias reading on pin5 was -36.2v and when ppimv turned down pin5 would read -46.3v, is this right??
No, that's not right. No current flows thru the mv pot or the grid stopper so no voltage can be dropped. Whatever voltage you have at point "#" should also be present at pins 5. Something is not wired correctly with your mv pots. Your schematic is correct.
That's an odd bias supply. I've never seen a bias supply tapped into a voltage doubler quadrupler like that. Where did you get this circuit? Have you used it successfully before?
PS... Where is the 1Ω resistor? Is it common to both 6L6s? Or do you have one resistor for each 6L6?
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Thanks Sluckey, I was not thinking that the PPIMV circuit was wrong,so i will check it again. As for the PSU/bias circuit i pinched that from one of Darryl's builds,the PSU works well with cheep local parts. :icon_biggrin:
The 1r resistors are on the build just didn't draw on schem.