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Old 2nd February 2008, 06:33 AM   #31
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Join Date: Jan 2002
Originally posted by Steven-H
Onto the power tube. Most schematics I have seen grid bias the tube @ -90v (blue line). As Gluica helped me out with earlier; I would be right there @ -89v if I used 7 of those 12v batteries (Q=750v@25mA). However, in looking at the curves dead center (grey line), I think that in triode mode anyway; I would be better off with my Q=580v@50mA:

Click the image to open in full size.

Nope. Assuming you're intending to bias at say 900V B+ and 85mA, find that point on the curves, then draw the 6K loadline. It will swing well above the 900V B+. Eyeballing your curves, at the Q point I mentioned Vg = -95V.
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Old 2nd February 2008, 07:13 AM   #32
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Brett -
Thanks, as usual I was doing something wrong...

Click the image to open in full size.

So, now my bias point isn't neatly fitting into multiples of 12 (Q=900v@90mA). Once I get the PSU breadboarded and find out my true B+ I suppose the best way to get the bias where I need it to be would be by adding some 1.5v batteries.
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Old 2nd February 2008, 07:25 AM   #33
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That's better. get it working with a variable DC supply to the grid, then worry about batteries. If you do a sectioned filter as per Morgan Jones' book, it'll be quiet and clean and you'll never have to mess with batteries.
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Old 2nd February 2008, 10:59 AM   #34
Gluca is offline Gluca  Italy
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That picture I posted is from an amp running the GM70, ok ... I was cheating . I changed a while ago the output tube.

When I used the 813, it was running at 750V/100mA and I believe I used 6 batteries. In case you cannot fix the bias with multiples of 12V you can still use a center tap resistor at the filament (bias would be just like AC, i.e. 6*12V=72V) or a small resistor under the cathode. I usually don't care if the plate current is +/-5ma away from ideal. Another option is to ground the +terminal of the DC filament so the bias will be 6*12-5=67V. Old datasheets suggest to tie the -terminal of the DC filament to ground but I can't see any difference and I did it without any issue.

So you can bias the 813 at 77V, 72V or 67V using 6 12V batteries.

Fine tune with a cathode small unbypassed resistor if you like. Keep in mind the whole filament current will be passing through the resistor so it needs to be fractions of ohm (lets say 0.47Ohm) but high power (0.47*5.1*5.1=12W). That's why I prefer to over/under bias the tube. If you use the classical hum pot at the cathode, the resistor will be passing only the plate current so it will not be dissipating such a huge power.

Easier to do than to write.

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Old 3rd February 2008, 04:39 PM   #35
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Thanks again for the help. So I guess I will clearly have a whole lot more swing than necessary....
I am going to start to work on my PSU. With 141 ohm DC resistance in my OPT's I am shooting for about 915v (When I PSUD'd the design I was getting about 916v; got lucky so far...)
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