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Fixed bias PSU questions
Fixed bias PSU questions
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Old 11th January 2018, 11:51 AM   #1
Tyimo is offline Tyimo  Hungary
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Default Fixed bias PSU questions

Hi

In the case of using fixed bias what are the requirements of the power supply?

- what power transformer? How many VA? e.g. for 6C33CB tubes
- What kind of quality rectifier, caps, resistors etc?

greets:

Tyimo
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Old 11th January 2018, 01:05 PM   #2
kodabmx is offline kodabmx  Canada
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In my KT88 amp I use a 12VA 48V transformer (Triad F48-250-C2), HER208 in a bridge, CRC filter (220u/1k/220u). Works fine for me. Generally 250ma is more than enough current.
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Old 11th January 2018, 01:12 PM   #3
Eli Duttman is offline Eli Duttman  United States
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A C- (bias) supply is a low current situation. Only a few mA. flow, when the NET voltage on the grids approaches zero. Combine those few mA. with the losses in the trim pots. and you have the requirement. From a practical standpoint, rail voltage (which is design dependent) is the critical number.

Many power trafos have a bias tap on the rectifier winding that is 1/2 wave rectified. A spare filament winding can be voltage multiplied to generate the volts needed. As this is a negative rail, reverse the diode polarity shown in the uploaded graphic. Use Schottky diodes to minimize forward drop losses, along with being "noise free".
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File Type: jpg Half Wave Parallel Multiplier.JPG (77.4 KB, 151 views)
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Old 12th January 2018, 01:20 PM   #4
Tyimo is offline Tyimo  Hungary
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Thank you!
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Old 12th January 2018, 02:49 PM   #5
goldenbeer is offline goldenbeer  Germany
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You should not operate 6C33C toobs in in pure fixed bias.
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Old 12th January 2018, 03:36 PM   #6
Tyimo is offline Tyimo  Hungary
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Why not?
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Old 12th January 2018, 05:03 PM   #7
Eli Duttman is offline Eli Duttman  United States
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Look at the 6C33C datasheet. Notice, the 200 Kohm grid to ground resistance limit, which may be for self biased arrangements. The type is extremely vulnerable to runaway. "Fixed" bias exacerbates the runaway character of tubes. If you employ combination bias, with a relatively small portion of the TOTAL bias voltage coming from a RC network, the operating point is stabilized.

Arrange your driver circuitry so that a 47 Kohm grid to gound resistor is not problematic and "stand" the cathode on a 100 Ω/470 μF. network. The RC network stabilizes the operating point and provides a convenient "idle" current test point.
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Old 13th January 2018, 05:32 AM   #8
PRR is online now PRR  United States
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Quote:
Originally Posted by Eli Duttman View Post
...200 Kohm grid to ground resistance limit...
It is a Pass Tube. In the intended application, the load it is powering is in-effect a giant cathode resistor. The pass tube can't flow more than the load will take. And typically a pass tube driver has large down-swing to throttle a runaway.

Considering that lesser tubes spec 50K in fix-bias, I agree this monster should not be near 200K except with mostly cathode bias.
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Old 13th January 2018, 11:31 AM   #9
Tyimo is offline Tyimo  Hungary
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Quote:
Arrange your driver circuitry so that a 47 Kohm grid to gound resistor is not problematic and "stand" the cathode on a 100 Ω/470 μF. network. The RC network stabilizes the operating point and provides a convenient "idle" current test point.
Thank you!
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