Fixed bias PSU questions

Tyimo · 11th January 2018, 11:51 AM

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

kodabmx · 11th January 2018, 01:05 PM

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.

Eli Duttman · 11th January 2018, 01:12 PM

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".

Tyimo · 12th January 2018, 01:20 PM

Thank you!

goldenbeer · 12th January 2018, 02:49 PM

You should not operate 6C33C toobs in in pure fixed bias.

Tyimo · 12th January 2018, 03:36 PM

Why not?

Eli Duttman · 12th January 2018, 05:03 PM

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.

PRR · 13th January 2018, 05:32 AM

Quote:

Originally Posted by Eli Duttman

...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.

Tyimo · 13th January 2018, 11:31 AM

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!

11th January 2018, 11:51 AM	#1
Tyimo diyAudio Member Join Date: Nov 2002 Location: Hungary	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 __________________ "Replacing any coupling capacitors with a well designed and correctly matched driver or interstage transformer yields an almost shocking improvement in sound..." Peter Qvortrup

12th January 2018, 01:20 PM	#4
Tyimo diyAudio Member Join Date: Nov 2002 Location: Hungary	Thank you! __________________ "Replacing any coupling capacitors with a well designed and correctly matched driver or interstage transformer yields an almost shocking improvement in sound..." Peter Qvortrup

12th January 2018, 03:36 PM	#6
Tyimo diyAudio Member Join Date: Nov 2002 Location: Hungary	Why not? __________________ "Replacing any coupling capacitors with a well designed and correctly matched driver or interstage transformer yields an almost shocking improvement in sound..." Peter Qvortrup

12th January 2018, 05:03 PM	#7
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Monroe Township, NJ	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. __________________ Eli D.

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11th January 2018, 01:05 PM	#2
kodabmx diyAudio Member Join Date: Oct 2011 Location: Toronto	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.

12th January 2018, 02:49 PM	#5
goldenbeer diyAudio Member Join Date: Jan 2014	You should not operate 6C33C toobs in in pure fixed bias.

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