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#1 |
diyAudio Member
Join Date: Nov 2002
Location: Hungary
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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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"Replacing any coupling capacitors with a well designed and correctly matched driver or interstage transformer yields an almost shocking improvement in sound..." Peter Qvortrup |
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#2 |
diyAudio Member
Join Date: Oct 2011
Location: Toronto
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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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#3 |
diyAudio Member
Join Date: Apr 2004
Location: Monroe Township, NJ
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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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Eli D. |
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#4 |
diyAudio Member
Join Date: Nov 2002
Location: Hungary
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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 |
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#5 |
diyAudio Member
Join Date: Jan 2014
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You should not operate 6C33C toobs in in pure fixed bias.
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#6 |
diyAudio Member
Join Date: Nov 2002
Location: Hungary
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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 |
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#7 |
diyAudio Member
Join Date: Apr 2004
Location: Monroe Township, NJ
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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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Eli D. |
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#8 |
diyAudio Member
Join Date: Jun 2003
Location: Maine USA
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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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#9 | |
diyAudio Member
Join Date: Nov 2002
Location: Hungary
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Quote:
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"Replacing any coupling capacitors with a well designed and correctly matched driver or interstage transformer yields an almost shocking improvement in sound..." Peter Qvortrup |
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