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Old 27th July 2007, 10:41 PM   #1
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Default Fixed Bias 6BQ5?

Hi! I cannot seem to find any schematics for a hi-fi fixed bias EL84 amp! Sure there are plenty of guitar amp schematics out there... mainly claiming 20 watts output.


I am wanting to build a ST-35, but if I can find a fixed bias circuit for a little more output I will do that instead.

Maybe into 7189 territory here?
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Old 27th July 2007, 11:50 PM   #2
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Max. plate voltage is higher for the 7189, but the max. plate dissipation is the same as the 6BQ5/EL84.

The Scott 299A and 299B used "fixed" bias on the 6BQ5s. Scott 299B schematic here.

Starting with the Scott design, you could do something quite good. The Russian 6p14p-ev/EL84M is a true 7189 equivalent. Run 7189s at highest allowed anode voltage in pentode mode. Regulate screen grid B+ with a 2X 0B2 stack. That will be quite linear, on its own. Loop NFB will take care of the damping factor issue.
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Old 30th July 2007, 03:50 AM   #3
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hey thanks! I know the 7189 is not capable of higer power, but it can handle higher voltages better.


I wonder if I could use the same ST35 schematic with added fixed bias? Does the PI have enough drive?

BTW, instead of OB2's couldn't I just use a solid state regulator? I mean, tubes would be cool but I am trying to keep the parts to a minimum.
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Old 19th August 2007, 03:41 AM   #4
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The Knight KN734 and KN737 both ran EL84 in fixed bias.
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