• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Fixed Bias 6BQ5?

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ThSpeakerDude88

Member
Joined 2005
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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Eli Duttman

Member
Joined 2004
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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ThSpeakerDude88

Member
Joined 2005
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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