Fixed Bias 6BQ5?

ThSpeakerDude88 · 27th July 2007, 09:41 PM

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?

Eli Duttman · 27th July 2007, 10:50 PM

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.

ThSpeakerDude88 · 30th July 2007, 02:50 AM

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.

Ty_Bower · 19th August 2007, 02:41 AM

The Knight KN734 and KN737 both ran EL84 in fixed bias.

27th July 2007, 09:41 PM	#1
ThSpeakerDude88 diyAudio Member Join Date: Jun 2005 Location: USA	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? __________________ always preach the gospel- and when necessary use words.

27th July 2007, 10:50 PM	#2
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	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. __________________ Eli D.

30th July 2007, 02:50 AM	#3
ThSpeakerDude88 diyAudio Member Join Date: Jun 2005 Location: USA	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. __________________ always preach the gospel- and when necessary use words.

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19th August 2007, 02:41 AM	#4
Ty_Bower diyAudio Member Join Date: Mar 2007 Location: Newark, DE	The Knight KN734 and KN737 both ran EL84 in fixed bias.

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