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Old 19th April 2009, 07:08 PM   #1
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Default SE 7868 Guitar Amp - some issues...

I've put together an SE guitar amp from some junk parts i had around and, for the most part, it seems to work pretty good. The amp sounds great except for this distortion I'm getting. Instead of going from clean to a slight overdrive, it goes from clean straight into this nasty, buzzy, raw heavy distortion... It just sounds very bad and unnatural... I'm hoping you guys can point out something I'm missing here.

I'm running it at a 230Vdc B+, very low power, with a 6.6K primary load. The OT is a 35W unit intended for P-P operation... It's my understanding that P-P output transformers can work for SE if they are over sized enough. The primary CT is left unconnected.

Here is the schematic:

Since the drawing I've changed the 1K screen resistor to a 3.2K. It's just a Fender 5E1 preamp with a 7868 output stage.

plate = 225Vdc
cathode = ~7.4V
screen = 217Vdc
heaters = 6.4Vac
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Old 19th April 2009, 08:53 PM   #2
PRNDL is offline PRNDL  United States
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In my experience, the 7868 is very sensitive to bias.
I set the bias initially to what was shown on other schematics, but it had a strange distortion similar to what you describe. I fine tuned it by ear while playing guitar, and once set correctly it sounds great.

I built something similar to an Ampeg Portaflex with PP 7868's, which I call The Tank.
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Old 28th April 2009, 07:33 PM   #3
Zibi is offline Zibi  Poland
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Location: Bialystok
Use Ra=3k output transformer with air gap. Change R9 value to increase quiescent current.
Click the image to open in full size.
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