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Update on DIY 7591 PP bias cap issue!
Update on DIY 7591 PP bias cap issue!
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Old 11th January 2006, 02:20 PM   #1
mr mojo is offline mr mojo  United States
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Join Date: Jun 2005
Location: Iowa
Default Update on DIY 7591 PP bias cap issue!

Howdy folks!

A couple weeks back I was having some issues with the coupling caps used in my PS to develop my bias supply.

Many thanks to all the members who so graciously helped me out!

I replaced the Sprague Orange Drops (600VDC/300VAC rating) which were in series with the 400VAC HV secondary and eventually shorting-causing my bias voltage to plummet and my plate voltage to soar-with some Cornell Dublier 3000VDC/1000VAC poly caps.

After an hour, the caps are just barely warm to the touch, bias is rock solid. Schematic lists 475v on the plates of the 7591s at 33ma per tube, or .9v drop on the four cathode resistors to ground at the DC balance test points.

Now, at 33ma per tube, I'm at 474v on the plates-I'd say that's close enough, and the amp is finally working correctly for the very first time!

The sound is first-rate and I can't keep myself from continually checking DC and AC balance because I get such satisfaction out of getting the same exact reading over and over!

Thanks again for the help folks-this site and its' members ROCK!!!

mr mojo
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Old 11th January 2006, 11:22 PM   #2
SHiFTY is offline SHiFTY  New Zealand
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Congrats But you forgot the pics

I have half a dozen 7591s that will eventually become my guitar amp
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Old 11th January 2006, 11:32 PM   #3
anatech is online now anatech  Canada
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Join Date: Jun 2004
Location: Georgetown, On
Update on DIY 7591 PP bias cap issue!
Guitar amp? 7591's? okay, I normally see 6L6GC's or EL34's there.

I only cry 'cause my Eico ST-70 needs a set, which I will buy. And you are using them for a guitar amp. Easy to drive though.

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