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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Sun Jan 01, 2012 5:45 pm
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:oops: You're right, the PI is the other half. I have the same schematic, just that I mixed up the halves. And they are even aimed at each other, DOH! Sorry about that.

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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Mon Jan 02, 2012 12:31 am
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Shimmy, No Problem. Kinda unusual to see so many functions broken up between the two triode halves. Is that 1/2-12AX7 to the left of the phase inverter half, part of an old school cathodyne PI circuit? Thanks!

http://www.freewebs.com/valvewizard/cathodyne.html


http://www.webphix.com/schematic%20heav ... aa1164.pdf


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Mon Jan 02, 2012 7:48 am
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The PI itself would be the concertina/cathodyne, using only half of the tube. The half of a tube that is to the left of the PI is just a gain stage where feedback is tied in. The long-tailed PI seen in many other amps would use both halves of a tube.

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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Mon Jan 02, 2012 10:30 am
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Shimmy, Thanks! I noticed that on many this is the only push-pull amp that uses this arrangement. And have seen mods on the Net to use a full 12AX7 as the PI, in the classic long-tail arrangement. But I'm guessing this would NOT work if the PI of the PR is truly cathodyne. Not without modifying that first half.

Could explain some crazy overdrive seen in "hotrodded Princetons." ???

http://www.webphix.com/schematic%20heav ... _schem.pdf


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