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Old 18th May 2010, 11:41 PM   #1
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Join Date: May 2010
Default Precision Electronics S-35 help,

[IMG]file:///C:/DOCUME%7E1/Owner/LOCALS%7E1/Temp/moz-screenshot.png[/IMG] Howdy. Sorry in advance for the long-winded post without an introduction.

I bought an old, non-functioning Precision Electronics S-35 PA as a guitar/bass conversion project. I was hoping it would be very similar to the Bogen CHB-35B. Thus far I've had a ton of fun on the project and learned a lot about electronics and tube amps (before we go further, yes, I'm discharging the things that can kill me before I go poking around-- just wanted to get that out of the way). In the future I'd really like to do more of this tinkering. Super fun. But I digress.

I've all ready got the amp up and running. I changed the input/output jacks over to 1/4" and replaced the rectifier, replaced the phase inverter tube, installed a three prong socket, removed the record-playery nonsense, removed most of the feedback circuit, and replaced the filter capacitors. It works fine now and produces a clean sound, but it's a bit bland, a bit mid-range heavy, and impossible to push into overdrive.

So I wanted to get more of a guitar-friendly sound out of it. I know from what I've read that having the master volume placed so early in the circuit isn't good... I've tried placing it after the EQ, but I get a LOT of hum... so that's no good. I'm very interested in changing the tone-stack over to the one pictured lower, removing some of the capacitors on the input side, and placing the master volume lower.

So I've discovered these two threads by a member of this forum; one where the poster tries and modifies the existing circuit and one where he (successfully) re-wires it entirely. I'm more interested in keeping the amp closer to stock.
Precision PA conversion help needed
7868 amp sounds good but low volume

I wanted to ask to see if this schematic he proposes is usable-- like I said, if I move anything around, I tend to get his results... quiet, scratchy distortion. I'm thinking his problem was a bad PI tube since his amp was nonfunctional to start with... my amp is working but my problem is a lack of knowledge.

So without further ado, would this work? Thanks so much for your guys' help. I've learned a ton just by lurking around on the forum.
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This is what I'm running currently:
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Old 20th May 2010, 03:37 PM   #2
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Join Date: May 2010
Just a bump before I tear out the 6EU7's and put in 12AX7's to make this: Just curious is that first schematic would work or would result in the weird, scratchy distortion.

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