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Divide and Conquer

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4/14/2003 6:52 PM

Divide and Conquer

You have two different channels with two independent tone stacks. Does this "brittleness" sound the same on both sides when the eq settings are set to the same levels? If it does, then it's not in your tone stack or eq.

Pull your reverb tubes... Still the same? Then it's not in your reverb send/return circuitry.

Pull your tremolo tube... Still the same? Then it's not in your Tremolo ciruitry.

Got new speakers in there...? (Good chance this is your culprit). Then plug the amp into another cab with worn in speakers. If the brittleness goes away, then you know your speakers need some break in time.

Running a GZ34 as the rectifier? This'll pump up the treble. Try rebiasing with a 5U4GB and see if this is the culprit.

Run a known good 12AX7 down the line one position at a time. See if you have a crappy tube in there that's corrupting the signal path.

If all that fails, get your AB763 layout and start checking all the component values start to finish. There's always the possibility that an incorrect value component has found it's way in there either from the crappy QA of the original CBS era construction or done later on by a tech that wasn't paying close attention during a routine repair.

All things being equal, there's nothing magic about the AB763 design that would make one amp noticeably more trebly than another.

4/15/2003 6:15 PM

OK guys, awesome input. I forgot to mention that the amp does sound much better with my old 12's from my BM, not as bright, so the new P10R's are definitely contributing to the problem. I don't have an option changing these, though, so the user will have to just live with 'em / break 'em in. But, to my ears, the amp still doesn't sound quite right even with my favorite 12's.

Yes, both channels have the same excessive treble. I realized some time ago that the problem would likely be somewhere between the 220K mixing resistors (at the PI input) and the speaker jack. I guess this would also imply that the reverb section wouldn't be the culprit either, although the reverb channel does sound a bit thinner than the normal channel.

I've been thru the whole amp, all parts are tested and within spec. Tried the 12AX7 tube replacement as well.

The GZ34 (5AR4 in this case) is a very, very interesting point. I did replace the original 5U4GB with a new 5AR4, trying to get a bit more B+ (it was low, someone's apparently replaced the PT). Does the 5AR4 really pump up the treble.....? THis I gotta investigate. The problem I gave myself here is that I've previously put in 100uF Atom filter caps in the power supply, which are fine for the 5AR4, but a tad much for the 5U4. Has anyone run 100uF filter caps for extended periods on a 5U4 without shorting it out ? Kinda concerned about taking the chance...

Thanks again,
Jack

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