From blit–(at)–lash.net Sat Oct 3 13:04:21 CDT 1998
Article: 130128 of alt.guitar.amps
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From: Danny Russell <*NOSPAM*blit--(at)--lash.net>
Newsgroups: alt.guitar.amps
Subject: Re: 2 vs 3 spring reverbs ?
Date: Sat, 03 Oct 1998 09:33:53 -0700
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Tremolux wrote:
>
> > The only available change is to
> >raise the 470 ohm shared Rk to 1k or 1.5k (which it probably should be
> >anyway)
>
> Huh? 470?? The cathode bias resistor on the 12AT7 in my amps is 2.2k! That’s
> the BF value.
>
> > then bypass with .22 or .47.
>
> OK, that will reduce the drive at low frequencies.

Yeah, some of the blackface amps did use 2.2k bypassed with a 25/25, but
not universally though. Actually on the ones with those values, lotsa
times you’ll get that snapping, crackling oscillation from the driving
12AT7 when the amp is cranked up. Sounds similar to the O.T. arcing
over or on it’s way out. I believe that was the reason for the 560pf
ceramic connected from plate to cathode on later silverface, supertwin,
75 etc. (frequency-bubble band-aid.) Re-arranging the order of the
pin-jumpers cures the problem better (as incorrect order is the source
of the problem). Fender has the cathodes tied across first closest to
the socket, next the grids are tied across above the jumped cathodes.
Lastly the plates are tied together around the outside. The cathode and
grid jumpers need to be reversed. (grids together at the bottom, and the
cathodes jumpered together above them.) The plates are O.K. around the
outside. -Danny