From SRSN81–(at)–rodigy.com Sat Oct 7 21:13:16 CDT 1995
Article: 4233 of alt.guitar.amps
Path: geraldo.cc.utexas.edu!cs.utexas.edu!howland.reston.ans.net!newsfeed.internetmci.com!news.sprintlink.net!in1.uu.net!prodigy.com!usenet
From: SRSN81–(at)–rodigy.com (Joseph Pampel)
Newsgroups: alt.guitar.amps
Subject: Re: Info on Fender Pro Reverb wanted
Date: 7 Oct 1995 04:01:46 GMT
Organization: Prodigy Services Company 1-800-PRODIGY
Lines: 58
Distribution: world
Message-ID: <454u3a$1q7--(at)--senetw1.news.prodigy.com>
References: <44hefu$sl--(at)--anix3.panix.com> <44kb3f$h5--(at)--arcia.efn.org> <44m8oq$ns--(at)--anix3.panix.com> <44pctt$3p--(at)--nramp.arc.nasa.gov>
NNTP-Posting-Host: inugap5.news.prodigy.com
X-Newsreader: Version 1.2
selkir–(at)–elsci.arc.nasa.gov (Rennie Selkirk) wrote:
2. Certainly the above brown amps, and I believe the brown amps in
general,
> had lower plate voltages in the preamps, usually under 200 V, but
with
> 7025 phase inverters with fairly high plate voltages, up around 300 V
> or more. In contrast the AA and AB763 amps of the same name had 200+
> voltages in the preamps and 12AT7 inverters, with voltages under 250.
>
> To an amateur like me, the higher preamp voltages in the black amps
> spell something like cleaner and twangier. Maybe also richer. (Is
that
> the “spank” you’re talking about?) Certainly not “browner!”
>
The brown schematics show lower plate volts on the pre-amp, but in
practice (and esp with 120V coming out of our modern sokets) the pre-amps
are hit with 200+ volts at the plate. The Phase inverter is always the
highest pre-amp plate voltage becuase you want the largest voltage swing
to drive the output tubes. The bias was running about -52 or so back then,
which means that a 104V p-p signal would drive for full power (assuming
everything else could keep up..) The PI would have more than 300V on the
plates, but it must be remembered that the cathode is biased up to about
40V on the Brown amps, so the true plate volts are 40V (or so) lower than
they appear when read at the plate. All else equal, higher voltages
sound brighter and punchier. The Brown amps actually run about the same
voltages (in general) as the BF amps.. The output plates are at 460VDC
quiesent. One major difference I’ve noticed in goofing around with amps
is that the brown amps have much lower gain pre-amps in general (check
the split plate loads in the Vibroverb and 6G12-A Concert for example..)
and then have a higher gain phase inverter to pick up the gain in the
power amp. (using a 6.8K “tail” resistor where the BF amps are running
27K or so, much lower gain.)
> Don’t know what the difference the 12AT7 design makes, though I know
> that the sound of my black Tremolux gets somewhat muddier when I
replace
> the 12AT7 with a 12AX7.
>
The 12AT7 was subbed for the 7025 for its lower gain, lower rp and its
ability to deliver power to drive the outputs. When the output grids
draw current (as grid potential approaches zero) you need to provide
power from the driver stage to keep the waveform together. That’s also
part of what those series grid resistors are about. They are for stopping
osc, but also they develop extra bias when the output tubes draw rid
current. In Hiwatt circuits, they use a 10K?(20K? I forget..) in this
position to help keep things loud and clean.
Joe P.
