From: META000 <META@UNB.CA>
Subject: RE: Q: Bassman tube amp mods?
Organization: The University of New Brunswick
Date: Wed, 12 Aug 1992 21:40:11 GMT

In article <1992Aug12.182352.15670@bnrmtl.bnr.ca> barsz@bmtlh168.BNR.CA (Peter Barszczewski) writes:
>I am modifying an old Bassman (Jan'63) for a guitarist friend of mine.
>He wants an overdrive channel (like Boogie or Marshall), and a master
>volume control.  I have a couple of ideas on how to do this (first go
>to EL34s instead of 6L6s), but I'm sure someone has done this before
>(like Randall Smith ;-).  Does anyone know of a good source of
>information for hot-rodding old Bassmans?  I have Pittman's classic
>Tube Amp Book, but it doesn't have any Boogie schematics.
>
>Basically we want a nice tube distortion out of a cheap (money-wise
>cheap, not quality-wise) tube Bassman.
>--
>Peter A. Barszczewski         |           .   .   .   .   .   .   .
>(barsz@bnr.ca)                |        :    :    :    :    :    :    :
>Bell-Northern Research, Ltd.  |     3     3     3     3     3     3     3
>Montreal, Canada              |  D      D      D      D      D      D      D
>.
three suggestions:

(1) yes, you may switch tubes to the "marshall variety", but you will
have to adjust (or have adjusted) the bias voltage control inside the
amp.  you will notice a change in sound, but this will not, by itself,
result in more crunch, merely a different sort of crunch when you get
crunch.
(2) if you look in the pittman book, you will see that many tube amps
have a resistor (anywhere between 4.7k to 100k) that connects the
speaker side of the output transformer to an earlier driver stage or to
the power stage itself.  this resistor provides negative feedback in the
amp to keep distortion at bay and provide a clean sound at most volume
settings.  if you lift this resistor, you should find that, although,
overall, there is not more distortion, there is a bit more "edge" to the
sound at lower volume settings (due to a little taste of harmonic
distortion).  i've done this to my 59 princeton, and i like it.(3) if
you look at the schematic of virtually any fender made between 59 and
1968 or so, you will see that the tone controls ultimately end up being
tied to ground.  in amps without a midrange control the bass control is
tied to ground through a 6.8k resistor, and in amps with a
midrange control, this resistor is simply replaced with a 10k pot.  if
you lift the ground connection from the resistor coming off your bass
control on the bassman, you will find that none of the tone controls
function anymore, but there is an added boost to the volume.  since the
fender tone controls work by selectively shunting off certain
frequencies to ground, then disabling them sends more of the signal
through to the volume control and subsequent stages.  i did this to an
old tremolux head, and installed a tone bypass switch along with the
tremolo on-off switch in a footpedal.  the "flat" sound is not to
everyone's liking, but it is reasonably good if you like the
unadulterated sound of your guitar, or if you have an eq box to doctor
it up a bit.the boost is not immense, but an extra 3db here and there
helps by the time it gets to the speakers.
(4) you can install a master volume control in a bassman or similar
design amp very easily.  i did this to the tremolux, and got a pleasing
overdrive not unlike an older hiwatt i played once.  a nice throaty
grunt.