ASKAMPMAN
MarshMan!
Jeff,
I have a ‘ 66 Bassman head and I’d
like to modify the Bass channel for
smooth overdrive tones. The Normal
channel is what I gig with, going from
a Strat or Tele to my effects then to
the Bassman, but I’d like to explore
some natural amp overdrive and keep
the Normal channel set for maximum
clean headroom.
I’ve seen some so-called Dumble
schematics on the internet, but I don’t
really want to turn the whole head into
a Dumble clone; I just want to incorporate a few modifications to get a nice,
smooth overdrive.
Can you help me accomplish this?
Thanks for your consideration,
Todd Lynch
Hi Todd,
Not only do you have one of the nicest
amps around for straight-ahead rock n’ roll
guitar tones, but the chassis also holds a
secret weapon for generating some great
overdrive tones. The tone that comes from
the Normal channel of a Bassman head
connected to a 4x12 cabinet is classic rock
in its purest form, but you should definitely
be able to kick the Bass channel up a
notch. The reason? The amp already has an
extra stage of gain that is unused – there
is one half of a 12AX7 preamp tube sitting
there, waiting for our bidding.
While I’m not going to provide you with
the exact schematic of the proposed
modification, I will try to give you enough
information to take to an experienced tech
so they will be able to modify the amp for
your desired results. In order to do this,
they will need a copy of the Bassman’s
schematic, along with a copy of the schematic for a standard Marshall 2203 or 2204
JEFF BOBER
preamp. If dealing with an experienced
tech, odds are they will already have these
in their schematic collection. For the sake
of simplification, I’ll refer to the Marshall
schematic as “the schematic.”
First, the number two input jack on the
Bass channel should be removed and
replaced with a 1M-A pot. This will be
used as your preamp volume control. Next,
have him wire the output of the first gain
stage to the aforementioned preamp volume control using the appropriate coupling
caps and resistors shown in the schematic.
Disregard the “low” input jack on the schematic as it is just used in this instance as
a signal pass thru. From there, the signal
should be sent to the input of the unused
half of the available 12AX7 and the appropriate components installed to replicate
the second gain stage of the schematic.
The output of this stage would then go
back over to the second half of the initial
12AX7, utilizing the appropriate circuitry per
the schematic. The output of that second
half should then be connected to the tone
controls and original Volume control, which
will now act as your Master Volume. Just
to clarify things, the last part of the schematic will be different at this point. Since
there is no fourth preamp section available
in the Fender chassis, the tone stack we
will end up with is plate-driven, as opposed
to a cathode-driven stack. Other than that,
you pretty much have a Bassman with
the front end of a 2203 Master Volume
Marshall in place of the Bass channel.
I’d like to add a few suggestions for your
tech. Remember to change the value of
the caps in the tone stack from 0.1uf to
0.022uf and remove the unused deep cap.
If you want to get creative, since there’s no
midrange control, you can use the deep
switch to parallel a resistor across the
existing midrange resistor, thereby converting the deep switch to a mid shift switch.
Feel free to experiment with the values
of the resistors, allowing you to adjust the
midrange to taste. You can also change the
bass pot from a 250K to a 1M for more
bass range.
Want to really go over the top? Instead of
using it as a mid-shift switch, repurpose
the deep switch, using it to lift one end
of the midrange resistor. It will now be
a tone defeat switch – once engaged,
the tone stack is defeated and the gain
increases dramatically. I would leave off
the .001 cap across the preamp volume
control; I find it to be too brittle at lower
settings. If the modified channel is still too
bright, remove the 470pf cap that parallels
the 470K resistor prior to the preamp volume control. If the channel is too woofy
at higher gain settings, change the value
of the first cathode bypass cap from 25uf
to something substantially smaller, possibly a 0.68 or 1.0uf.
One last suggestion would be to consider
using shielded cable on the long signal
runs that go to and from the additional
tube and preamp circuitry to help eliminate
the possibility of any internal feedback or
oscillation. Shielding the signal from the
input jack isn’t a bad idea either.
There you have it. With a little personal
tweaking by both you and your tech, you
should be able to get a plethora of tones
from your new MarshMan.
Jeff Bober
Co-Founder and Senior Design Engineer – Budda Amplification
jeffb@budda.com or www.budda.com
©2007 Jeff Bober
