ASK AMP MAN
Souping Up a Bassman 10
JEFF BOBER
Hi Jeff,
I was recently given an early-’70s Fender
Bassman 10 by a dear friend of mine who
said he couldn’t get it to overdrive. I told
him it was never designed to overdrive—it
was built to stay clean. He said he’d lose
money trying to sell it and he’d rather give
it to someone who would use it.
I’m definitely using the amp, after several
tube changes. It’s very sweet, but it’s just a
little too bassy. The stock speakers were shot
when my friend got it, and he replaced them
with some off-brand 200-watt speakers.
I don’t want to make a radical change in
tone, I just want to add some highs without losing the low-end “thunk.” I love the
percussiveness of the strong low end, but
I need more cut on top. My idea is to mod
the tone controls on one set of inputs to be
more trebly and leave the bass side alone. I
usually run two cables (left and right) from
my pedalboard—one to each of the amp’s
channels. My warped logic is, if I can make
one channel brighter, I can blend the two
channels for a (hopefully) perfect mix.
Any suggestions? Warnings? Am I wasting my time futzing with the tone controls
when I should be saving up to replace the
speakers again? I’m open to suggestions.
Having played semi-pro (usually got paid,
though never made a living at it) for the last
43 years, I’ve played through many old amps
of various flavors. And I always enjoy your
articles, especially your takes on amps I used
to own. Everybody has that “Holy Grail”
sound in their head, so thank you for helping
me and so many others in our quest!
Kirby
Thanks for reading and thanks for writing.
I’m glad the column is helping you in your
quest for the Holy Grail of tone. Speaking of
the Holy Grail of tone, you actually have in
your possession a very good candidate for
modification. With a little effort, your amp
can approach one of my Holy Grails of tone,
the Fender Super Reverb. There are many
similarities between the two amps, and with
a few simple component changes you could
have a great-sounding amp on your hands.
Also, an added benefit of this particular
amp over a Super Reverb is that you can
definitely run a signal to both the Normal
and Studio channels and blend them. This is
possible because both channels in this amp
are in phase, as opposed to those in the
Super Reverb (and most other blackface and
silverface Fender amps), which are out of
phase due to the additional reverb circuit on
the second channel. I’m going to focus on
leaving the Normal channel intact and just
modifying the Studio channel.
As always, this work is very dangerous—even
lethal. So if you are not familiar with the
inner workings of a tube amplifier and the
possible hazards involved, please have this
work performed by someone who is.
Getting Down and Dirty
Now let’s get started. First, locate the capacitors in the tone stack of the Studio channel.
They should consist of a 0.01 uF, a 0.047 uF,
and a 500 pF capacitor all grouped together.
In order to replicate the tone stack of a
Super Reverb, you will need to remove the
0.047 uF cap and replace it with a 0.022 uF
600V capacitor. You will also need to remove
the 500 pF cap and replace it with a 220 pF
or 250 pF (1kV or 500V) ceramic capacitor.
These changes will place the mid and treble
frequencies more in the guitar range.
Next, you’ll need to replace the capacitor
located on the Studio channel’s Bright switch.
Some schematics list this cap as either a 220
pF or a 330 pF, but you’ll want to remove
whatever the existing cap is and replace it
with a 120 pF ceramic cap. This will give you
a more subtle bright boost that is focused in
the upper frequencies of the guitar range.
Another change you can make is to locate the
0.0047 capacitor that connects the Treble pot
wiper to the CW leg of the Volume pot. This
cap is limiting some of the Studio channel’s
frequency range, and removing it will give the
channel additional punch.
Finally, we need to open up the response
and feel of the output stage a bit. To do this,
you’ll need to change the grid and plate
resistors in the phase inverter. Locate the two
330k resistors that are connected to the grids
(pins 2 and 7) of the phase inverter tube, V3
(12AT7). Remove these and replace them each
with a 1M (meg) resistor. Next, locate the two
47k resistors that are connected to the plates
(pins 1 and 6) of V3. Remove these and, in
their place, install an 82k resistor in the position connected to pin 1, and a 100k in the
position connected to pin 6. Half-watt carbon
film or carbon-comp resistors are fine for these
changes, but I generally use 1-watt resistors in
my repairs, as well as in all production amps.
Making a Case for New Speakers
Those are really the only modifications necessary to bring the circuit into Super Reverb
territory. Of course, it will not be exactly the
same as a Super Reverb, seeing as how there
are other items that are not the same—such
as the output transformer and rectifier—but it
should be very close. In order to get the most
out of your new acquisition, there is one other
item I’d recommend checking: the speakers.
You mentioned that the original speakers were
bad and that they had been replaced with
some “off-brand 200-watt speakers.” This could
definitely be an issue. My guess is that these
speakers may not even be musical instrument
speakers. These amps originally had 32-ohm
speakers that were all wired in parallel, for a
total impedance load of 8 ohms. In the musical-instrument world, 32-ohm speakers are almost
unheard of. The only 32-ohm speakers I know
of were installed in early Ampeg SVT 8x10
speaker cabinets, and they were definitely not
200-watt speakers. Sounds like you may have
some home or car audio units inhabiting your
vintage prize. I’d recommend replacing them
with decent 10" guitar speakers, of which there
are far too many to mention. Just be sure that
you install 8-ohm speakers and have them
wired in series/parallel so the resulting total
impedance remains at 8 ohms.
Now have fun with your new Super
Bassman 10!
Jeff Bober
Jeff Bober, one of the godfathers of the low-wattage amp
revolution, co-founded and was the principal designer
for Budda Amplification. Jeff has just launched EAST
Amplification. He can be reached at pgampman@gmail.com
