TECH TIPS > ASK AMP MAN
ROCKIN’ A BASSMAN 50 BY JEFF BOBER
Jeff,
In the December issue, I read about the mods you can make
to a Fender Bassman 10 [“Souping Up a Bassman 10,” Dec.
2010 PG]. I’d like to know if those mods could also apply
to an early ’70s Bassman 50? The amp has already been re-tubed and re-capped (including filter caps), but all the tone
is focused in the mid frequencies. It lacks the typical Fender
high-end and low-end. Can you help? —Dennis
Hi Dennis,
Glad you enjoyed the column
on the Bassman 10. Just FYI—
and to anyone else that is considering those modifications—
we published a correction for
that column with regards to a
.0047 µF capacitor. The corrected instructions should read:
Another change you can make
is to locate the .0047 µF 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. Removing it
and replacing it with a short wire
will give the channel additional
punch. So, with that out of the
way, let’s get to your question.
After an extensive and frustrating search, I have to tell you
that a schematic for the amp
sold as the Fender Bassman
50 does not appear to exist.
While there are schematics for
Fender Tweed-era Bassmans,
Bassman 10s, Bassman 70s,
Bassman 100s, Bassman 135s,
12-watt bass amps, 200-watt
bass amps, and even Fender’s
behemoth 300-watt bass
amps—a schematic specifically
labeled “Bassman 50” is simply
nowhere to be found.
But after seeing a picture of
a Bassman 50, I can surmise
that it most closely relates to a
standard-style Bassman of the
’60s and ’70s. Since there are at
least five or six schematics that
encompass this era, it’s impossible to tell which might be the
appropriate schematic for your
amplifier. The best advice I can
48 PREMIER GUITAR MAY 2011
give you is to go through the
modifications in my Bassman 10
column, and see which are applicable to your particular amp.
Even though I believe most
guitarists who play through
Fender Bassmans do so specifically because they don’t sound
and respond like a typical
Fender guitar amp, there are a
few more suggestions I can give
you regarding ’60s and ’70s
Bassmans that may make them
to your liking.
As always, this work is very
dangerous—it can even be 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.
First, locate the 100k plate
resistors on pin 1 of the first
two preamp tubes (V1 and
V2). These resistors may have a
capacitor in parallel (mounted
across them). The Normal channel may have a 500 pF and the
Bass channel may have a 0.01
µF capacitor. Removing these
caps will immediately brighten
up their respective channels.
Now locate the components
attached to pin 6 of V1 and
V2. Each should have a 100k
plate resistor attached to them.
You can leave these alone. Also
attached to pin 6 on most
models will be another resistor.
These are the channel mixing
resistors. In most cases, these
will both be 220 kΩ resistors,
and they can also be left alone.
If the resistor associated with
the Normal channel is a 470
A Bassman 50 rig as pictured on page 52 of Fender’s 1972 catalog.
Photo courtesy of FMIC and vintageguitars.org.uk.
kΩ, change it to a 220 kΩ.
This will give that channel a
bit more punch and fullness.
By the way, in most of
these Bassman-style heads,
both channels will be in phase,
which means that you can run
your guitar signal into both
channels simultaneously—
and blend the two together
for the best overall tone and
response. If, however, the mixing component from pin 6 of
the Bass channel is a capacitor,
the channels are out of phase
in this model, and you’ll get
phase cancellation at various
frequencies if the two channels
are combined.
Now let’s move a bit further
down the line to the phase
inverter. Most amps will have a
resistor and one or two 0.01 µF
or 0.1 µF capacitors connected
to pin 2 of V4. If your amp is
like this, it’s fine and can be
left alone. If your model has a
500 pF cap connected to pin
2, this should be changed to a
capacitor in the 0.01–0.1 µF
range. This will give the amp a
considerable increase in fullness
and body.
This same area is where
the negative feedback loop is
employed. There is a lead coming from the tip terminal of the
output jack. Follow it to the
first resistor it is connected to.
If it is connected to an 820 Ω
resistor, there is nothing more
to be done. If it is connected to
a 47k resistor, in most models
this resistor will have a 100 pF
capacitor in parallel with it.
This cap is adding additional
high frequencies to the negative feedback signal. Removing
this 100 pF cap will bring just
a tad more brightness back to
the amp.
There you have it. These
changes, along with any applicable changes from the “Souping
up a Bassman 10” column should
give you a pretty good sounding
Bassman 50. Hey, it will probably sound better—after all, it’s
40 more than 10! Enjoy.
JEFF BOBER, one of
the godfathers of the
low-wattage amp revolution, co-founded and was
the principal designer
for Budda Amplification.
Jeff launched EAST
Amplification in 2010, and he can be
reached at pgampman@gmail.com.
premierguitar.com
