Hi Jeff,
I have a Supro S6420 that I recently had serviced by a local
tech. I brought in the amp because the bottom end sounded
like mush. It broke up at low volume, and for a 35-watt amp,
it was not very loud. In fact, my ’66 Princeton Reverb has a lot
more volume and headroom.
So, my tech replaced the filter caps, removed a non-original
output cathode bypass cap, checked all power and plate-load
resistors, checked the transformers, and adjusted the bias (VP
425, 45 mA bias). All preamp and output tubes are good and
the speaker is good. Still, it has no volume. My Princeton
blows it away. What’s causing this lack of power?
—Bob,
Townsend, Tennessee
Hi Bob,
Thanks for writing. Well, it
sounds like your very cool
Supro S6420 is not living up
to your expectations, and from
your explanation of the servicing—
most particularly removing
a non-stock output cathode
bypass cap—it seems likely that
its previous owner may have
felt the same way. However,
I’m not necessarily agreeing
there is something “wrong”
with the amp. If you plug in a
Tele, crank it up, and you suddenly
sound like the first Led
Zeppelin album, the amp may
indeed be functioning properly!
Comparing it to a Fender
Princeton Reverb is quite
honestly an apples-to-oranges
scenario, but let’s see if there
may be an explanation for your
findings and better yet, a fix.
Regarding the Fender having
a better bottom end, this may
be due to a couple of factors.
First: Most Fender amps, due
to the type and position of the
tone-stack circuitry, have a substantially
reduced mid content.
This results in the amps’ top
and bottom end being more
pronounced. Coupled with the
substantially more closed-back
design of the Fender cabinet,
this may very well explain why
the Princeton’s bottom end
sounds fuller.
Even though the Supro has
a 15" speaker, the fact that the
baffle board and cabinet are not
much larger than the speaker
itself and the rear of the cabinet
is virtually wide open, the enclosure
is basically a baffle with
sides. There’s not much low end
being developed by the enclosure
itself. If you’ve ever compared
a 10" speaker mounted
in a nice enclosure with a 15"
speaker mounted on a piece of
plywood, you’d know which one
would win the battle of the bass.
Another consideration would
be the speaker itself. Just because
it “works” or doesn’t have any
buzzing or tears in the cone,
doesn’t necessarily mean it sounds
good. Speakers get old and tired,
and I can’t tell you how much
a new speaker can completely
change an amp for the better. A
worn-out speaker along with the
cabinet design could easily be
responsible for the fact that the
amp is not as loud as you believe
a 35-watt amp should be.
You mentioned that the
tubes are “good,” but didn’t
explain how you determined
this. Just because a tube
checks well, does not necessarily
mean that it sounds good.
Substitution is always the best
way to confirm how the tubes
are holding up, so if you haven’t
already done so, swap out the
tubes for a quick check.
You also mentioned that the
amp breaks up early and does
not have much headroom. This
too could be the result of a
tired speaker, but it could also
be design-related. Looking at
the schematic, I see the final
stage of the power supply—
which supplies power for the
first preamp stage—is fed by a
100k resistor. This is a far larger
resistor than is typically used in
such basic amps, and it would
substantially reduce the voltage
supplied to this stage. Though
this is great for producing
compression and overdrive, it
reduces headroom. Decreasing
the value of this resistor to 27k,
or even 10k, should increase the
headroom in this stage.
There is one other aspect
of the comparison to your
Princeton that I’ve not yet
addressed: The Princeton has
an extra gain stage in its basic
amplification section. By contrast,
this could easily make the
Supro sound anemic. The good
news is there’s an unused half
of a 12AX7 in this Supro that
you could press into service.
Your tech would know how to
turn this into a working gain
stage. Placing this extra stage
ahead of the existing first stage
would yield more gain and
saturation, while placing it after
the volume control would yield
more sensitivity.
If you’re not willing to go
that far at this point, there are
a couple of quick changes you
can implement to make the amp
more “full sounding.” Just be
aware that the fuller the amp
becomes, the more difficult it
may be for it to stay clean at
higher volumes. If you’re willing
to give it a try, I’d suggest changing
the input cap and resistor
of the first gain stage. These are
located immediately after the
input-jack resistors. Currently,
the capacitor has a .005 μF value
and the resistor measures 270k
Ω. Try changing the cap to a .01
μF or even a .047 μF, and the
resistor to a 1M Ω. This should
give the amp a fuller response,
and the mod is easily reversible if
you don’t like the result.
If all this fails to bring the
amp to what you believe is an
acceptable performance level,
I’d recommend a quick substitution
of a similar output
transformer. Again, something
may appear to be good, but
a quick, temporary substitution
is the best way to know
for sure. I hope you get your
Thunderbolt thundering!
Jeff Bober is one of
the godfathers of the
low-wattage amp revolution,
co-founded and was
the principal designer for
Budda Amplification. Jeff recently launched EAST
Amplification, and he can be reached at
pgampman@gmail.com.