GOOSING A SUPRO THUNDERBOLT S6420 BY JEFF BOBER
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 sub-
stantially 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.
2
1. A beautiful 1966
Supro Thunderbolt
S6420. 2. The Thunderbolt’s cab is almost
completely open.
Photos courtesy of
amparchives.com
1
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, one of the
godfathers of the low-watt-age amp revolution, co-founded and was the principal designer for Budda
Amplification. Jeff launched
EAST Amplification in
2010, and he can be reached at pgamp-man@gmail.com.
