60 PREMIER GUITAR NOVEMBER 2009 www.premierguitar.com
JEFF BOBER
Jeff Bober
Jeff Bober, Godfather of the low wattage amp
revolution, co-founded and was the principal
designer for Budda Amplification. He can be
reached at pgampman@gmail.com.
ASK AMP MAN
Making a Supro Super
Hi Jeff, I have a question for you. Recently,
I was given an old Supro amp. As far as
I can tell it’s a 1948 model, all-original
except for the power cord, which I
replaced. There are no model numbers on
the amp. Of course, it’s an all-tube amp,
with what appears to be a 10" speaker,
two inputs, an on/off toggle switch and
a 2-amp fuse. When I turn on the amp, it
fires fine and sounds pretty good. After it’s
been on for about 10 minutes, it starts to
lose a little volume, but the sound just gets
better … sort of a nice fuzzy, bluesy sound.
I noticed last night that the speaker frame
was getting hot (after about an hour of
playing). Is this normal for an old amp like
this? Is it okay to use it without fear of it
“blowing up?” The amp is in excellent condition except for the handle. Any idea what
the value of this amp might be? Also, I was
thinking of replacing the transformer with
one of the new ones on the market. Would
it be worth doing? Thanks for your time,
and I hope you have a great day. Love your
articles in Premier Guitar.
Jim Edens
Hi Jim,
I don’t know that I’ve ever worked on that
particular model amp, but from the research
I’ve done it looks to be a very cool amp. It
seems that there are quite a few different
versions of the amp, but the common thread
appears to be a bottom-mounted chassis
with inputs and controls accessible from the
lower rear. The speaker is mounted above
the chassis and could have been either a
Jensen or a Rola, the latter of which came in
PM (permanent magnet) or field coil versions.
I’d have to assume that the field coil version
would have been the earliest version of the
amp, as the field coil speaker predated the
PM speaker. (Just FYI, in a field coil speaker,
the magnetism needed for the operation of
the speaker was generated by running the
B+ (high voltage) of the amp through a coil
on the rear of the speaker, essentially turn-
ing it into a large electromagnet.) The tube
configuration in these amps seems to change
as well. While all of the versions utilize a 5Y3
rectifier tube, the preamp section consisted
of either a single 6SL7, or a 6SC7 along with
a 6J7. Also, from the information I’ve seen it’s
possible that the amps were built using either
a single 6V6 for the output stage, or two
6V6s in a parallel single-ended design. That’s
it for the history of the amps. Now let’s get
to your questions.
You mention that the amp loses some vol-
ume and starts to sound a bit fuzzy/bluesy
after approximately 10 minutes of opera-
tion, and that the speaker frame was getting
hot. Remember that the speaker is mounted
directly above the entire chassis. On that
chassis are the mains transformer and any-
where from three to five tubes, with an
output stage running in Class A mode. This
in and of itself is probably enough heat to
warm the speaker frame, but excessive heat
could be the tell-tale sign of a problem. The
symptom of the amp dropping in power and
becoming “dirtier” is a somewhat typical sign
of an output tube that’s shorting internally
once it gets nice and warm. This produces a
substantial amount of extra heat that would
add to the warmth of the speaker frame.
There may be, however, an additional cause.
If the speaker is of the field coil variety, the
excessive current that the malfunctioning
tube is consuming is being pulled straight
through the coil windings on the rear of the
speaker. This may not only cause the coil to
produce additional heat, but could be bad
for the longevity of the coil as well. It can also
cause the mains transformer to run warmer
than it typically would. With that diagnosis
in mind, I would suggest replacing the 6V6
output tube(s) and see if the performance of
the amp is improved. If it continues to lose
power and become buzzy, I’d have it looked
at by a local tech to see if the filter caps need
to be changed. The only reason I wouldn’t
tell you to just have them replaced is that you
mentioned the amp was almost 100 percent
original. Since it’s functioning rather well,
replacing the caps may reduce the value of
the amp on the vintage market—should you,
of course, decide to sell it.
Which brings me to your question regarding the replacement of transformer(s). While
there appears to be replacement transformers
made for these amps, and installing one could
improve the sound of the amp, the vintage
value of the amp would be compromised. The
decision should be based on tone vs. value.
If you plan on keeping the amp and making
it the best it can be, I’d say go ahead and try
it. In my opinion, you’d probably get a bigger
bang for your buck by replacing the speaker—
almost always the weak link in older amps.
Just remember, if the speaker is the field coil
type, you’ll need to install a choke to take the
place of the magnet winding, as it’s an integral
part of the power supply. I hope this will help
you make your Supro amp super.
While there appears to
be replacement trans-
formers made for these
amps, and installing one
could improve the sound
of the amp, the vintage
value of the amp would
be compromised
