In last month’s column, John
Gilbert wrote in to ask about
restoring his mid- to late-1950s
Supro combo amplifier, and I
began describing different steps
he could take to get the amp up
and running. We discussed retubing,
and also replacing the line
cord and grounding capacitor. If
you missed that column or just
want to refresh your memory,
and give it a quick read (the photos
are pretty cool, too). Now let’s
continue with the overhaul, picking
up where we left off.
John, once you’ve added a
grounded plug as I outlined last
month, a good electrical cleaning
comes next. Spray each tube socket
one at a time with an electronic
contact cleaner that does not
any lubricant, and then repeatedly
cycle the tube in and out of its
socket (6-10 insertions and removals
are usually sufficient).
Once that’s done, use a similar
method to clean the input jacks,
but this time use an electronic
contact cleaner that does
lubricant. Again, move the plug
in and out of the jack to get rid
of any corrosion or dirt.
Now, if there are any openings
on the body of the front-panel
potentiometers, spray the lubricant
cleaner inside the pot and rotate
the control numerous times. This
is a good time to make sure all the
jacks, nuts, and screws are tight.
Tight hardware assures the safest
and quietest operation.
Filter capacitors are another
item to address. Most often, I
make the decision whether or
not to replace an amp’s filter caps
based on the condition of the
caps, how the amp sounds, how
the amp looks on the bench test
gear (various meters and scopes),
and whether a customer is concerned
about maintaining as
much originality as possible.
Because I can’t physically see
your Supro or play through it,
and your intention is to make it
a reliable and safe practice amp,
I’d recommend replacing these
capacitors. These will be the
largest capacitors inside the amp,
so they shouldn’t be too hard to
find. According to the schematics,
the amp should have either
three 10 μF 450V or two 10 μF
450V and one 40 μF 450V caps.
If these are all discrete components,
then any currently available
electrolytic filter capacitor
will yield more than adequate
results for this project. If these
are multi-section caps (that is,
with more than one capacitor
inside the enclosure), then
Antique Electronic Supply (tubesandmore.
com) will probably be
the place to source these.
Incidentally, if you need
multi-section caps but can’t seem
to find any to purchase, you can
also use individual capacitors in
place of the multi cap. Connect
the positive end of each cap to
the appropriate positive connection
of the original capacitor,
and remember to connect all the
negative connections to the same
place that the original capacitor’s
negative was connected.
Once you’ve done all this, you
should be ready to reassemble and
check out the amp. If the amp
powers up, has a decent output,
and is relatively noise free, then I
believe a declaration of “job well
done” is in order.
If the amp fires up but emits
background noise—such as lowlevel
popping or a sound like a
crackling bowl of Rice Krispies—
I’d suggest replacing the 6SL7 preamp
tube (if you haven’t already).
If the noises still persist with a
new tube, it’s time to start replacing
some of the only remaining
components in this basic amp.
I’d first concentrate on the
preamp-tube plate resistors.
These resistors might have a
value of 100k (brown-blackyellow)
or 220k (red-red-yellow)
or 270k (red-violet-yellow).
Replacing them with any carbon-
film or carbon-composition
resistor will be fine. I prefer to
use 1-watt resistors in tube amps,
but traditional 1/2-watt types
will work, if necessary.
While replacing these resistors,
I’d also suggest replacing
the associated signal caps. When
working on significantly old
equipment, it’s a good idea to
minimize the number of times
you remove and reattach components
on the terminals of the
aging tube sockets. Since this is
not a “keep everything original”
project, let’s do it all at once.
Reviewing the schematics, I
see these caps consistently seem
to be labeled 0.05 μF. Since a
0.05 μF cap is not a readily available
value, feel free to substitute
any quality 0.047 μF 600-630V
DC capacitor. Again, a Sprague
Orange Drop or similar poly-cap
will work fine.
If the amp still sounds
like breakfast, I’d replace the
preamp-tube cathode resistors.
These may be older values, such
as 2k (red-black-red), which are
not as readily available, but it
would be fine to substitute a
2.2k or 1.8k in these instances.
Again, if there are cathode
bypass capacitors associated with
these resistors, I suggest replacing
them at the same time. They
will more than likely be of the
25 μF 25V variety, and any currently
available 22 μF or 25 μF
25V cap will suffice.
Well, if you’ve come this far,
you have an almost completely
rebuilt Supro—and you’ve been
a willing participant in the process,
not just a spectator. That
can yield a lot of satisfaction,
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