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 re-tubing, 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 contain 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 contain
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
John Gilbert’s 1950s Supro combo
from the back. Considering the amp
is more than half a century old, it’s in
pretty good shape.
In an old amp, you’re likely to find cobwebs (look at the transformer,
located down below the power tubes) and a lot of dust and debris.
Use a Swiffer cloth to gently clean out the cabinet’s interior and wipe
off the speaker magnet and frame. Photos by Matt O’Harver
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 (
tube-sandmore.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.
JEFF BOBER one of
the godfathers of the
low-wattage amp revolution, co-founded and was
the principal designer for
Budda Amplification. Jeff
has just launched EAST
Amplification, and he can be reached at