Hello Jeff,
I enjoy your amp column every month—please keep up the good work! I’ve just acquired a mid- to late-1950s
Supro combo amplifier, most likely a Spectator model, and have every hope of making it usable. If not for the
stage, then as a studio tool.
The tubes look very clean and are probably original, although I’m going to take them to my local electronics
store to have them tested. The speaker has serial codes that match several years from the ’50s models. I haven’t
plugged the combo in yet, probably due to your July 2010 “Turning Your Amp On” column!
Could you cover restoring an amp like this? I don’t necessarily want to restore it to original condition—just
make it safe and playable. Oh, and clean—there was a wasp nest inside! I’d like to keep any removed parts for
the sake of collectability, but realize that the amp probably needs a grounded plug and maybe other safety precautions.
I appreciate any information you might offer. Thank you for your time.
—John Gilbert
John Gilbert’s 1950s Supro combo. Photo by Matt O’Harver |
The Supro’s top panel features two instrument inputs, a microphone input,
Volume and Tone knobs marked “Low” and “High” at their extremes,
a pilot light, and a fuse receptacle. Photo by Matt O’Harver |
Hello John,
Thanks for reading the column
and for your comments. Let’s see
if I can shed some light on your
restoration question.
Not being familiar with this
particular Supro model, I went
hunting for a schematic. Once I
found it, I realized that this very
basic amp would be an excellent
candidate for a first-time restoration.
Actually, I’ve seen a couple
of different versions of a schematic
for this model, one with a simple
Volume control or a Volume and
Tone setup. Yours is obviously the
latter, which has three inputs, a
6SL7 preamp tube, a 5Y3 rectifier
tube, and a single 6V6 or a pair of
6V6 output tubes running parallel
in a single-ended configuration.
Either way, this is a pretty basic
amp. As far as sourcing parts, most
typical replacement parts that I
discuss here should be available
from the many tube amp parts
suppliers on the web, including
Mojo Musical Supply, Antique
Electronics, and Amplified Parts.
First, regarding tubes and
commercial tube checkers: They
may have been okay to keep your
television running in the early days
of black-and-white broadcasts, but
they don’t really provide a comprehensive
way to check the quality of
audio tubes. My suggestion would
be to at least replace the two 6V6
output tubes, as fresh tubes in
the output stage can bring a huge
improvement in sound. Generally
speaking, once the restoration is
finished, if the amp functions fine
with the 5Y3 rectifier tube, it’s
really not necessary to replace it.
As far as the 6SL7 preamp tube is
concerned, if the amp is producing
any popping or crackling noises
after the initial overhaul, replace
this tube. If it continues to generate
extraneous noises, you may
need to replace additional components.
I’ll cover this in Part 2 of
this restoration series.
Replacing the line cord is definitely
a good idea, especially since
there’s a good chance that it’s dry
rotted and possibly crumbling
in your hands. Unsolder the two
existing line cord connections,
remove the old cord, and replace
it with a new 3-conductor
grounded cord. Referring to the
Supro schematic at hand, I’d recommend
connecting the black (or
brown) lead to the power switch
and the white (or blue) lead to
the fuse holder.
You should also see a 0.05 μF
600V capacitor with one lead
attached to a fuse holder terminal
and the other lead attached
to the chassis, possibly using a
solder lug. I would recommend
replacing this capacitor with a
currently available 0.047 μF 600V
cap. A Sprague Orange Drop
or any other poly-cap would be
fine for this location. Once that’s
replaced, attach the green (with
yellow stripe) wire to the chassis
at this point, either using a separate
solder lug or soldering directly
to the existing capacitor lead
connection. This will add a safety
ground to your chassis, as well as
leave the common line of the AC
power tied to ground and through
the grounding capacitor.
Next month, in the second half
of this Supro restoration column,
we’ll discuss how to give the amp
a good electrical cleaning, evaluate
its filter capacitors, troubleshoot
noise, and more.
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
pgampman@gmail.com.