RESUSCITATING A SOUND CITY CONCORD COMBO BY JEFF BOBER
Last month, I described beginning what seemed like a normal repair of a Sound
City Concord 2x12 combo [“Rare Bird—the
Sound City Concord Combo,” April 2012].
Blown fuse, scorched tube sockets—these
signs indicated typical high-voltage havoc.
Normally I’d replace the tube sockets, check
for and replace any damaged grid and screen
grid resistors, install and bias a new set of
output tubes, give the amp a thorough bench
check, and then rock out with it for a while
to see if it’s ready for the stage.
Well, once I pulled the chassis on this cool
vintage combo, I was stunned to see a couple
of melted ceramic resistors. Now I’m no
chemist, but doing a bit of research I learned
that ceramic can have an operating temperature of between 3,300 and 5,400 degrees
Fahrenheit. This means it melts at an even
higher temperature! How could this have
happened without incinerating the amp?
My initial inspection revealed that the
output tube sockets would need replacing, but what the heck was going on with
these ceramic resistors? Also in the area of
the resistors were two axial filter capacitors,
along with the chassis-mounted can capacitors. The can capacitors looked like original
equipment, the axial caps definitely did not.
Whoever installed these replacement caps also
installed 220k voltage-divider resistors in parallel with the capacitors—a good thing to do,
as these resistors produce an even voltage split
across each of the two capacitors in series.
But looking at the schematic, I realized these
resistors didn’t belong in the circuit. And crazier yet, they were wired so that all the voltage and current needed to supply the entire
amplifier had to flow through the resistors.
My first step was to pull out everything
that was cooked or simply didn’t belong there.
Can caps and voltage-divider resistors were
burnt, so out they went. Axial “add-ons”—
gone. Rectifier diodes, out. And since I was
replacing the two main filter caps anyway, I
decided to replace the third dual can, too.
Now, the big question: Are the cool
Partridge transformers okay? To test the
mains transformer, I disconnected all loads
to the secondary and pulled all the tubes
to remove the load from the filament line.
Disconnecting the low-voltage winding
from the bias supply removed that load,
and for the high voltage … well, that was
easy, because the leads on the diodes were
melted clean off the capacitor connection.
1. Ceramic resistor meltdown? This amp was on fire! 2. Inspecting the chassis—schematic in
hand—I spot improper caps and resistors. (Hint: Look at the yellow cylindrical caps.) They’ve got
to go! 3. The new dual 100 µf caps and divider resistors are installed. 4. Sweet! Top view of the
Concord’s chassis after I’ve installed the new caps and tubes.
After installing a new fuse and connecting the amp to a Variac, I was able
to bring up the mains voltage slowly and
monitor the output of each secondary
winding. Everything looked good, so it
seemed the mains transformer survived.
Next, I ran a quick test of the output
transformer. Checking the resistance of the
primary and secondary windings, I found
nothing open and all readings looked relatively typical. I checked all windings with
reference to ground to see if there were
any internal shorts to the case. All looked
good, so I was optimistic that the transformers would function.
I replaced the filter capacitors and volt-
age divider resistors, output tube sockets,
grid resistors, and rectifier diodes. One
more detail needed to be addressed—burnt
wires. A few, including the screen grid sup-
ply and bias supply wires, were barbecued
in the meltdown. Removing and replacing
the screen grid supply wire wasn’t a prob-
lem, but replacing the bias supply wire
looked like it might require removing a few
extra components from the terminal strip.
Considering the age of the terminal strips
and component leads, I opted to leave the
original bias wire connected to the strip and
simply splice it down the line.
JEFF BOBER, one of the godfathers
of the low-wattage amp revolution, co-founded and was the principal designer
for Budda Amplification. Jeff launched
EAST Amplification in 2010, and he can be
reached at firstname.lastname@example.org.