✪ CAUTION. The reverb circuits on this page aren't guitar stomp boxes! Don't try
to adapt them for guitar. Guitar reverbs require a reverb tank driver on the input of the tank. (This tank
driver is an amplifier with a high impedance front end.)
What? It doesn't work? Let's get that classic car reverb up and running! I feel your pain: Ouch! Here are
some pointers.
Testing a reverb unit: The intended input for the unit is the speaker amplifier in a 1960's (or earlier) car
radio. A good way to test the reverb is to connect it to a working AM radio from that era. The intended output for your unit
is two 8-ohm (or 10-ohm) speakers, front and rear. Of course, you'll also need a 12-volt (or better, 13.8-volt) power
supply for the radio.
▲ WARNING. This page assumes the use of a classic radio receiver
(3 Wrms output, maximum) with the reverb. Use the reverb with
negative-ground vehicles only! The term “classic” refers to
a period radio and reverb from the 1960s. Never connect a classic reverb
unit to a contemporary radio. The two are incompatible. Connecting them will
likely cause a fire. Personal injury and equipment damage could result.
Mechanical Considerations. Many reverb parts are unavailable today. Yet most of these parts, including the transistors,
are reliable, even over the long term. If the spring tank is mechanically sound, it will probably work. You should
clean the dirt and cobwebs out of it. Also make sure that you install it with proper damping (spring mounts, please).
Otherwise, bumpy roads can cause jarring noises from your sound system. Of course, you can simply turn off the reverb until
the pavement smooths out.
Dust off the cobwebs! Classic Gibbs 9" tank and OC 14" tank. From 1960s Kustom guitar amp in
the Craig Barnard Collection. 2019 photo by James Barnard.
Switch. Another mechanical consideration: The on-off switch might have failed from frequent use. After all,
it's several decades old. (There's no shame on the manufacturer there! No switch lasts forever.) On
pre-1966 reverbs, this switch will be a standalone unit. If so, you might find a contemporary replacement. On post-1966
reverbs, the switch will be part of the push-pull fader assembly. If you can't separate the switch from this assembly,
you'll have to mount a new switch somewhere on the reverb front panel.
Capacitors. Probably the most failure-prone electronic parts are capacitors, particularly electrolytic ones.
Capacitors dry out and can disturb operation. In consumer equipment, failures occur after about 10 or 15 years. Here are
three symptoms of bad capacitors: An erratic or dead power supply, missing low frequencies, or low volume. The Ford
Studiosonic® reverb has three electrolytic capacitors...
C1, 100 uF, emitter bypass for the preamplifier transistor
Recommend redesign. Example: Allied reverb eliminated 1-interstage
autotransformer by direct-coupling (DC) reverb preamp & power amplifiers.
Today's DIY maker can build similar DC circuit. See:
Speaker autotransformer
Recommend redesign. For single-ended circuits, RC couple speaker. For lower losses
through collector resistor, substitute load transistor. Bias load transistor at
50% on. Better: Substitute dual-ended (complementary) stage with RC output.
Balance control (5W rheostat w/pull-on feature)
Recommend redesign. 5W rheostats are still available. Separate rocker switch would be
almost as convenient as pull-on feature. BUT: Only use 5W rheostat with
classic (1960s) radios. NEVER use such rheostat with modern car stereos. Otherwise,
expect fire.
Preamp transistors, germanium
Replace parts with silicon, “general purpose” devices. Rebias
for silicon parts. See:
Lamp (in use as thermistor)
Allied model 19B4192 reverb used #1855 pilot lamp as positive temperature
coefficient (PTC) thermistor. Lamp seldom fails, but may after several
years. (In reverb, lamp doesn't light. Normally, lamp drops
reverb operating voltage to about 10.8VDC. Symptom of failure is no power to
reverb. To test lamp, remove it. Check using multimeter on lowest "Ohms"
scale. Only replace bulb if it tests as high resistance ("OL").
Power transistors, germanium
Replace parts with silicon, “high-gain power” devices. Devices in
TO-220 package will work. Heat sink as necessary. Match output power, beta, current, & minimum
input current. Power voltage should be several times supply voltage (13.8V). Rebias for silicon.
Suggested parts:
Comment
Replacement transistors. Although most germanium transistors are obsolete,
NTE sells approximate replacement devices. These replacements tend to be
expensive. Yet for this application, replacements will likely work well.
Replacements can be germanium just like the originals. No rebiasing is
necessary. Given that fact, price may be acceptable. Order by original part
number (Example: Delco DS-503). Fry's sells NTE transistor line. See...
▲ WARNING. Anyone who builds these circuits does so at his own
risk. I take no responsibility for your success or failure. If you
injure yourself, damage your favorite oscilloscope or burn your house
down, you pay the damages.
If you aren't an advanced builder and experimenter,
don't even attempt this project.
From time to time, I will update and attempt to improve the information on
this page. I will make changes without notice.
I assume no responsibility for errors or inaccuracies that might occur on
these pages.
♦ NOTICE. If you'd like to contribute ideas or suggestions,
email me. Submitted ideas and suggestions become the property of
Hawes Amplifier Archive. I try to mention the source of
anything I use.
♦ NOTICE: FURTHER INFORMATION. The details that I have appear on these
pages. As I learn more, I'll add more data. Please don't email me for more
information.
♦ NOTICE.None of the reverb units on this page is for sale by me.
You might find one on eBay, though.
♦ NOTICE: Repairs, Modifications. Buy a Sams PhotoFact (schematic)
set on eBay. I don't have, nor do I provide schematics. I don't repair or modify
reverb units.
