Classic Reverb Kit for Cars. Daniel Meyer (“Demco”) and
Popular Electronics Magazine offered the first car reverb kit in 1966.
(Magazines had published earlier car reverb projects. But Meyer's Demco
reverb is the first manufactured parts kit.) We've pieced together the installation
instructions for use with classic cars.
Two-part reverb This kit pre-dates the release of the mini-spring tank by
Gibbs. For that reason, the kit installs in two pieces: The under-dash control module
and the trunk-mount tank and recovery amplifier. The kit also requires a rear speaker,
which your car might already have. Unfortunately, the kit PC board is unavailable
today. But an ambitious maker can scrabble the circuit together on perfboard. Back in
the day, that's how we built our projects.
✪ 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.)
✪ CAUTION: Instructions are Approximate. This page can't tell you exactly how to
connect the unit in a classic car. Each car manufacturer has its own wire colors and standards. By
studying drawings of reverb units, one can puzzle out the approximate instructions. Our educated
guesses appear below.
▲ WARNING. This page assumes the use of a classic radio receiver
(3 Wrms output, maximum) with the kit. Use the kit 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.
In-car wiring, 1966 reverb
♦ NOTICE: Connection Numbering. In these instructions, wire connections appear as
numbers (“1”, “2”).
✪ CAUTION: Speaker Dos and Don'ts (1966). Only use this kit with negative-ground
vehicles. The kit can operate 4, 8, or 10-ohm speakers.
Do ground speakers to the car chassis, as necessary.
Don't use this kit with 40-ohm or other types of speakers!
Don't use this kit with a speaker that carries a DC voltage on either of its leads.
Otherwise, kit malfunctions or equipment damage might result.
Rear Speaker Assembly
If necessary, mount the 6-by-9 rear-deck speaker and speaker grille. (Only 4, 8, or 10-ohm
speaker speakers are compatible with this unit!) Follow
instructions that came with the speaker and grille.
Under-Dash Module
Turn off the ignition accessory switch, so that no power flows to the car radio.
Disconnect the speaker from the car AM radio.
Connect Reverb Terminal 1 to the car radio's "SPEAKER +" terminal (Wire 1).
Connect the car radio's "SPEAKER -" terminal to the car ground terminal (Wire 2).
On the front speaker: Connect the "SPEAKER +" wire to Reverb Terminal 3.
On the front speaker: Connect the "SPEAKER -" wire to the car ground terminal.
Connect wires 4, 5, and 7 to the Under-Dash Module.
Trunk Module
Run wires 4, 5, and 7 from the Under-Dash Module to the trunk.
Connect wires 4 and 5 to their terminals on the Trunk Module.
Connect Trunk Module Terminal 6 to the car ground terminal (Wire 6).
On the rear speaker: Connect Wire 7 from the Under-Dash Module to the rear speaker.
On the rear speaker: Connect the "SPEAKER -" wire to the car ground terminal.
Connect Reverb Terminal 8 to +13.8V through F1, a 1ASB fuse. (13.8V is available at
the accessories terminal of the car's ignition switch.)
Mount the Trunk Module inside the trunk. Hardware: Use mechanical dampers (rubber washers,
springs, etc.) This hardware reduces spurious sounds when rough road pavement jostles the
reverb springs.
Optional Alignment
Turn on the car radio.
On the reverb, cut out the reverb by toggling the switch.
On the reverb, turn the fader rheostat knob and listen.
Answer these questions...
Does turning the control vary the volume at the speakers?
Does the front volume increase to maximum at one extreme position?
Does the rear volume increase to maximum at the other extreme position?
Does the rear volume increase as the front volume decreases?
NO to any of these questions: Check and repair your work. YES to all
questions: Go to the next step.
On the reverb, cut in the reverb by toggling the switch.
Is the volume about the same as before? YES: Go to the next
step. NO: Service your work as necessary, until you can answer YES to
this step.
Using the radio volume control, set the radio to your normal
listening level.
Set the fader rheostat knob about halfway. Rear sound should probably be slightly
quieter than front sound (a matter of taste).
Level trimmer. Inside the reverb, trimmer resistor R4 (1K) adjusts preamp gain. Adjust
trimmer resistor R4 while listening to the rear speaker. Set R4 for your preferred level of
reverberation on the REAR speaker.
Turn the radio off. (Keep the reverb in the circuit.)
Output bias. With your DVM, measure the DC voltage between ground and the
collector of Q5. (On the 1966 kit, the collector is the transistor case.) Popular
Electronics says that the target voltage is 6V. But: This voltage is half of your car
battery voltage during a typical drive. Depending on your power supply, this voltage could
be somewhere between 6V and 7V.
Voltage is below 6V: Use trimmer R8 (25K) to increase the voltage to 6V.
Voltage is above 6V: Use trimmer R8 to decrease the voltage to 6V. Voltage is
about 6V: Go to the next step.
Screw the top on the reverb unit.
Using sheet metal screws, mount the Under-Dash Module beneath the dashboard.
▲ 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.
