FAQ

Q At what current level should I
drive the reverb’s input coil?

AThat’s an important
question. Too little current and the output will be noisy, too much current and
the core will saturate producing a muddy sound. A maximum current of 30
amp-turns is just about right (see Tech
Note)

Input Impedance Code

Number of Turns

Max current (milliamps)

A

124

242

B

538

56

C

600

50

D

688

44

E

1100

27

F

1700

18

Q There’s a lot of hum on
the output of my unit. How can I reduce it?

A Hum is often the thorniest
problem to solve when designing with spring reverbs. The reverb unit’s
coils, especially the output coil, are very susceptible to picking up hum from
any power transformer. You can test to see if this is your problem by
unscrewing the reverb unit and moving it around. You
will almost certainly note that the hum gets loudest when the reverb is closest
to the power transformer. This problem is very difficult to complete eliminate,
but you can usually get it down to acceptable levels by the following:

Mount the reverb unit and
power transformer as far away from each other as possible.

Mount the transformer and
reverb unit on opposite sides of the chassis so the chassis can provide
some shielding.

Try turning the power
transformer at different angles–some should produce less noise than
others.

Try various orientations of
the reverb unit.

Try some shielding. Mu metal
is the best material to use, but even thick aluminum (1/16" or
thicker) can help. It is often most effective to eliminate the hum at its
source. Try placing pieces of metal around the transformer. Try different
size pieces at different spots and orientations both near the transformer
and near the reverb unit. By playing around with shielding, you should be
able to produce a substantial reduction in hum.

Use a toroidal power
transformer. This type of transformer produces much less hum. You can
order toroidal transformers from Plitron www.plitron.com.

Beware of ground loops. If
the reverb tank is mounted on a metal part of the chassis, try isolating
it. Try connecting a 0.1 uF capacitor to a test lead and connect one end
to the metal body of the reverb tank. Touch the other end to different
ground points on the chassis.

Q When I turn
up the reverb control I get feedback. How can I stop it?

A The
reverb unit can act like a microphone–especially when it is mounted in
the same box as the speaker. Vibrations via the air or through the panel to
which the unit is mounted can cause the reverb’s springs to vibrate,
thereby setting up feedback. The first thing to try is to mount the reverb
resiliently with rubber grommets or the like. This tactic will help isolate it
from panel vibrations. It may also help to mount it on its own little subpanel
to further isolate it from vibrations. If feedback via the air is the problem,
you can purchase a special bag in which the reverb unit can be mounted. They are
available from many of the places the sell the reverbs themselves.

Q I have an
existing amp that doesn't have reverb. Should I try to add reverb to it?

A It
depends. Do you have enough space for both the tank and the circuitry? If space
is tight, will you be forced to mount the tank too close to the power
transformer, causing excess hum? Also, does your amp's power supply have the
necessary voltages and current available? If you have the space and the
voltages, you could give it a try.