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The Accutronics reverb tanks found in Hammond organs and Fender amplifiers the world over,
as well as countless other brands, are a true classic. But there are a bewildering number of
models to choose from, all looking very similar. So how do you pick between them? The answer
is in the seven-digit model code assigned to the various types. Each of the seven digits refers to
a specific characteristic of the design, and we’ll explain those here.
Let’s start by explaining what each digit refers to:
1 - tank type
2 - input impedance
3 - output impedance
4 - decay time
5 - connector type
6 - locking device
7 - mounting orientation
1 - Tank Type
There are three common types of tank, denoted by a digit:
Type 4 - the classic industry standard as found in the Fender Twin reverb. Four counter-wound
and coupled springs in a tank measuring 16.75” long, 4.735” wide and 1.313” high
Type 8 - a smaller tank designed to give the best sound quality in smaller amplifiers or
standalone units, the type 8 consists of three single springs in a tank measuring 9.25” wide,
3.375” wide and 1.313” high.
Type 9 - the same physical size as the type 4, but containing six counter-wound and coupled
springs (in three paired rows). This gives the fullest and richest reverb of all the tank designs.
2 - Input Impedance
A - 8 Ohms for the type 4 and 10 Ohms for type 8 and type 9
B - 150 Ohms for the type 4, 190 Ohms for type 8 and type 9
C - 200 Ohms for type 4, 240 Ohms for type 8 and 9
D - 250 Ohms for type 4, 310 Ohms for type 8 and 9
E - 600 Ohms for type 4, 800 Ohms for type 8 and 9
F - 1475 Ohms for type 4, 1925 Ohms for type 8 and 9
3 - Output Impedance
A - 500 Ohms for the type 4 and 600 Ohms for type 8 and type 9
B - 2250 Ohms for the type 4, 2575 Ohms for type 8 and type 9
C - 10,000 Ohms for type 4, 12,000 Ohms for type 8 and 9
REVERB TANK MODEL NUMBERS DEMYSTIFIED
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4 - Decay Time
1 - Short (1.2 to 2.0 seconds)
2 - Medium (1.75 to 3.0 seconds)
3 - Long (2.75 to 4.0 seconds)
5 - Connectors
A - Input Grounded/Output Grounded
B - Input Grounded/Output Insulated
C - Input Insulated/Output Grounded
D - Input Insulated/Output Insulated
E - No Outer Channel
6 - Locking Devices
1 - No Lock
7 - Mounting Orientation
A - Horizontal, open side up
B - Horizontal, open side down
C - Vertical, connectors up
D - Vertical, connectors down
E - On end, input up
F - On end, output up
So we can work out from this that the classic 4EB2C1B model number features the following:
4 - this is a type 4 tank
E - the input impedance is 600 Ohms
B - the output impedance is 2250 Ohms
2 - the decay time is medium
C - the connectors are input insulated/output grounded
1 - there is no locking device
B - the tank should be mounted in the horizontal orientation with the open side facing down
This model is highly suitable for feeding from a headphone amplifier, the output of the tank then
being fed into an instrument (guitar) level input on an audio interface, or a good quality DI
(Direct Inject) box. This will give a clean and full tone that can then be EQ’d to taste in your DAW
of choice. For stereo reverb you will need two tanks. Each can then be driven via a single mono
source, or fed a stereo feed from the le and right headphone outputs independently.
Care needs to be taken when soware monitoring is enabled to avoid a feedback loop
occurring in your DAW! This could potentially damage your speakers and/or other
equipment in the chain!
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