Reverb Tanks Explained

Author: Kevin  

Would you like to know what those numbers on a reverb tank mean?

Evatco carries a wide range of reverb tanks covering both MOD and Accutronics. We also carry digital reverb solutions with the Accutronics Bricks but let’s stick to the tanks for now.

There are essentially 3 types of tank but you will probably recognise these as large and small when you first look at them,

Large, which often start with the number 4 or 9 are approximately (assuming you are looking at these flat on a table)

  • 425mm Long
  • 108 mm wide
  • 38mm high

Small, which often start with the number 8 are approximately (assuming you are looking at these flat on a table)

  • 235mm Long
  • 86 mm wide
  • 38mm high

So why did I place the disclaimer (assuming you are looking at these flat on a table), well that’s because some tanks are designed to be installed on the base of an amplifier and others on the side or vertically, so which is top bottom etc vary.

Lets understand the first digit.

  • Type 4
  • Type 8
  • Type 9

Type 4

Reverb tanks that start with a 4 have 4 counter wound and coupled springs and have been used in quite a few famous amps over the years

Type 8

Reverb tanks that start with an 8 have 3 springs in them and are designed for compact situations, don’t let the size fool you, these are my favourite reverb tanks. The size does not make that much difference and really does cover the key sounds you come to expect from a spring reverb.

Type 9

Reverb tanks that start with a 9 have 6 springs counter wound, coupled and arranged in parallel and are ultimately designed for use in amps that require a large luxurious sound e.g., pedal steel guitars.


What are the numbers on a reverb tank?

The numbers on a reverb tank haven’t really changed over the years, some models designed for specific companies may have unique numbers from time to time but these numbers should cover the majority.

The numbers found on most reverb tanks are 7 digits long and contain both letters and numbers

These numbers represent the following

  • 1st digit      Reverb Type
  • 2nd digit     Input impedance
  • 3rd digit      Output impedance
  • 4th digit      Decay
  • 5th digit      Connectors
  • 6th digit      Locking device
  • 7th digit      Mounting plane


1st Digit – Reverb Type, reverb

Generally, this will be either a 1,4,8 or 9


2nd Digit Is the impedance device, represented by a letter

  • A       8 Ω   – 10 Ω
  • B       105 Ω – 190 Ω
  • C      200 Ω – 240 Ω
  • D      250 Ω – 310 Ω
  • E       600 Ω – 800 Ω
  • F       1,475 Ω – 1,925 Ω

3rd Digit Output Impedance, represented by letter

  • A       500 Ω – 600 Ω
  • B       2,250 Ω – 2,575 Ω
  • C      10,000 Ω  – 12,000 Ω


4TH Digit – Decay Time, represented by a number

  • 1       Short        1.2 to 2 Seconds
  • 2       Medium    1.75 to 3 Seconds
  • 3       Long          2.75 to 4 Seconds


5th Digit – Connector type, represented by a letter

  • A       Input Grounded / Output Grounded
  • B       Input Grounded / Output Insulated
  • C      Input Insulated / Output Grounded
  • D      Input Insulated / Output Insulated
  • E       No Outer Channel


6th Digit – Locking Device, represented by a number

  • 1 - No Lock


7th Digit – Mounting Plane, Represented by a letter

  • 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 let’s look at an example


Digit 1


Tank type (8 is small)

Digit 2


8 – 10 Ω Input Impedance

Digit 3


2,250 Ω – 2,575 Ω Output Impedance

Digit 4


Medium Decay 1.75 to 3 Seconds

Digit 5


Input Grounded / Output Grounded Connector

Digit 6


No Lock

Digit 7


Horizontal, Open Side Down Mounting Plane


Well, that’s about all you need to know.

What if I cant find my number?

There are, from time to time, tanks that either don’t have a number or don’t quite match, consider your options through the key information provided e.g.

  • Digit 1 is Optional – While ideally you should match, Its not mandatory
  • Digit 2 is Important – You need to be in this range
  • Digit 3 is Important – You need to be in this range
  • Digit 4 is Optional – While ideally you should match, Its not mandatory
  • Digit 5 is Important – Match required
  • Digit 6 is Optional – Not required
  • Digit 7 is Ideally matched but I have seen plenty of amps with this option ignored. I personally match where available .


Good luck