Does Your Reverb Tank Need Replacing?

A surprising number of people order replacement reverb tanks when the tank itself has nothing to do with why their reverb isn't working.  They end up paying for a tank and shipping, and it doesn't solve their problem.  First, it's important to take your amp to a qualified tech to actually diagnose why your reverb isn't working.  It isn't always the reverb tank.  If you've properly ruled out the rest of the reverb circuit and confirmed for certain it's the reverb tank, please read on...

Are You Ordering the Right Tank For Your Amp?

A surprising number of people purchase anything labelled "reverb tank", thinking it will be a drop-in replacement for their existing tank.  Unfortuantely, it isn't as simple as that.  There are not only different sizes, spring counts, and decay lengths, but different input/output impedances, grounding schemes, and mounting configurations.  It's important to understand that reverb tanks are not universal.  To make your life easier, it would be ideal to match up the proper tank code when seeking a replacement.

That said, if you understand the codes properly, you are free to possibly experiment with other tanks that will work but have a slightly different characteristics.

Reverb Tank Codes Explained

A reverb tank code is always 7 characters long, looking something like this: 4AB3C1D.  Each character represents an important detail of how the tank is designed and works.  Here's a quick guide for how to understand reverb tank codes (including what characters are allowed to be different while still working with your amp):

1st Character = Tank Type (SHOULD match original tank; swapping 2 & 8 are the ONLY exception)

Type 2 = Short 9" tank with 2 springs
Type 4 = Long 17" tank with 2 springs
Type 8 = Short 9" tank with 3 springs
Type 9 = Long 17" tank with 3 springs

2nd Character = Input Impedance @ 1kHz (MUST match original tank)

Tank Type	A	B	C	D	E	F
4	8 ohms	150 ohms	200 ohms	250 ohms	600 ohms	1475 ohms
2, 8, or 9	10 ohms	190 ohms	240 ohms	310 ohms	800 ohms	1925 ohms

Line up the tank type in the left column with the corresponding first letter to understand this chart.  For instance, a 4Axxxxx tank would have an 8-ohm input impedance, where as, an 8Cxxxxx tank would have a 240-ohm input impedance.

3rd Character = Output Impedance @ 1kHz (MUST match original tank)

Tank Type	A	B	C
4	500 ohms	2250 ohms	10000 ohms
2, 8, or 9	600 ohms	2575 ohms	12000 ohms

Line up the tank type in the left column with the corresponding first letter to understand this chart.  For instance, a 4xBxxxxx tank would have a 2250-ohm output impedance, where as, an 8xCxxxx tank would have a 12000-ohm output impedance.

If your input and/or output impedances are off, your reverb will either be distorted or paper thin sounding.  And, if off by enough, could damage the reverb circuit in your amp.

4th Character = Decay Time (does not have to match original; choose to your preference)

1 = Short Decay (1.2-2.0 seconds)
2 = Medium Decay (1.75-3.0 seconds)
3 = Long Decay (2.75-4.0 seconds)

5th Character = Grounding Scheme (SHOULD match original tank; but can be modified to suit by a qualified individual)

A = Input Grounded, Output Grounded
B = Input Grounded, Output Insulated
C = Input Insulated, Output Grounded
D = Input Insulated, Output Insulated

Note, we added the caveat that this CAN be modified, but only by qualified individuals.  You will immediately void the warranty on your tank if you modify it.  But, it's good to know this can be changed if you have the skill to change it.  That means you can get a tank with any grounding scheme and modify it to suit your amp, as long as the other key specs match.

6th Character = Locking Device (MUST match original tank)

1 = No Lock (for many decades, this is basically always "1")

7th Character = Mounting Configuration (SHOULD match original tank; but can be modified to suit by a qualified individual)

A = Horizontal, Open side up
B = Horizontal, Open side down
C = Vertical wall, Long axis horizontal, Connectors up
D = Vertical wall, Long axis horizontal, Connectors down
E = Vertical wall, Long axis vertical, Input up
F = Vertical wall, Long axis vertical, Output up

Note, we added the caveat that this CAN be modified, but only by qualified individuals.  You will immediately void the warranty on your tank if you modify it.  But, it's good to know this can be changed if you have the skill to change it.  That means you can get a tank with any grounding scheme and modify it to suit your amp, as long as the other key specs match.

Quick Summary

1st Character (number) = Tank Type (MUST match)
2nd Character (letter) = Input Impedance (MUST match)
3rd Character (letter) = Output Impedance (MUST match)
4th Character (number) = Decay Time (choose your preference)
5th Character (letter) = Grounding Scheme (SHOULD match, but can be modified by a professional)
6th Character (number) = Locking Device (MUST match)
7th Character (letter) = Mounting Configuration (SHOULD match, but can be modified by a professional)

Interesting Implications To Understand

• You CAN change a "short" 2-spring tank for another "short" 3-spring tank, because the impedances will match.
• You CANNOT change a "long" 2-spring tank for another "long" 3-spring tank, because the impedances will NOT match.
• You can choose a tank with a different decay time to get a different reverb character, as long as the rest of the specs match.
• For MOST people, that means ONLY the 4th character (decay time) can be changed.
• If you do NOT know your tank code, you NEED to know all of the required specs to know what replacement to purchase.  We might be able to assist, but not always.  Unfortunately our industry is not like the auto industry.  Not all amp manufacturers publicly release the specs required.
• If you are qualified, you can modify the grounding scheme and mounting configuration to suit your needs.  In that case, you only need to match the 1st, 2nd, 3rd, and 6th characters to your original tank.  You will void your warranty, but you will also have much more freedom!
  • That said, if you do that, please scratch out the original code and write the NEW code on your tank with permanent marker so you don't confuse your future self (or a future owner of your amp).

A Very Colourful Surprise

There is one recurring issue that comes up when people install replacement tanks.  Not every amp uses the same colour code for the reverb input and output connections.  For some amps, white is input and red is output.  For others, the colours are switched.  If you've properly ruled out the reverb circuit and know for certain the replacement tank is needed, but it isn't working after installation.  The FIRST thing to try is swapping the input/output connections on the amp or the tank.  This almost always solves the problem.

In the rare instance you've actually received a defective replacement tank, it is of course replaceable under warranty.

Hope that helps!  Happy building/modding/repairing!