FIF : Replacing A Reverb Tank

Written by
Logan Tabor
Published on
January 19, 2021 12:37:34 PM PST January 19, 2021 12:37:34 PM PSTth, January 19, 2021 12:37:34 PM PST
So you're having trouble getting the reverb to work properly in your amp?  Maybe it works intermittently, or maybe the reverb is totally gone.  In any case, you're in luck!  Today we are going to cover the steps involved in diagnosing a bad reverb tank, as well as replacing your tank with a working unit.  
While replacing a reverb tank is a pretty straightforward repair to make, there are a few other things we need to test for before we can say for certain that our issue is, in fact, the reverb tank.  The only tool we will really need today is a screw driver to free our reverb tank from the cabinet -- apart from that, you will want to have an extra 12AT7 or 12AU7 9-pin tube so we can troubleshoot our reverb driver tube.

Step 1 : Testing The Reverb Driver Tube

Alright, first thing's first, let's rule out our reverb driver tube as the culprit.  This simply involves removing the existing driver tube and replacing it with a new tube (in many cases you'll have to remove your upper backpanel, so I'm going to go ahead and remove both of my backpanels, as seen in Image 1A).  If possible, use a tube of the same value as your current reverb driver tube, this way you maintain a good frame of reference for what your reverb sounds like when it is working properly.  Note : Reverb driver tubes of lesser or greater gain factors will almost always noticeably change the way your reverb sounds.


In the amp I'm currently working with, the driver tube is a 12AT7 -- so I have grabbed another 12AT7 from my pile of spare tubes, and now I'm ready to troubleshoot.  Note : If you are unsure which tube to replace, try to find a schematic or wiring diagram for your amp, but more often than not, the reverb driver tube is located closest to the actual reverb controls.  Carefully remove the existing tube and drop in the new one (See Images 2A and 2B).

Once the new tube is in, just turn on your amp as you normally would, plug in your guitar and start jamming.  If your reverb is working properly, the issue was your tube all along and you can stop right here!  In my situation, the reverb still isn't working, so I'm going to put my original tube back in and keep testing around.


Step 2 : Testing The Recovery Stage

Next up on our troubleshooting list is the recovery stage of our reverb circuit.  This is a stage that handles the reverb return and integrates it back into your signal.  The recovery is usually handled by half of a 12AX7 that is also assigned to another portion of the preamp.  To test this, we simply need to unplug the "return" lead from our reverb tank and tap on the end of it (See Images 3A and 3B).  If there is a pronounced hum or buzz, this is an indication that the recovery stage is working and it's time to move on.  If there is no buzz, it may be time to take your amp into a tech as it could require a deeper look.  Right now, my amp is putting out a fair amount of buzz when I tap on the return lead, so it's time for me to replace my reverb tank.  


Step 3 : Replacing The Reverb Tank

A worn out reverb tank may seem like something that is hard to come by, but it actually happens more often than one would think, especially to amps that are out gigging often.  Think about how much a gigging amp gets tossed around, bumped, moved by the drunk guy at the bar who just wants to help, etc.  The transducers inside your tank are made with some very thin wire that can wear out and even break over time (See Image 4A).


So, the first thing we need to do here, is determine what value of reverb tank is going to be right for our amp.  Often times you'll be able to see the value printed on your tank, but sometimes our amps are old enough to where we can no longer just read the tank and see what value it is.  For these situations, you should be able to find this information by visiting your amp manufacturer's website or contacting their customer support team.  Lastly, if you happen to know a few more technical details about your amp and it's reverb circuit, you can use Mojotone's Reverb Tank Key to determine which value tank you'll need.  

Once we have determined the appropriate tank for our amp (in my case I'm going with a Mojotone 4AB3C1B), we can swap the tank.  To do this, we first need to remove the old tank by removing any screws that are fastening the tank to the inside of our cabinet (See Image 5A).  Note : My reverb tank is inside a tolex bag, but yours may not be.



Next, disconnect the RCA style plugs from the tank and set the old tank aside.  Now, connect the RCA style plugs to the corresponding "send" and "return" jacks on the new tank (See Image 6A).  Before fastening down the new reverb tank, it is best to go ahead and run a test, so let's turn on our amp and see what we get.  If the issue was your reverb tank, you will now have a working reverb circuit and you can go back to playing all the Dick Dale songs your heart desires.  If you still are still experiencing issues, it is time to have an experienced tech take a look at your reverb driver transformer.  It is rare that these will malfunction, but it does happen.  In my case, the replacing the reverb tank did the trick and now I'm jamming way too hard!  Now all that's left to do is fasten my tank back down and put my back panels back on.


Thanks for checking out this installment of our Fix It Friday series.  Check back in with us in two weeks for another article!