How do Reverb Tanks Kill Amps?

Telfer

Member
Messages
235
So recently I decided to replace the 9 inch Accutronics 8EB2C1B tank in my Fender Princeton 112 Plus (solid state) with a longer 9EB2C1B made by MOD.

It worked for a few minutes, although there was a higher noise floor, then it started into an extremely high-pitched squeal and went dead...no sound at all. I tried putting the original tank back...but it didnt work...my amp has been murdered.

I was assured this was a compatible replacement model...so HOW does this happen??? It could be that the IN/OUT cables were mismatched with the tank jacks?

Many thanks for your expert help! -Telfer.
 
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PremiumPlus

Member
Messages
1,150
If it were a tube amp I'd be suspecting the reverb driver tube or the reverb transformer. Solid state is going to need a trip to the amp doctor...sorry. It sounds like the reverb drive section started oscillating and took out a driver transistor (or chip).
EDIT: Are you sure that you installed the new tank in the proper orientation, in to in and out to out? If you reversed them that may have been what initiated the failure.
 

Telfer

Member
Messages
235
If it were a tube amp I'd be suspecting the reverb driver tube or the reverb transformer. Solid state is going to need a trip to the amp doctor...sorry. It sounds like the reverb drive section started oscillating and took out a driver transistor (or chip).

EDIT: Are you sure that you installed the new tank in the proper orientation, in to in and out to out? If you reversed them that may have been what initiated the failure.
Thats possible...I recall connecting the red plug to the red jack, but I re-connected several times and may have made a mistake. Nevertheless, the dealer told me that switching the cables would not damage the amp.
 

woof*

Member
Messages
8,803
Thats possible...I recall connecting the red plug to the red jack, but I re-connected several times and may have made a mistake. Nevertheless, the dealer told me that switching the cables would not damage the amp.
They wont work if switched but not gonna kill anything. As i mentioned the amp will work even without the tank connected. Check and try a new reverb driver....other than that its time to go to the tech.
 

woof*

Member
Messages
8,803
Reverb driver = one of the preamp tubes that runs the reverb circuit, usually a 12ax7
 

Papanate

Member
Messages
19,820
A solid state amp reacts differently than a tube amp to mismatched reverb tanks.
In your case I'd suspect that the amp circuit expected a isolated input or a grounded input
and the new tank was the opposite - result would be a short or near zero input impedance,
which would stress the circuit and the amp would be damaged.

If the problem was simply an impedance mismatch the amp would not sound right or might
just be silent - and putting the original back would restore the amp.
 

drbob1

Gold Supporting Member
Messages
30,228
To start with, the way that a reverb circuit is implemented is slightly different between tube and SS versions. With a tube version, the reverb "driver" is actually acting like an output tube: a very low output amp sending a signal to the 8 ohm or so tank. It requires a transformer and is relatively durable, like all tube amps, to short term changes in impedance or even to being disconnected long term. And if it does fail, it's usually the tube, which is a simple fix. Other problems in the circuit are usually simple electronic failures or bad solder. And since the reverb section is essentially a separate amp mixed in with the dry signal, the rest of the amp should go on merrily even if you pull the tube from the reverb circuit.

A SS reverb circuit is going to be implemented a bit differently in that the output comes directly from a power transistor to drive the tank. There's no shielding there, and too low an impedance can basically wreck the driver transistor in a shortish period BUT it is the same in that the reverb is a separate path in the amp and if it dies it shouldn't take the rest of the amp with it. It's likely that, with a 20 year old SS amp, something was on the ragged edge and changing the tank pushed it over somehow. You've got a dead power circuit in there and a tech is the only one who can fix it. It will be a simple fix, likely, since all the electronics in that amp are thru hole parts. The bench charge will kill you though ($70 to trouble shoot, $30 to repair)! Is the amp worth another $100 to get working?
 

Telfer

Member
Messages
235
A SS reverb circuit is going to be implemented a bit differently in that the output comes directly from a power transistor to drive the tank. There's no shielding there, and too low an impedance can basically wreck the driver transistor in a shortish period, BUT it is the same in that the reverb is a separate path in the amp and if it dies it shouldn't take the rest of the amp with it.

You've got a dead power circuit in there and a tech is the only one who can fix it. It will be a simple fix, likely, since all the electronics in that amp are thru hole parts. The bench charge will kill you though ($70 to trouble shoot, $30 to repair)! Is the amp worth another $100 to get working?

I thank you for the technical diagnosis Dr.Bob. The 8EB and the 9EB have the same input/output impedance...that's why the new tank was recommended as a compatible replacement.

I hate the idea of old Fender amps being tossed in the trash just because of the bench charge. Most of the time spent on the bench is probably just unscrewing dozens of bolts holding it together. So, yes its worth it!

Is it possible I will NEVER be able to upgrade the tank? The stock 8EB model is very weak and shallow.
 




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