Make the Wrong Reverb Right?

Telfer

Member
Messages
137
Is there any way to determine what type of reverb tank will work with a specific amp without the schematic? I have a 1970s Gibson amp with what I suspect is the wrong tank...doesn't work at all.
 

zenas

Member
Messages
8,228
You need to know the impedance of the tanks input and output side. Hopefully that's on the schematic but it doesn't sound like you've got one anyway. Maybe if you mentioned the specific amp someone here will know what tank it takes.
Lots of other things can kill a reverb too. I've had dead ones that came back to life after just changing all the electrolytic caps. On amps that old that's always the first thing I do.
 

HeavyCream

Member
Messages
3,204
Have you tried swapping the reverb tube(s)?

What specific amp and what’s the make/model of the tank?

Have you tried flopping the RCA’s around. If someone bought the right tank but have the RCA cables backwards it won’t work. One time I bought a replacement tank for an amp and put the cables on with the in/outs oriented the way the original one was (the original one wasn’t broke, I just wanted less decay), and it didn’t work at all. I triple checked to make sure I bought the right tank, figured I must have got a dud but couldn’t see anything wrong with it. Grasping at straws, I switched the RCA’s so the In/Outs were backwards and it worked! Worked perfectly normal, nothing weird. I’m not sure if the tank jacks were mislabeled by the factory or what but it worked. It’s probably a long shot but it’s easy to try and worth a shot, especially if it’s not the original tank.
 

Telfer

Member
Messages
137
You need to know the impedance of the tanks input and output side. Hopefully that's on the schematic but it doesn't sound like you've got one anyway. Maybe if you mentioned the specific amp someone here will know what tank it takes.
Lots of other things can kill a reverb too. I've had dead ones that came back to life after just changing all the electrolytic caps. On amps that old that's always the first thing I do.
Thanks for your comments Zenas. The amp is very rare...a Gibson G115 from the 1970s made by CMI Electronics in El Monte California. I've tried googling a schematic but nothing comes up. It has four Eminence alnico speakers and a beautifully balanced crystalline tone.

The existing tank is an Accutronics two spring 4FB2A1A. I tried switching the wires but its dead either way. I have another 9EB2C1B tank laying around, but I dont know if its safe to try it???

You're probably right about the old caps.
 

Telfer

Member
Messages
137
Have you tried swapping the reverb tube(s)?

What specific amp and what’s the make/model of the tank?

Have you tried flopping the RCA’s around. If someone bought the right tank but have the RCA cables backwards it won’t work. One time I bought a replacement tank for an amp and put the cables on with the in/outs oriented the way the original one was (the original one wasn’t broke, I just wanted less decay), and it didn’t work at all. I triple checked to make sure I bought the right tank, figured I must have got a dud but couldn’t see anything wrong with it. Grasping at straws, I switched the RCA’s so the In/Outs were backwards and it worked! Worked perfectly normal, nothing weird. I’m not sure if the tank jacks were mislabeled by the factory or what but it worked. It’s probably a long shot but it’s easy to try and worth a shot, especially if it’s not the original tank.
Its solid state so there's no reverb tube. I tried doing the wire switcharoo...didnt work. Thanks!
 

zenas

Member
Messages
8,228
Here's some reverb tank info, that's an amp I've never worked on so I can't help much. https://www.amplifiedparts.com/tech-articles/spring-reverb-tanks-explained-and-compared

Wouldn't hurt to check the resistance of the input and output. That'll be DC resistance in ohms not impedance ohms, like a speaker the reading you get will be lower than impedance but it'll give you a ballpark number anyway. Also if it's way off like shorted or open, you'll know if the tanks just dead.
 

RyanPitch

Supporting Member
Messages
270
Wait... granted I know more about RF circuits than audio, but you’re telling me people design their tanks for different impedances? I’d think that would be a standard. I guess not.

The reverb tank in my ‘80s Carvin X100B is dead. How do I tell what impedance it was designed for?
 

Telfer

Member
Messages
137
Here's some reverb tank info, that's an amp I've never worked on so I can't help much. https://www.amplifiedparts.com/tech-articles/spring-reverb-tanks-explained-and-compared

Wouldn't hurt to check the resistance of the input and output. That'll be DC resistance in ohms not impedance ohms, like a speaker the reading you get will be lower than impedance but it'll give you a ballpark number anyway. Also if it's way off like shorted or open, you'll know if the tanks just dead.
Good link! I'll pull out the tank tomorrow and check both RCA jacks with a multimeter. I can fix a broken contact...but I cant fix a dead transducer.
 

Tone_Terrific

Supporting Member
Messages
31,764
Good link! I'll pull out the tank tomorrow and check both RCA jacks with a multimeter. I can fix a broken contact...but I cant fix a dead transducer.
First make sure the send is sending a signal out and the return is returning a signal injected into it.
Then look at the tank. Check the springs and fine wires going from the transducers to the jacks.
The amp is likely close enough compatible with other tanks designed for SS amps.
 

zenas

Member
Messages
8,228
Wait... granted I know more about RF circuits than audio, but you’re telling me people design their tanks for different impedances? I’d think that would be a standard. I guess not.

The reverb tank in my ‘80s Carvin X100B is dead. How do I tell what impedance it was designed for?
If you look at that link I posted above there's a list of different impedances. Be nice if they were all standard! Most of my reverb amps are old tube Fenders or Ampegs, they take different tanks because they use different reverb circuits in the amps.
There should be codes on the tank in your Carvin, that link tells how to read them. Hopefully there's a compatible one on the market today. Also on a 40 year old amp there could be something else going on in the amp.
 

Telfer

Member
Messages
137
I just tested the in and out jacks with a multimeter set at 2000 Ohms. Both jacks gave a reading of approx 165 Ohms. I also tested a different tank with slightly different impedances and it didnt work with the amp either.

So I guess the problem is inside the amp. What should I look for when I open it? Are there obvious signs of a dead cap???
 
Last edited:

zenas

Member
Messages
8,228
Am electrolytic cap that old can look like new and not work well or at all. Or you might see bubbles, bumps or leaks. You can test um for capacitance with a lot of multi meters but that doesn't tell the whole story because the voltage isn't even close to what a lot if them run at. Same thing with the little ESR meters but maybe they're a little more informative. The old way was substitution and that's still the best bet. I just replace them all in old amps. Often that alone fixes all sorts of problems and makes vintage amp repair seem easy. Unfortunately it just as often doesn't, then you gotta dig in deeper and that takes awhile to learn.
Might be time to take it to a tech.
 

mlj_gear

Member
Messages
2,907
I found a schematic. It indicates a 4FB2A1A.

The schematic says G-105 at the upper left of the page, but in the title box it says G105-115; presumably it's the right schematic and the difference between the 105 and 115 is just the 2x12 format of the 105.


I'd also take it to a tech if the other pan didn't work. Could be the IC that drives the reverb or any number of things in addition to caps as mentioned above.
 
Last edited:

Telfer

Member
Messages
137
I found a schematic. It indicates a 4FB2A1A.

The schematic says G-105 at the upper left of the page, but in the title box it says G105-115; presumably it's the right schematic and the difference between the 105 and 115 is just the 2x12 format of the 105.


I'd also take it to a tech if the other pan didn't work. Could be the IC that drives the reverb or any number of things in addition to caps as mentioned above.
Brilliant...at least now I know I have the right tank! Thanks!
 

Telfer

Member
Messages
137
Am electrolytic cap that old can look like new and not work well or at all. Or you might see bubbles, bumps or leaks. You can test um for capacitance with a lot of multi meters but that doesn't tell the whole story because the voltage isn't even close to what a lot if them run at. Same thing with the little ESR meters but maybe they're a little more informative. The old way was substitution and that's still the best bet. I just replace them all in old amps. Often that alone fixes all sorts of problems and makes vintage amp repair seem easy. Unfortunately it just as often doesn't, then you gotta dig in deeper and that takes awhile to learn.
Might be time to take it to a tech.
Excellent advice...what kind of cap has the shortest life span in a guitar amp?
 

Telfer

Member
Messages
137
Have you tried plugging into the Reverb footswitch and see if that does anything?
Yes first thing...with several different switches.
Can you tell from the schematic if 'reverb on' is the default...without any footswitch plugged in?
 

Telfer

Member
Messages
137
Just watched a video from Steve Carr.

He says if you thump the amp enclosure and hear the reverb splashing...its probably the driver side of the reverb circuit that's dead. Make sense???
 

zenas

Member
Messages
8,228
Excellent advice...what kind of cap has the shortest life span in a guitar amp?
The electrolytic caps generally have the shortest life span in modern amps. Back in the 1950s and earlier any cap could be bad, but normally since the 60s it's the electrolytic caps that go bad with age. There are always exceptions though!
 




Trending Topics

Top