REverb going out on the Twin

[redir]

On my '72 twin the reverb is acting funny. I fire up the amp and it works just fine for about 3 minutes then it just fades off to zero.

What I did:

1) If I shake the amp I can hear the reverb clang through the speakers so I am assuming the tank is OK and that the output to the amp is good from the tank.

2) Removed the tank and inspected all the leads. All the solder joints look good.

3) Switched the output and input cables from the amp to the tank to verify that one of the cables is not bad. Same results, I can hear the clang but nothing goes to the input of the tank.

4) Switched the Reverb tube 12AX7 with the Vibrato 12AX7 and Voila it worked. But then oddly so did the vibrato so...

5) I plug in the guitar start playing everything works with vibrato and reverb going strong then all of a sudden the reverb just phased out to zero volume after just about 3-5 minutes playing.

Any ideas? It seems to me that it couldn't be the tube since the vibrato works well after switching tubes and if it was the tank then why would the clang output work.

I'm thinking that perhaps there is a bad solder joint somewhere so that when what ever it is expands due to heat it moves away from the connection or something? IDK?

[muscmp]

try a chopstick or wooden stick to poke around near the reverb section including the tube's pins. that may alert you to where the bad area is.

when did it start doing this? was it before, during or after a gig? something may have come loose with those movements.

you may also want to post on the tdpri, tele, forum. there are some pretty good people there that may be able to help you out. it would be the amp tech section under the amplifier discussion area. if you are not registered, you will need to prior to posting. here is the main link.

http://www.tdpri.com/forum/index.php

play music!

[clintj]

Well, it sounds like both tubes are good. If you swapped a tube with one bad triode to the vibrato position, you'd have weak sound or no vibrato - the other half of that tube is the driver that reamplifies the signal for that channel. Could be a bad cap in the cathode circuit, or a defective grid leak resistor, or a few other things.

If you're comfortable with working around dangerously high voltages, I can have you check a few things with a multimeter to start isolating the problem. If not, it should be a fairly quick troubleshooting session for a tech with the amp on their bench.

Sent from my SM-G920V using Tapatalk

[redir]

Ok cool. I just wanted to hear from you guys just in case it was a quick and easy, which I didn't suspect it was. I got a guy who's a wiz with this stuff and when I emailed him yesterday he said the same thing as clintj, probably a cap.

I got bit by an amp once so I don't do high voltage anymore

I'll take it to my local pro.

[jonfields45]

Isn't the reverb driver a 12AT7??? Sounds like you lost the drive side of the reverb and you might have swapped the wrong tube. No problem, for a brief test, swapping a T with an X.

[muscmp]

Originally Posted by jonfields45

Isn't the reverb driver a 12AT7??? Sounds like you lost the drive side of the reverb and you might have swapped the wrong tube. No problem, for a brief test, swapping a T with an X.

jon: you may be correct. see enclosed from fenderguru.

http://fenderguru.com/amps/twin-reverb



play music!

[clintj]

Those should run a 12AT7. The reverb drive circuit needs the extra current they can provide, but a 12AX7 will work in a pinch.

That delayed fade out with either tube to me indicates that either a cap is dying, or something else is causing that tube's bias to drift.

That's an older amp. When was the last time it had a full recap and service done?

Sent from my SM-G920V using Tapatalk

[redir]

Recap and new tubes was done about a month ago. Perhaps this cap that is used for the reverb is not one considered too important to change out?

I may have stated the wrong tube in my OP. While looking at the back of the amp, and counting from the left I swap the 6th (Vibrato) and 7th (Reverb) tubes to run those tests.

Could it be that new caps and eletronics could send more current down the line and fry out older parts?

It's kind of like taking your car in for service and the thing they fixed causes problems on down the line in the system.

Don't get me wrong, my amp tec is a great guy and did a fine job, the Twin sounds better then it ever has. He'll take care of it but I was just hoping that the wisdom of the Internet could just say, do this dummy it's an easy fix, then I would not have to load up the amp, drive it across town and wait a week to get it back.

Also since some one asked, I no longer gig with this amp. I did for a good ten years but now it's a home studio amp and has not left the house in 3 years except for the recap and I let some one borrow it for a gig a time or two.

Regards.

[clintj]

When it was recapped, were all the smaller electrolytics on the main board replaced as well? Some people only replace the main filter caps, and forget that the smaller ones have the same life expectancy and need to be done as well.

Anyway, shouldn't be too complicated to pin down.

Sent from my SM-G920V using Tapatalk

[jonfields45]

Originally Posted by redir

Recap and new tubes was done about a month ago. I may have stated the wrong tube in my OP. While looking at the back of the amp, and counting from the left I swap the 6th (Vibrato) and 7th (Reverb) tubes to run those tests.
Regards.

There are 1.5 tubes involved with reverb. Looking at the back of the amp, the 8th tube, to the right of the reverb driver transformer (similar function to the output transformer except driving the reverb spring), is a 12AT7 with both triodes wired in parallel to get enough current to drive the reverb spring (via that transformer). That is the tube I suspect.

[Aaron Smith]

Take it back to the amp tech and have him fix it. It has worked fine for over 40 years, now it doesn't.

My money is on a bad 12AT7. Try a different brand new tube and see if the problem goes away. I've bought plenty of new tubes that had intermittent issues, especially the cheap chinese ones.

Originally Posted by redir

Recap and new tubes was done about a month ago. Perhaps this cap that is used for the reverb is not one considered too important to change out?

I may have stated the wrong tube in my OP. While looking at the back of the amp, and counting from the left I swap the 6th (Vibrato) and 7th (Reverb) tubes to run those tests.

Could it be that new caps and eletronics could send more current down the line and fry out older parts?

It's kind of like taking your car in for service and the thing they fixed causes problems on down the line in the system.

Don't get me wrong, my amp tec is a great guy and did a fine job, the Twin sounds better then it ever has. He'll take care of it but I was just hoping that the wisdom of the Internet could just say, do this dummy it's an easy fix, then I would not have to load up the amp, drive it across town and wait a week to get it back.

Also since some one asked, I no longer gig with this amp. I did for a good ten years but now it's a home studio amp and has not left the house in 3 years except for the recap and I let some one borrow it for a gig a time or two.

Regards.

[redir]

Originally Posted by jonfields45

There are 1.5 tubes involved with reverb. Looking at the back of the amp, the 8th tube, to the right of the reverb driver transformer (similar function to the output transformer except driving the reverb spring), is a 12AT7 with both triodes wired in parallel to get enough current to drive the reverb spring (via that transformer). That is the tube I suspect.

Oh ok I'll have to try that tonight. So as mentioned earlier will an AX7 work just to test? I'm not sure what old tubes I have laying around.

[Gypsyblue]

Originally Posted by redir

Oh ok I'll have to try that tonight. So as mentioned earlier will an AX7 work just to test? I'm not sure what old tubes I have laying around.

The 12ax7 has a little more gain than a 12at7 but it'll work.

Your Fender also uses a 12at7 for the phase inverter or "driver" tube...right next to your four 6L6 power tubes.

Marshall uses a 12ax7 (more gain) in that position and Marshalls have more grind...partially because of that.

[jonfields45]

Any update?

[redir]

Didn't get it done before the inlaws are in for a visit this week. So I'll get on on it asap and get back to you all. Thanks for the help so far.