Fender ProTube Twin-Amp reverb feedback

[jim p]

Dude, I'm afraid to even look at a capacitor!

If it comes down to modifying the circuitry the way you describe, then I'll take it to someone like you that knows what an ohm is.

Well thought it might help. On the chance changing a tube could have an effect changing V3 may have an effect. This is used as a cathode follower for the clean signal to the reverb and the output. If its cathode impedance was higher then normal it could cause a problem like this. Hope this helps.

[Mick]

Okay, I tried swapping out all of the preamp tubes, one at a time.

No joy...

I'm about 99.999% positive that it's not a microphonic preamp tube causing the feedback.

So, I pulled out the reverb tank...



...but I didn't see any obvious problem with the tank, like a loose wire, broken spring, or whatever. So I cleaned the RCA connectors where the cable goes into the tank, then put it back in the amp.

I still have nasty feedback when the reverb is turned up.

Could the reverb transformer be causing the feedback? Is there some way to test it to see if it's within specs?

What should I try next?

[oldguy]

I think Jim P gave a fair evaluation of his prognosis after perusing the schematic of the amph. I would suggest taking it to an amph tech to confirm (or disprove) his theory as to the cause of the feedback.

[Mick]

I think Jim P gave a fair evaluation of his prognosis after perusing the schematic of the amph. I would suggest taking it to an amph tech to confirm (or disprove) his theory as to the cause of the feedback.

Pay someone that knows what they're doing to fix it, eh? That's quite a concept you got there!

Nawt yet... Methinks I'll try to verify that all the existing components are within specs before I throw in the towel.

I checked the tubes.
! checked the tank.

What's next? Maybe pull the chassis and have a look inside? How do I do that without electrocution? How do you drain capacitors safely? Lick the ends?

[Mick]

C'mon people! Doesn't anyone have any helpful suggestions?

[Gutmann]

Some prednisolone for your tongue
No other idea than the tubes, the tank and the tech - sorry.

[Mick]

Some prednisolone for your tongue

I found a website that tells you how to drain capacitors without using your tongue.

How to drain the filter caps

What I'd like to see is detailed instructions on safely removing the chassis on a Fender Twin. Ever done that?

[Mick]

I wonder if my problem might be similar to this...

Cold solder joints causing amp to "feedback" whether a guitar is plugged in or not.

Hmmm...

[Mick]

In case anyone is following this...

Someone from another website suggested replacing the RCA cable. Apparently, they had had a situation where a cheap unshielded cable caused reverb feedback. So, I removed the tank and connected it with a new RCA cable. It worked! With the reverb tank sitting on a table next to the amp and the reverb dial at max, no feedback!!!

Unfortunately, putting the reverb tank back in the amp case causes it to feedback as before. Even with the new RCA cable installed.

Somehow, the magnetic field of the two 12" Eminence speakers is interacting with the reverb tank to cause the feedback. It's like the tank is microphonic, but only when it's near the speakers. I tried placing various materials between the tank and speakers (rubber mat, wood, etc.), but nothing helped. Although a 1/2" thick piece of oak did change the frequency of the feedback.

The search for a cure continues...

[Mick]

I installed a new reverb tank and that fixed the problem. No more feedback, even with the reverb set to maximum. I'm happy.

With the new tubes and reverb tank this amp is awesome! I have to be careful with the volume though because If I turn it up too much it'll blow out every eardrum in the place, and the windows too!

[markb]

Glad you cracked it, Mick

I have to be careful with the volume though because If I turn it up too much it'll blow out every eardrum in the place, and the windows too!

Yup, that's a Twin

[Gutmann]

That's great. Have fun with this great amp!

[aeolian]

Your determination paid off. Good work!

I'm in Portland, but where the heck is Deer Island. I've lived in Oregon for 30 years and have never heard of the place before.

[Mick]

Thanks guys!

Your determination paid off. Good work!

I'm in Portland, but where the heck is Deer Island. I've lived in Oregon for 30 years and have never heard of the place before.

The huge metropolitan portion of Deer Island is located on Hwy 30 between Portland and Astoria. It is 36 miles from downtown Portland, and 60 miles from Astoria. It consists of one gas station/convenience store on the highway and a small church. Part of Deer Island actually is an island on the Columbia River. Most of the island is cattle ranch. My wife and I wanted to move away from the hustle and bustle of big city life, so we built our house a few miles away from town. It's about a mile from the highway, in the hills overlooking the river. We have no neighbors close by, so I can play my guitar as loud as I want without anybody complaining. Which is a very good thing considering how well I play.


Here's some pics that I took of our house this winter, and my wife Becky, and me, and Mr. Kitty (a stray tom cat we took in just before it started snowing).

[aeolian]

Here's some pics that I took of our house this winter, and my wife Becky, and me, and Mr. Kitty (a stray tom cat we took in just before it started snowing).

Now I know where Deer Island is. Less than 40 miles from me and yet I've never heard of it! You do know that Eurotubes is in Portland near McLaughlin Blvd, right? But if you have to drive that distance to pick something up you may opt for Bob shipping the tubes to you.

I remember the December snow we had, I have a few pictures of a lounge chair with snow piled up to the seat in my backyard. I checked out your photo albums, beautiful photography. Do you do that for a living? I enjoy taking pictures too, but if I come up with a decent one it is more due to luck than to skill.

[oldguy]

Glad to hear you got the Twin fixed, Mick. You should post a pic or two of it sometime. With your photography skills, you could make it look sweet.

[Mick]

Now I know where Deer Island is. Less than 40 miles from me and yet I've never heard of it! You do know that Eurotubes is in Portland near McLaughlin Blvd, right? But if you have to drive that distance to pick something up you may opt for Bob shipping the tubes to you.

Yup, I bought a complete set of tubes from Eurotubes. I put in my order on Friday and the tubes got here the very next day. Talk about fast service! Bob was very helpful on the phone too. A great guy.

I remember the December snow we had, I have a few pictures of a lounge chair with snow piled up to the seat in my backyard. I checked out your photo albums, beautiful photography. Do you do that for a living? I enjoy taking pictures too, but if I come up with a decent one it is more due to luck than to skill.

I'm glad you like the pics. Nice if you to say so. No, I'm not a professional photographer, although I've been doing it for almost 40 years. I'm really just an avid amateur. I have done a few paid photo shoots though, and I've sold usage rights to some of my photos to various companies and even some well known journalists. I actually make my living as a senior software engineer. Our company creates communications and network management/reporting software for large corporations.

I just recently took up the guitar again after a long time away. I learned and played for a couple years when I was a kid (late 60's), but I never got very good. I didn't really play again until my son bought me a guitar and amp for my birthday a year ago last March. Since then I bought myself an American Deluxe HSS Strat, this Twin amp, and just today my UPS buddy brought me a pedal board and my first two pedals. A Big Muff Pi and a Boss ML-2. Way fun!

BTW, if you like making digital photos then you might enjoy DPChallenge.com. They have several photo challenges going on all the time with various subjects. No prizes, just virtual ribbons, but it can be a lot of fun if you have a thick skin and don't take it too seriously. You can register and enter open challenges for free, or buy a membership if you want to enter member-only challenges and have a portfolio. My dpc user name is Mick.

[Mick]

Glad to hear you got the Twin fixed, Mick. You should post a pic or two of it sometime. With your photography skills, you could make it look sweet.

Thanks oldguy! I'll give it a try soon. I've actually been thinking about trying to recreate some of Fender's ad shots.

[oldguy]

That sounds like a cool idea. I've always liked this one, even though it came out two yrs. before I was born................don't know if it's the dachsund or the raybans...................