Fender ProTube Twin-Amp reverb feedback

[Mick]

Hi People,

I just discovered this place and this is my first post. I'm hoping you folks can :confused: help me diagnose a feedback problem that I'm having with my Fender ProTube Twin-Amp.

I just bought this amp off eBay. It's a used amp, but it looks almost brand new. According to the folks at Fender the amp was manufactured in April of 2003.

When I got the amp the tubes were in a cardboard box, individually wrapped in bubble-pack, so I carefully unwrapped each tube and installed it in the amp according to the tube chart inside the amp. Unfortunately, two of the 12AX7 preamp tubes were missing. Since some of the tubes were missing, and all the ones I had appeared to be the original Fender Groove Tubes (relabeled Sovteks), I decided to go ahead and replace all of tubes. So I ordered a complete set of JJ tubes from Eurotubes.com. The retube kit for the Pro Tube Twin-Amp consists of a matched quad of the JJ 6L6GC's, one ECC81 to replace the 12AT7 reverb tube in V7, and seven ECC83S's, with one of them balanced for the phase inverter in V4 to replace the 12AX7's.

I ordered the tubes on Friday and they arrived on Saturday afternoon. The first thing I did was install two of the new ECC83S tubes to replace the missing 12AX7's. Then I gave the amp a quick test. With the mix of old tubes and new tubes the amp seemed to work okay. Unfortunately I didn't test everything. I was just relieved that the amp worked at all. Up to that point I didn't know for a fact that it would work. Then I pulled out all of the old Groove Tubes and replaced them with the new JJ tubes, then set the bias and balance using a bias probe and meter that I bought from Eurotubes

With the new tubes installed, I plugged in my '06 American Deluxe strat and started checking things out. The 100w/25w switch is a nice feature. The clean channel sounds awesome! I love it! The distortion channel sounds great too. The tremolo works great a sounds terrific. The reverb however has a problem. With the amp in 100w mode, if I turn the reverb knob up past 5-6 or so it immediately starts feeding back. A high pitched tone starts out low and quickly builds until it blows out your eardrums. I verified that it's not the guitar. It will feedback like this even with no guitar attached. It also feeds back with the amp in 25w mode. It just takes a bit more reverb knob. It feeds back in both the clean and distortion channels.

After speaking with the guy at Eurotubes, I tried swapping out the reverb tube in V7 with the original 12AT7 tube. That didn't help. Then I tried swapping out the phase splitter in V4 with one of the original 12AX7 tubes. That didn't help. Then I tried swapping all of the preamp tubes around. I'm not sure, but I think that might have helped a tiny bit. I can turn the reverb up to 6-7 now before the feedback starts. Then I bought a can of contact cleaner and cleaned all of the preamp and power tube sockets. That didn't help.

That pretty much brings us up to date. I love the amp. It sounds awesome... as long as you don't turn the reverb up too much. But the reverb is one of the main reasons why I bought a Fender Twin in the first place. Would someone help me find out what's wrong?

What causes feedback like this anyway? Is it only caused by microphonic tubes, or is there something else that could be causing it?

If it has to be microphonic tubes, would it be preamp tubes or power tubes, or could it be both types?

Anyone?

[Gutmann]

Feedback without attached guitar probably is caused by a microphonic tube.
Try to hit the tubes slightly with a pen while the amp is running. If one is making a strange noise, it might be a microfonic one. It could be a preamp or a poweramp tube.

Also it could be the reverb tank causing the problem - but I don't know how to check this.

Good luck - this is an excellent amp!:AOK:

[tot_Ou_tard]

Feedback without attached guitar probably is caused by a microphonic tube.
Try to hit the tubes slightly with a pen while the amp is running. If one is making a strange noise, it might be a microfonic one. It could be a preamp or a poweramp tube.

Yep, I had the same increasingly loud squeal with no input on my Gries 5.

tap, tap with a pencil eraser & lo & behold the 12AX7 was obviously microphonic. I changed that & problem solved.

[oldguy]

Welcome to TheFret, Michael.

I agree you should check the preamp tubes to start with........even new ones can be microphonic occasionally.

Your photography is stunning, btw, esp. the wildlife.

[Mick]

Thanks for the replies guys! Extra thanks to oldguy for checkin' out my pics. I'm glad you like 'em.

So, I just tap on the tubes and listen for strange sounds, eh? I guess that sounds easy enough. Even for me.

In fact, I'll go give that a try right now.

[tot_Ou_tard]

Thanks for the replies guys! Extra thanks to oldguy for checkin' out my pics. I'm glad you like 'em.

So, I just tap on the tubes and listen for strange sounds, eh? I guess that sounds easy enough. Even for me.

In fact, I'll go give that a try right now.
A microphonic tube will be obvious, kind'a the dental hygenist hitting the exposed nerve.

[Mick]

A microphonic tube will be obvious, kind'a the dental hygenist hitting the exposed nerve.
Well, I removed all of the metal shields (vibration dampers?) from the preamp tubes and let the amp warm up. Then I tapped each tube several times, including the power tubes. All I heard was a tiny bit of noise that sounded like small vibrations coming from the springs in the reverb tank. And they all sounded the same. I did the test several times, with the reverb dial set higher each time. None of the tubes had any obvious difference. Oddly enough, this time I was able to turn the reverb dial up to 8 without getting the feedback. But if I turn it any higher the feedback starts right up.

Is it possible that I have the bias set too hot? Can that cause feedback? The bias setting instructions on Eurotubes.com says "Super's and Twin's have a higher plate voltage so with the JJ 6L6GC's we see them come out of crossover distortion at about 34mA but we typically like to bias them between 38 to 44mA and most players like them best at 38 to 40mA." I set the bias on this amp to 39mA.

What should I try next?

[Moshe]

What should I try next?[/QUOTE]


I would swap out the tubes one at a time until the feedback goes away.

[Mick]

I would swap out the tubes one at a time until the feedback goes away.
Okay, I'll try that...

But what if the problem is actually a combination of two or three bad tubes? This amp has a total of 12 tubes (seven 12AX7's, one 12AT7, and four 6L6GC's) and any of them might be microphonic, right? I could be swapping tubes in different combinations until hell freezes over!

When I was a kid they used to have vacuum tube testing machines at the store. I remember my dad taking tubes from our TV in for testing. Why don't they have something like that nowadays?

[markb]

Okay, I'll try that...

But what if the problem is actually a combination of two or three bad tubes? This amp has a total of 12 tubes (seven 12AX7's, one 12AT7, and four 6L6GC's) and any of them might be microphonic, right? I could be swapping tubes in different combinations until hell freezes over!

When I was a kid they used to have vacuum tube testing machines at the store. I remember my dad taking tubes from our TV in for testing. Why don't they have something like that nowadays?

So that people can charge you extra for doing the testing for you. But really, without the cynicism, a tube tester like that won't pick up a microphonic tube. For that you need an audio amplifier like, say, a guitar amp. Get two new 12AX7s and a new 12AT7 and swap each one with the new ones. If you swap one out and there's no difference you can say that's a good one. The problem won't be in the power section so you can save yourself the cost of the 6L6s and a bias adjustment straight away. And when you've found the bad one and thrown it away, you'll still have a spare or two for when the next one goes bad :AOK:

[Mick]

So that people can charge you extra for doing the testing for you. But really, without the cynicism, a tube tester like that won't pick up a microphonic tube. For that you need an audio amplifier like, say, a guitar amp. Get two new 12AX7s and a new 12AT7 and swap each one with the new ones. If you swap one out and there's no difference you can say that's a good one. The problem won't be in the power section so you can save yourself the cost of the 6L6s and a bias adjustment straight away. And when you've found the bad one and thrown it away, you'll still have a spare or two for when the next one goes bad :AOK:
I have two extra 12AX7's. I ordered two Ruby 12AX7A's from Guitar Center before I decided to replace all of the tubes with JJ's (Note: The Ruby tubes I got from GC are just JJ tubes with a Ruby label added). I don't have an extra new 12AT7, but I do have the original Groove Tube 12AT7 that came with the amp. I'll swap those in and see what happens. If that doesn't work, then I'll buy a new 12AT7.

[Mick]

Why does the amp only feedback when the reverb is turned up?

If it's a microphonic preamp tube causing the feedback, then shouldn't it feedback even when the reverb is turned off?

Why does the reverb setting have any effect on the feedback?

If there's no feedback when the reverb is turned off, doesn't that indicate that the problem must be somewhere in the reverb circuit?

[markb]

Why does the amp only feedback when the reverb is turned up?

If it's a microphonic preamp tube causing the feedback, then shouldn't it feedback even when the reverb is turned off?

Why does the reverb setting have any effect on the feedback?

If there's no feedback when the reverb is turned off, doesn't that indicate that the problem must be somewhere in the reverb circuit?

That was my thought, Mick so it should be the reverb driver tube, assuming that that model Twin has a tube driven reverb. Another thing you could look at is the way the tank is positioned and mounted. The older Fender amps had their reverb tanks in a vinyl bag (which damped them a bit) screwed to the bottom of the cabinet with rubber isolating washers. You could still turn round and kick the amp for the whipcracks in Rawhide, say but they were quite well damped.

I'm not familiar with the more recent models. The most recent Fender tube amp I've owned was a 1982 Concert which was built the old way.

Oops, that's not true! I had a Pro Junior (no reverb) and a Blues Junior (naked tank, no isolating washers) and the BJ was touchy if you turned up the 'verb. Your fix could be the price of some rubber washers and slightly longer fixing screws.

[Mick]

That was my thought, Mick so it should be the reverb driver tube, assuming that that model Twin has a tube driven reverb. Another thing you could look at is the way the tank is positioned and mounted. The older Fender amps had their reverb tanks in a vinyl bag (which damped them a bit) screwed to the bottom of the cabinet with rubber isolating washers. You could still turn round and kick the amp for the whipcracks in Rawhide, say but they were quite well damped.

I'm not familiar with the more recent models. The most recent Fender tube amp I've owned was a 1982 Concert which was built the old way.

Oops, that's not true! I had a Pro Junior (no reverb) and a Blues Junior (naked tank, no isolating washers) and the BJ was touchy if you turned up the 'verb. Your fix could be the price of some rubber washers and slightly longer fixing screws.
This amp does have a tube-driven reverb. I think the 12AT7 tube in the V7 position is for the reverb. The tank is mounted as you described, in a vinyl bag attached to the bottom of the cabinet with screws. I can't tell if there are any rubber isolating washers. I haven't done anything with it yet. If swapping out the tubes doesn't fix the problem, then I'll probably pull the tank out and have a closer look.

[Gutmann]

Right, the 12at7 normally is used as the reverb driver and if this one's microphonic it feedbacks when you push it. You already did swap the AT7 as you said in the first post?
One Thing you could check: Sometimes the reverb tank ist attached via 2 small round plugs, a red and a black one here they are called cinch-cable.
Check if there's a marker to show where the red one has to be plugged in - fender doesnt mark this, but a tech sould do that before unplugging when he takes the amp out - maybe someone didn't take care and swapped the two cables - I'm not shure what the effect would be but this could be it.

I hope we can help you - but here's another address you could contact: http://www.thevintagesound.com/ffg/

Greetings from Germany

[jim p]

Do you have a schematic for this amplifier? I was looking at the Pro Reverb schematic at the Fender web site and see that this is an all tube amp including the reverb drive and recovery circuits with an 8 ohm input tank, nice. From what you have posted the reverb will start to feedback with no signal into the amplifier just turn up the reverb level pot. By looking at the schematic for the Pro Reverb if the signal level out of the reverb is high enough it could feedback across R76 to its input and then oscillate (squeal). This is probably at a frequency greater then you need for the range of the guitar. One way to prevent this would be to limit the bandwidth of the recovery amplifier the 12AX7 V5-A. If you add a 300pf 500volt cap with 10k ohms in series across the plate load resistor R71 this will roll off the maximum frequency from the recovery at approx 5 kHz and may stop the oscillation. Because this oscillation happens with no mechanical excitation (no input to amplifier) I kinda don’t think it is a microphonic tube that is the cause.

[Mick]

Right, the 12at7 normally is used as the reverb driver and if this one's microphonic it feedbacks when you push it. You already did swap the AT7 as you said in the first post?
Yes, I swapped it with the original tube that came with the amp--a Groove Tube 12AT7. It didn't fix the problem. In fact, it might have made it worse.


One Thing you could check: Sometimes the reverb tank ist attached via 2 small round plugs, a red and a black one here they are called cinch-cable.
Check if there's a marker to show where the red one has to be plugged in - fender doesnt mark this, but a tech sould do that before unplugging when he takes the amp out - maybe someone didn't take care and swapped the two cables - I'm not shure what the effect would be but this could be it.
The cable from the reverb tank looks like a normal RCA cable. It has two wires and the connectors on the ends are color-coded (red & white). They plug into color-coded sockets on the chassis. I verified that the colors matched when I cleaned the connectors and sockets with the spray contact cleaner.


I hope we can help you - but here's another address you could contact: http://www.thevintagesound.com/ffg/

Greetings from Germany
Thanks! Hello from Deer Island, Oregon USA!

[Mick]

Do you have a schematic for this amplifier? I was looking at the Pro Reverb schematic at the Fender web site and see that this is an all tube amp including the reverb drive and recovery circuits with an 8 ohm input tank, nice. From what you have posted the reverb will start to feedback with no signal into the amplifier just turn up the reverb level pot. By looking at the schematic for the Pro Reverb if the signal level out of the reverb is high enough it could feedback across R76 to its input and then oscillate (squeal). This is probably at a frequency greater then you need for the range of the guitar. One way to prevent this would be to limit the bandwidth of the recovery amplifier the 12AX7 V5-A. If you add a 300pf 500volt cap with 10k ohms in series across the plate load resistor R71 this will roll off the maximum frequency from the recovery at approx 5 kHz and may stop the oscillation. Because this oscillation happens with no mechanical excitation (no input to amplifier) I kinda don’t think it is a microphonic tube that is the cause.
Dude, I'm afraid to even look at a capacitor! :eek:

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.

[mrmudcat]

Where the he..............hec is DR. TUNG:master:

[jim p]

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

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...

http://micknewton.smugmug.com/photos/517158959_ZKLn4-Th.jpg (http://micknewton.smugmug.com/photos/517158959_ZKLn4-L.jpg) http://micknewton.smugmug.com/photos/517161588_pnMRg-Th.jpg (http://micknewton.smugmug.com/photos/517161588_pnMRg-L.jpg)

...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. :D

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 :rolleyes:
No other idea than the tubes, the tank and the tech - sorry.

[Mick]

Some prednisolone for your tongue :rolleyes:
I found a website that tells you how to drain capacitors without using your tongue. ;)

How to drain the filter caps (http://studentweb.eku.edu/justin_holton/caps.html)

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. (http://studentweb.eku.edu/justin_holton/5Wresistors.html)

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. :D

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.
:rockon:

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).
http://micknewton.smugmug.com/photos/446747857_azfud-Th.jpg (http://micknewton.smugmug.com/gallery/6980017_tamev/1/446747857_azfud)

[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.

http://home.comcast.net/~kitn13/photos/purple1.jpg

[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. :D

[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 (http://www.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 (http://www.dpchallenge.com/profile.php?USER_ID=16230).

[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. :D
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...................:D


http://www.thefret.net/imagehosting/1154a045a6f730d0.jpg (http://www.thefret.net/vbimghost.php?do=displayimg&imgid=1093)

[Mick]

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...................:D


http://www.thefret.net/imagehosting/1154a045a6f730d0.jpg (http://www.thefret.net/vbimghost.php?do=displayimg&imgid=1093)
Oh yeah, that's a good one!

I also like the “You Won’t Part With Yours Either” ad campaign photos by the late Bob Perine. Cool stuff!

[sevenfingersamurai]

Just recently picked up an old Peavey Deuce as it's the closest thing I have found to a Twin in my price range. Had the same issue described here. I pulled the reverb tank from my crappy 80s Fender Princeton Chorus and plugged it in, problem solved. Then I pulled the tank out of the deuce. It is clearly a new tank, so I was surprised to find that inside it there were blocks of light foam (think packaging for a retail product) in the center of the tank. Pulled those out and the issue was immediately resolved.

My theory here is that the blocks were acting the same way your finger does when you play the harmonic on a guitar string at the fifth, seventh, 12th (and so on) frets of a guitar.

Wish my Deuce were your Twin, but now that it doesn't feedback it is exactly what I was looking for.