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 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!:

[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.

[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]

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!

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:

[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?

[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!

[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]

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.

[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.