Buy from MF and Support the Fret!


Musician's Friend Stupid Deal of the Day
Page 1 of 3 123 LastLast
Results 1 to 19 of 40

Thread: Fender ProTube Twin-Amp reverb feedback

  1. #1

    Default Fender ProTube Twin-Amp reverb feedback

    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?

  2. #2

    Default

    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!:

  3. #3
    Regular Fretter tot_Ou_tard's Avatar
    Join Date
    Mar 2006
    Location
    Exterra Cognita
    Posts
    5,725

    Default

    Quote Originally Posted by 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.
    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.

  4. #4
    Lefty O'lman oldguy's Avatar
    Join Date
    Apr 2006
    Location
    NW Missouri,
    Posts
    4,086

    Default

    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.
    "you don't play in a sloppy manner and don't overplay when on a Uni; it will pull your pants down and ridicule you without hesitation." abraxas

    Guitars
    Wilburn Versatare, '52 FrankenTele(Fender licensed parts), Fender USA Roadhouse Strat, Charvel Model 6, Agile 3000 w/ WCR pickups, Switch Innovo w/EMG Selects, No-name (Madeira?) Strat copy, Ibanez MIJ V300 Acoustic, Yamaha acoustic, Squier Precision Bass,
    Amps
    Ceriatone Overtone Special,Jet City JCA20H, Musicman 212 Sixty-Five, Fender Super Reverb, Fender Super Champ XD, Traynor YCV-40 WR Anniversary w/ matching 1x12 ext. cab, Epiphone Valve Jr. head w/ combo of gold/fenderish mods, Epiphone SoCal 50w head w/ matching 4x12 cab (Lady Luck speakers), Avatar 2x12 semi-open back cab w/ Celestion speakers
    Pedals
    Danelectro-Daddy-O od, Tuna Melt, Corned Beef, PB&J, Chicken Salad, Hash Browns, Pepperoni, Zoom 505II, Digitech RP250, Digitech Bad Monkey, Digitech Jamman, DVM's ZYS, original Tom Scholz Rockman (w/Rockadapter), Goodrich volume pedal

  5. #5

    Default

    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.

  6. #6
    Regular Fretter tot_Ou_tard's Avatar
    Join Date
    Mar 2006
    Location
    Exterra Cognita
    Posts
    5,725

    Default

    Quote Originally Posted by 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.
    A microphonic tube will be obvious, kind'a the dental hygenist hitting the exposed nerve.

  7. #7

    Default

    Quote Originally Posted by tot_Ou_tard
    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?

  8. #8

    Default

    What should I try next?[/QUOTE]


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

  9. #9

    Default

    Quote Originally Posted by Moshe
    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?

  10. #10
    Regular Fretter markb's Avatar
    Join Date
    Nov 2007
    Location
    Kent, UK
    Posts
    2,356

    Default

    Quote Originally Posted by Mick
    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 :

  11. #11

    Default

    Quote Originally Posted by markb
    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 :
    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.

  12. #12

    Default

    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?

  13. #13
    Regular Fretter markb's Avatar
    Join Date
    Nov 2007
    Location
    Kent, UK
    Posts
    2,356

    Default

    Quote Originally Posted by 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?
    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.

  14. #14

    Default

    Quote Originally Posted by markb
    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.

  15. #15

    Default

    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

  16. #16

    Default Possible cause of reverb oscillation (squeal)

    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 donít think it is a microphonic tube that is the cause.

  17. #17

    Default

    Quote Originally Posted by 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?
    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.

    Quote Originally Posted by Gutmann
    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.

    Quote Originally Posted by Gutmann
    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!

  18. #18

    Default

    Quote Originally Posted by 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 donít 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.

  19. #19
    Regular Fretter mrmudcat's Avatar
    Join Date
    Oct 2007
    Location
    DixieFried in BAMA!!!!
    Posts
    2,502

    Default

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

Page 1 of 3 123 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •