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Reverb squeal / feedback

Discussion in 'Amp Tech Center' started by timbraun, Jul 10, 2013.

  1. timbraun

    timbraun TDPRI Member

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    Hello, folks

    I just finished re-capping my '65 Deluxe Reverb and now find that the reverb goes into feedback if turned up to 5 or so. I really don't expect a Fender amp to be unstable at all.

    I haven't started trouble-shooting this - I just wanted to play the thing last night. It's a nice little amp.

    Possible problems to look for?

    1. V3 Reverb drive tube (it's a new JJ ECC81, but maybe I'll put an old 12au7 in there). This failure might be due to too much gain in the drive or recovery circuit. But it was designed for the ECC81 type tube, so not likely the issue.
    2. Dirty reverb in/out jacks or tube sockets. Not sure how that would cause this. I should do a little cleaning though, I noticed some pots were pretty crackly.
    3. Reverb tank plugged in backwards. I thought that would give me no reverb output.

    The old bypass cap's were dried out so it's been a while since this amp has seen proper gain staging. The reverb circuit is a feedback circuit, so not too surprising that some failure modes cause it to become an oscillator.

    Just for interest's sake, here's the Deluxe Reverb glowing behind my latest Tele. :)

    Any ideas on what I should look for?
     

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  2. Chritty

    Chritty Tele-Afflicted

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    I was in a repair store the other day waiting to pickup a couple of pedals I had had serviced when a guy from a local music store came in with a customer's brand new Fender Reverb amp. I started eavesdropping on their conversation. Apparently the reverb squealed like a pig and the customer was irate!!

    The owner of the repair store said that it was luckily usually an easy fix. He said that the reverb tank is too close to the speaker. It basically starts off a chain reaction that causes a feedback loop. His fix? To strategically place rubber bump stops in the correct places.

    This may not be a solution to your problem but it very well could be. Hope it helps
     
  3. Mat

    Mat Tele-Meister

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    Chritty has nailed the most likely culprit given the age of the amp.

    I recently had this problem with 2 Goldentone amps (early 60's) which fed back like a teenager going back to the fridge!

    The isolating rubbers had both hardened to the point they were basically solid washers.

    Grabbed some grommets from the electrical store and put those underneath and problem solved.

    Sounds simple but before that I was also looking at gain stages, biasing replaing valves checking voltages etc - ah the road travelled to knowledge.

    Hope this fixes the issues for you.
     
  4. DSharp

    DSharp Tele-Holic

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    Did you put the new JJ tube in V3 as part of the recap? If so, I'd try a different tube in there first. Good luck!
     
  5. BottyGuy

    BottyGuy Tele-Meister

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    Assuming it didn'r feedback before the cap change I would look at the connections from the reverb tubes all the way to the tank. I had a an amp that I thought was isolation related and found that one of the solder connections to an RCA jack was not making a good contact and when the reverb was turned on it would shake loose and howl.
     
  6. BottyGuy

    BottyGuy Tele-Meister

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    Also if you can get it to feedback you should wiggle the cables at the connector and tank while it's feeding back to see if you can make it go away. Also try putting your hand on the tank to see if that makes it worse or better which might indicate an isolation issue.
     
  7. telex76

    telex76 Doctor of Teleocity Ad Free Member

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    Check this by pulling the tank out of the cabinet but leave it plugged in. If it doesn't squeal when out, it may need to be wrapped a little better.

    I've got an original 65 DR that I put a speaker with a larger magnet in and it squealed. I put some rubber under the screws holding the tank in place and that stopped it.
     
  8. honeycreek

    honeycreek Tele-Afflicted

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    My (then) brand new Kustom V100 squealed with the reverb turned up. I took it to a guy that was an authorized repair person. He basically wrapped the reverb tank in something like carpet/fuzzy amp covering. He made the comment "that is what we did back in the day with Fender's" and it "worked back then".
     
  9. timbraun

    timbraun TDPRI Member

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    I swapped V3 from the JJ back to the original RCA 12AT7 and the squeal is the same. Unplugged and plugged in the RCA cables for the tank. Tried the tank backwards.

    Time to pull the tank out of the bag, I s'pose. Ooh, I should try an external speaker. That would reduce the acoustic coupling from speaker to the reverb tank. I do like tests I can do without a screwdriver or soldering iron.

    I don't actually recall if it was squealing before the re-cap - but the whole amp didn't have much gain, the bypass caps were dried out. I wonder if lead position on the caps is significant? I doubt it - that would be a capacitive coupling issue. Mechanical or acoustic coupling is more likely.

    Thanks, Bottyguy, Telex76, honeycreek and other folks! I'm sure I'll get this. Acoustic coupling to the tank... that's my new theory. Maybe I moved the tank in the bag while pulling the chassis out or when I had the cabinet on it's side.
     
  10. brockw

    brockw TDPRI Member

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    Try replaceing V4 12ax7. My brand new 65 DRRI did the same out of the box and it was V4 GTECC82S 12AX7(JJ Solovak). Replaced with a Tung-Sol 12AX7, V4 is 1/2 reverb recovery and 1/2 gain for channel 2. Worked for me!
     
  11. timbraun

    timbraun TDPRI Member

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    Well acoustic coupling isn't it. Using an external cab made no difference to when the feedback starts on the reverb knob. I'll try swapping out v4, I had swapped v3, the reverb drive tube.

    On this circuit, v4 is indeed reverb return and another gain stage for the vib channel. With a shared cathode connection. What does that do? If there's a good bypass capacitor creating a virtual ground for the signals, it's irrelevant to the signal gain. That'd be one of the bypass caps I replaced.

    I think I will be opening the amp back up to fix this. First - v4 swap.
     
  12. timbraun

    timbraun TDPRI Member

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    Swapped v4 for an old made in Japan 12ax7 I have kicking around from who knows what. Realized I had already tried swapping v2 for v4 since there was one of those new Russian mullards in there. Didn't make the problem go away, but does move the onset of oscillation from 3.75 to 4 on the reverb dial.

    Really, does a blackface deluxe let you turn the reverb all the way up? Time to fire up my Garnet and my Traynor and twist some knobs.
     
  13. timbraun

    timbraun TDPRI Member

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    Neither my Garnet nor my Traynor amp gets unstable when the reverb is cranked. I didn't think they did, but who makes a habit of cranking reverb? Garnet Jammer I've had since '77. Traynor Custom Reverb head from '73 or so I found a couple of years back. I love tube amps.
     
  14. Andy B

    Andy B Friend of Leo's Silver Supporter

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    Lead dress, bad grounds, bad caps, bad reverb tank. These are the things I would look for.
    New caps can be defective too. Make sure cap polarity is correct.
     
  15. Chritty

    Chritty Tele-Afflicted

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    Any joy yet?
     
  16. bwacke

    bwacke Tele-Meister Gold Supporter

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    Based on what you've reported so far, I'm with AndyB. Look at the polarity of the cathode bypass caps for the driver tube and the return amp. Then drop the caps off the cathodes one at a time and see if your problem goes away. If it does, tack a new cap of any value over 10uf in place of the one you disconnected and check the operation. A shorted or reversed cap fools with the bias and keeps the gain high.

    Bob
     
  17. Foxy229

    Foxy229 TDPRI Member

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    A very common issue in older Fender Reverb amps. Try replacing the bypass cap at V4 (1/2 of this tube is the reverb recovery). I fixed many of them that way.
     
  18. Rifflicks

    Rifflicks TDPRI Member

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    Speaking of Squeals....can anyone please tell me why .... with all the knobs at zero on a 1968 Princeton Reverb that if you turn the Tremolo Intensity knob past 5 it puts out a Loud, High Pitched, Pulsating Squeal, even with volume down all of the way?
    I don't know where to look.
     
  19. Foxy229

    Foxy229 TDPRI Member

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    I answered that for you on another post, but here it is again:

    Not too certain about that. I would lift the negative feedback resistor just to make sure you are not getting parasitic oscillation. This is caused when the output transformer primary windings are reversed. I had a lot of bad luck with the color code of Hammond transformers having the blue and brown primary winding wires opposite of the Fender original transformers. The oscillation will often sound like a bad buzz and even a very high pitched noise, much like the tremolo issue you have stated.
     
    Lowellg likes this.
  20. timbraun

    timbraun TDPRI Member

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    Hey, guys. Thanks for the continued discussion and ideas. I went off on a vacation for a couple of weeks and haven't had a chance to open 'er up yet. I'm suspecting the cap's I re-capped - maybe I put one in backwards. The reverb circuit wiring is a little different from the pre-amp audio path - maybe I got fooled.

    No DR joy yet, but I had a good time...
     
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