Ampeg V4 HELP! Feedback, squealing

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by battlethesun, Feb 24, 2015.

  1. battlethesun

    battlethesun Member

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    Hey guys, I hope someone here can help this is my first post on TGP. Unfortunately, I'm having some issues with a 70-71 Ampeg V4. I just picked it up a couple weeks ago. Worked fine for the first couple hours I played on it. Sounded great. Still sounds great.. but there seems to be some feedback or interference happening. I power it up, play.. 5 mins in and it just starts squealing, almost popping and the best way I can describe it is a feedback kind of sound. If I tap on the top of the head, it goes away, sometimes. Also, the reverb knob makes a lot of noise.. I just leave it at 0. Sounds goes away. But ultimately over the course of an hour playing this noise happens 5-10 times. Sometimes I can tap the head and it stops, sometimes I have to turn it to standby and back on. That usually does the trick, temporarily. Does anyone have experience with this issue? Any help or advice would be greatly appreciated! Thanks!
     
  2. X–Ray

    X–Ray Member

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    Had four of them back in the early '70s but never experienced this problem and ran them hot and loud...

    Good luck!
     
  3. battlethesun

    battlethesun Member

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    It is DEF hot and loud haha. Thanks.
     
  4. Rockinrob86

    Rockinrob86 Supporting Member

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    It sounds like you have a microphonic reverb pan, or a tube. If it's a tube, you can gently tap them all with a chopstick to see if you can cause the noise to happen. If not, it is probably the reverb pan, but it sounds like you are not a tech, and I wouldn't recommend opening up this amp. The voltage is extremely high in these, over 600 volts even when off!

    Unfortunately, it could also be a signal cap in the amp. These used unique small caps with pretty weak legs, and are PCB construction. The caps can be fragile, especially 40 years on.
     
  5. Bruce Gerard

    Bruce Gerard Member

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    Your V4 could have more than one issue if it hasn't been refreshed by a competent solder monkey. The popping and squealing could be a tube or a failing solder junction somewhere in the amp. I say this because when you pound on it with your hand it will behave. Things like this are usually a mechanical problem (bad solder, tube failing, dirty socket, cracked component, etc. ) You do need to find out if some of the parts in the amp have been updated or not. Some of the capacitors in this need to be replaced if they are the original ones.

    The small electrolytic capacitors should have been replaced by now, or it needs to be done. Also, there are some horrible little coupling capacitors in there that look like chicklett gum with colorful stripes. These are called "tropical fish " caps in the tech vocabulary. If it has them still - they need to be replaced. These caps break down and get leaky. They can cause all sorts of screwy things to happen. These parts are located under the sheet metal covers that the small tubes pass through. They are all on printed circuit boards. The resistors need to be checked as well. I don't know your level of electronic knowledge, so I'll just say that unless you are sure of what you are doing - you can get yourself in a lot of trouble with these amps. The votages inside these amps can kill you.

    Hope this helps you figure it out.
     
  6. zenas

    zenas Member

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    Amp needs a tech. Maybe if say where you're at someone will know one near by.
     
  7. battlethesun

    battlethesun Member

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    Yeah believe me I have no experience with the inside of tube amps. And this one won't be my first haha. Thanks for the responses! I was hoping someone would suggest an easy fix. When I first bought it the guy told me it just came back from a tech and he replaced some things, the tubes are definitely new. But like I said I'm not opening this beast up! Perhaps I'll try tapping the tubes, I know the caps "look original" Anyways, thanks guys, I appreciate it.. amp is in beautiful shape, I'll get it to a tech when I can afford it lol.
     
  8. zenas

    zenas Member

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    Oh if you've worked on amps that's diffiernt. Then you already know that the old "just checked out by a tech" line from a seller to a buyer is a worthless line of crap.

    The only way to tell if the tech did his job or hacked it is to open it up and and look. I do that before I fork over the cash on vintage amps. (unless it's so cheap I don't care)

    If you don't want to open it up try subbing some tubes in it you might get lucky. New tubes go bad so new means nothing.

    If it's got original electrlytics I would replace them all. Old electrolytics aren't worth messing with.
     
  9. battlethesun

    battlethesun Member

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    So, I tapped the tubes while the amp was hot.. and the 6cg7 which I believe is the reverb driver? I pulled it out. Sounds great! No more noise? So I'm going to get a new 6cg7.. Does this hurt the amp in anyway to run it without this tube?
     
  10. Rockinrob86

    Rockinrob86 Supporting Member

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    If you take out the tube, the reverb circuit will no longer work, which makes it sound like it is probably the reverb tank and not the tube.

    If you tapped the tube, and that caused the oscillation to happen, then it is most likely the tube.

    The reverb pan can be replaced, but that requires opening the amp up, and you should know what you're doing with these amps.
     
  11. Bruce Gerard

    Bruce Gerard Member

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    As per above - if the tube is breaking down internally, a replacement will hopefully be all that's needed. If this has the original reverb, it has a lock on it for transporting. The lock is at the top rear edge of the right side (when facing at the BACK of the amp) It's a metal tab thing that almost looks home made. Push it inward and to one side and it should pop out 3/4" or so releasing the reverb springs.
     

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