Really WEIRD reverb issue

Discussion in 'Amplifiers' started by godwentpunk, May 18, 2020.

  1. godwentpunk

    godwentpunk New Member

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    I apologize for the long post....

    I noticed a couple months ago my amp was producing a hum that would slowing come and go. About 3 weeks ago the hum got noticeably louder and I tracked it down to the reverb. Regardless if the reverb was engaged or not, I would hear the hum and the only way to make it go away was to turn the reverb knob all the way off. So I decided to see if it was a tube issue. I took the back off, replaced the tube, put the back on and I still got the hum... so I decided to try a different tube... and OMG, the hum went away. So I screwed the back plate back on and to my amazement, the hum came back... WTH??? So while the hum was going, I unscrewed the back plate and as I slowly removed it, the hum went away.. as I brought the back plate back up, the hum came back.... What could be going on that the back plate is causing my reverb to act up?? Could it be some frequencies that is causing something to react weirdly? This is first time I've ever encountered this so I was hoping that maybe someone else heard of this happening before. Could my reverb tank be going bad??

    thanks for any insight.

    The amp is a PRS Custom 20 combo


    joe...
     
    #1 godwentpunk, May 18, 2020
    Last edited: May 18, 2020
  2. DreamTheaterRules

    DreamTheaterRules I'm sailing

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    What amp are we talking about?
     
  3. godwentpunk

    godwentpunk New Member

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    I updated my post...

    The amp is a PRS Custom 20 combo
     
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  4. DreamTheaterRules

    DreamTheaterRules I'm sailing

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    My first thought is, you have the amp near something (computer maybe) that is causing the hum, and for some reason the screen is acting like an antenna and picking it up... I don't know. I'll look at a pic of the back of that amp later when I get home.

    Have you recently moved the amp, or any potential noise source, that could be the cause of the noise?
     
  5. godwentpunk

    godwentpunk New Member

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    The amp is in the same position that it's been the last year... I'm going to try and clean the contacts on the reverb tank to just make sure that it's not something with that and maybe with the back on, things "reverberate" more causing this issue. I just played the amp for 30 minutes with the back off and it's all good... but I think I can remove that top screen to see if that make a difference. I don't gig with the amp so if removing the screen fixes things, that I'm good with that.

    thanks again for this idea.

    joe...
     
  6. andy474x

    andy474x Knows the Drill

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    Funny...

    I got my Custom 50 head second hand, and the reverb tank had been repositioned - previously had been mounted running lengthwise along the bottom of the head shell (chassis is on top), but appears to have been moved to be on an approximately 30 degree angle. I’ve tried moving it back, but it catches a lot of hum from one of the transformers, so I assume that’s why it was moved in the first place.

    I had also developed some other noise, which I think was coming from either the reverb tube or circuitry, or the phase inverter, sent it to the tech and he said it was noisy tubes along with low power tube bias. Of course, then the quarantine started, so it’s been at the tech for almost two months. I’m a little skeptical, because I had already tried new tubes, but we’ll see. On the Custom heads, you can go in through the front or back, and I hadn’t tried the back, I’ll have to see, if there is still noise, if the back cover makes a difference.
     
  7. godwentpunk

    godwentpunk New Member

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    So I removed the grate from the back.. still have the hum... Next I replaced the G12 Century Vintage Alnico speaker I had in it for the OEM speaker and hum went away... I can't believe a speaker would do this but who knows... maybe by virtue of me laying the amp down to replace the speaker put some loose part into place?? who knows... I'm going to see how long this works and if the hum goes away, I'll try to put the G12 back in and do some further testing,

    joe...
     
  8. JJJ

    JJJ asleep

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    tube amps can be sensitive to emi, phones, lights, mains noise, etc. It may not necessarily be a problem anywhere on the amplifier, not in the reverb or with any tubes, it's just picking up noise from somewhere that is exacerbated when you put the back on. Can you post a photo of the back plate? Is it shielded? It might be reflecting interference from somewhere.

    Try taking to amp to other rooms and seeing if the problem is the same.

    It could maybe be a design thing where the back reflects noise from a transformer or who knows.

    I'd only be concerned about hum if it was mains frequency hum that was super loud. Every amp I have owned picked up interference
     
  9. godwentpunk

    godwentpunk New Member

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    Ever since I replaced the G12 Century Vintage Alnico speaker, the hum has gone away. In fact, I've put in 2 different speakers and played each for about 30 minutes without any hum. I'm going to try later today to put the alnico speaker back in to see if the hum comes back. If it doesn't, then the only thing I can think of is that putting the amp on its side nudged some loose part into place... and maybe after a time of playing it, it shakes loose. I'm pretty sure it's something with the reverb so maybe I'll try and get a new reverb tank since I know the springs in that move, somewhat. I'll just need to figure out what tank to get.

    thanks again for everyone thoughts and advice, I can't thank you all enough.

    joe...
     
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  10. Tahlee

    Tahlee Tollywood Rhode Island

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    Good luck, Joe.
     
  11. Guitargonaut

    Guitargonaut Groovin' on a killer stump...

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    Could it be a case of the reverb wires being moved or reoriented?
     
  12. Em7

    Em7 deus ex machina

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    The G12 Century Vintage has a neodymium magnet, not an alnico magnet. I have a pair of these speakers. Neodymium magnets have the strongest magnetic field of all magnets. That is why neodymium speakers are much lighter than ceramic or alnico speakers.

    With that said, I have never encountered a non-field coil speaker that caused hum. I am willing bet that the problem lies in the circuit. The back panel and speaker are merely symptoms. Does the hum sound like 60 cycle buzz or is it a higher pitched hum (120 cycle)? The 2-Channel effects loop is notorious for hum. The Custom is derived from the 2-Channel.
     

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