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Problem with microphonic tubes that are brand new.

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  • Problem with microphonic tubes that are brand new.

    I just overhauled a vintage Fender Princeton Reverb (not a reissue) that was found in an attic and hadn't been operated in years, and of course replaced all of the tubes. It tests just fine on the bench but when I put it back in the cabinet, the preamp tubes become highly microphonic to the point where it just howls. I've tried swapping them around to no avail. These are all JJs. The power tubes are Tung-Sol and are just fine. The original used a 7025 for the input but I'm using a JJ ECC83S for economical reasons. Is this a tube problem or something else? Should I try different tubes or a 7025? Any help would be appreciated.

  • #2
    You say you "overhauled" the amp,did you recap the power supply?If the preamp caps are not up to par,they could cause oscillation problems.I have found JJ preamp tubes to have a higher level of microphonics,but it usually occurs in high gain circuits,not an amp like a Princeton,not saying it cant happen in a Princeton,its just more likely to happen in high gain situations.

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    • #3
      I have replaced every cap in the amp with the exception of the filter cap, which is a multi-section can type that tested okay with my ESR meter.

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      • #4
        Does the howl have anything to do with the reverb?
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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        • #5
          Originally posted by jeffects View Post
          I just overhauled a vintage Fender Princeton Reverb (not a reissue) that was found in an attic and hadn't been operated in years, and of course replaced all of the tubes...
          I don't know why you said "...of course replaced all of the tubes..." Was there a specific reason behind that? Consider that some, many or maybe even all of those old tubes may be better that the new ones you installed.

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          • #6
            No, I've unplugged the reverb and it still persists. Again, I can remove the chassis from the cabinet and it will be fine. I did put a new speaker in that's rated at a much higher wattage. (The owner wants a very clean sound.) I don't know if that would have anything to do with it. There's also a noticeable rattle from the tubes when you first turn the amp on.

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            • #7
              Tom, there were only two tubes still with the amp and they were not usable.

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              • #8
                Potentially the howl was there before the amp was "parked" ?
                Can you try with the NFB loop disconnected?
                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                • #9
                  g-one I will try that tomorrow when I get back to my shop and tell you what happens. Thanks.

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                  • #10
                    If you can eliminate all other generation points for the microphonics, and just be left with the input valves then that would be a start. Perhaps remove one of the input valves during testing to avoid it's influence. If you had a variety of other 12AX7 or variants (or even a 12AY7 or 12AT7 for a quick test), then swapping them would likely give you an idea of what a 'good' valve will achieve. All valves are microphonic, and NOS doesn't guarantee a good valve, however a good size population of old valves should have one that is not too bad to use for comparison. It may be that even with a good input valve, your speaker and the existing mechanical paths are too much, and so you would need to do something subtle in a mechanical way.

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                    • #11
                      Thanks trobbins. Right now I have no extra tubes to try in place of these but the fact that I need some is unavoidable so I will get some on order. I'm not sure what I can mechanically do once the chassis is back in the cabinet. Any suggestions on that?

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                      • #12
                        Can you narrow it down to a particular stage? I have a hard time believing every tube is microphonic. Tap lightly around the chassis and tubes to figure out where it's coming from. Then figure out if the problem stays with the tube or socket by swapping tubes at that location.
                        "Yeah, well, you know, that's just, like, your opinion, man."

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                        • #13
                          It may be that tightening the chassis screws is affecting a solder joint.

                          I would try reflowing the chassis ground solder joints.

                          Note: have you removed the 'death cap'?

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                          • #14
                            Good advice Jazz. Thank you for mentioning the "death cap" because it's still in there! I'll do the conversion tomorrow.

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                            • #15
                              You say the amp sat in an attic for years.I wouldnt try troubleshooting anything till I changed those filter caps.30+ year old caps in an amp that is used regularly are too old,in an amp that has sat unused they are dangerously old.A bad decoupling cap would cause the problem you describe and should be the first step.If you have a spare 10,16 or 20uf cap,clip it across the appropriate stage and see.

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