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Thread: Problem with microphonic tubes that are brand new.

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    Question 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.

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    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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    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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    Don't forget the joker g1's Avatar
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    Does the howl have anything to do with the reverb?

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    "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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    Old Timer Tom Phillips's Avatar
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    Quote 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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    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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    Tom, there were only two tubes still with the amp and they were not usable.

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    Potentially the howl was there before the amp was "parked" ?
    Can you try with the NFB loop disconnected?

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

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    Senior Member trobbins's Avatar
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    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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    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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    Supporting Member The Dude's Avatar
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    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.

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    Supporting Member Jazz P Bass's Avatar
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    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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    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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    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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    Came across a similar problem in an old Gibson that had a bad ground to a volume pot.

    It would sound like every tube was ringing, then be fine when the intermittent connection was making contact.

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    Okay, I have isolated the problem though I'm not sure what to do. I have a lamp current limiter that is plugged into a variac. Whenever the amp is plugged into the current limiter it is fine. If I plug it into the variac or any other outlet, it starts to howl. Turning the variac slightly down does not help. Interestingly, the three-prong conversion was done on this amp but the death cap was still connected, so I just clipped it out. What is my current limiter doing to mask this problem?

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    Last edited by jeffects; 12-23-2014 at 10:30 PM.

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    Did you try the NFB disconnect? I'm pretty doubtful about it but would like to eliminate the possibility of incorrect OT phasing.

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    "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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    g-one I haven't tried that yet. I got sidetracked when it started working properly. I definitely will tomorrow and let you know.

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    I'm also going to go back and check voltages with it plugged straight into the variac.

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    The current limiter limits the current,the variac lowers the voltage.Dont want to sound like a broken record,but the problem is current related and is most likely power supply related,filter caps.

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    Yes, what stokes said should be the first thing to try:
    Quote Originally Posted by stokes View Post
    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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    "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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    if I read correctly op said he replaced all the caps however, first thing I do when I get any amp 20+ years old or older is replace EVERY electrolytic cap in it regardless, they don't need to be puffy or leaking to be bad ( they could have just dried up and yes it happens I have seen it several times last was with a peavey VTM120 head).

    I personally have found more instances of bad electrolytic caps causing howl, hum, hissing, and buzzing than anything else. the other causes I have found would be cold solder joints ( you might call them bad :P) which are easily identifiable by dull crystalized looking solder, bad grounds, and faulty jacks/bad cables.

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    He said every cap EXCEPT electros.

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    see post #3.

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    "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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    Okay guys, problem solved. It seems that drewl nailed it. Had nothing to do with tubes or microphonics. The volume pot was not properly grounded and the hum caused the entire chassis to vibrate. Literally every component in the preamp section became microphonic. Could have caught this a lot sooner had my current limiter not masked the problem while on the bench. (Still not sure why that was the case.) Anyway, I resoldered the ground to the volume pot and also resoldered the wires coming off the preamp tube to the input and volume because they looked suspicious as well. It's now one of the cleanest amps I ever played through. Thank you so much for your input guys. This business is a never ending learning experience.

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    Supporting Member Jazz P Bass's Avatar
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    Glad you got it.
    (post #13)

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