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  1. #1

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    If I put on my SFVR to play some swing/bebop with an archtop, the sound is perfect. Low note are a little over-present but OK. After a few minutes, it looses dynamics and sounds a little harsh and stiff with lows disappearing. After half an hour of playing it still is not great. But If I leave it on (standby) or play it for at least a few hours, f.i. during sessions, it starts to sound very dynamic and fine. I had the same when I played a 64 Tremolux. I thought that it could be the power tubes getting old, but maybe it is normal?

    Do you recognize this? Thanks!

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  3. #2

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    Disappearing lows is a sign of power shortage. Rectifier tube is also a candidate.

  4. #3

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    If you play for 2-4 hours a day, expect to retube once a year.

    Also, sorry, what's SFVR ...? Some Fender Vintage Reissue?

  5. #4

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    Quote Originally Posted by starjasmine
    If you play for 2-4 hours a day, expect to retube once a year.

    Also, sorry, what's SFVR ...? Some Fender Vintage Reissue?
    Silverface Vibrolux Reverb. Fender secret code.

  6. #5

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    Dying power tubes or/and rectifier tube, as said above.

  7. #6

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    Does any background noise change along with the loss of dynamics after initial warm-up?

  8. #7

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    No background noise at al.

    Power tubes are two years old (JJ's), I played not enough to wear them out, I reckon, but playing a lot of 4-to-the-bar comping withe low E and A strings may have wear them out faster than playing in blues band? Can that be?

    Power transformer gets really hot after sessions...Maybe that indeed could be part of the problem?

    Thanks for your thoughts...very helpful

  9. #8

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    ...rectifier is fine, I put another in and it didn't solve the problem

  10. #9

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    My first instinct would be tubes and check the bias. Second would be power supply.


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  11. #10

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    90% of a tube amps problems are tube related.

  12. #11

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    tube rectifiers are basically pass-fail...they work or they are gone..like a lightbulb!

    could be power tubes..i love jj's...but they don't last like vintage tubes...could be one is faulty...try cleaning the tube sockets too

    luck

    cheers

  13. #12

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    What you are describing suggests that one or more components is changing its value or characteristic as your amp heats up. In addition to some ideas above, it could also be:
    -a very hot ( too hot to touch) power transformer suggests leaking electrolytic capacitors. If the are original caps in a SFVR, they are almost certainly leaking and drawing extra current.
    -a change on tone or volume could be due to varying bias, and also to old power tube grid leak resistors heating up and changing their value, which will change the sound if the resistors go too far out of spec. This has happened to one of my amps
    -if you are leaving the amp on standby for some time, this can cause cathode poisoning of power tubes.

    There could be other causes too. If a SF amp hasn't been serviced, it is almost certain that some components are out of specification by now. If you have tried changing all the tubes, including preamp and PI tubes, and that hasn't fixed it, take it to a good tech who can check all voltages, cap ESR, nominal component values etc, against the circuit. There is not a lot of point in trying to trouble shoot a SF amp that hasn't been checked and serviced, and any failing components changed as a matter of routine. It could be almost anything.

    I think it's unlikely to be the rectifier tube. They work, or they don't.

  14. #13

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    The electrolytic caps are replaced two years ago. They should be fine.

    The JJ 6L6 power tubes were replaced one year ago. Not two years ago as I stated above. I'm not playing so much and I guess they shouldn't be worn by now.

    Rectifier is not the problem. And all 12AX7 and 12AT7 tubes are checked and fine.

    That leaves -as far I can see- a not properly working power transformer. That's what some of you also suggest. My tech has checked all components and could not find a problem with the PT...

    Still a mystery...I'll check again with my tech when he's back from holiday.

    Thanks again for your comments. It is very much appreciated

  15. #14

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    Well, it would be unusual ( but not impossible I suppose) for a PT to have a heat-related fault; like rectifiers, they usually are fine, or they fail.
    Maybe ask your tech to check the voltages when it is still cool, and then after a couple of hours when it is hot. That should give a clue. A too- hot power transformer
    means that something, somewhere is drawing too much current.

    If nothing has changed during that test, and the bias stays stable, maybe time to look at the speakers? It's just possible that a failing or part-shorted speaker voice coil could cause the power tubes to draw too much current due to Output T impedance mismatch, thus heating up the PT. Admittedly it's a long shot, but just possible.

  16. #15

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    have all the power tube/socket resisitors checked..screen grid resistors etc...they can be way out of spec without actually burning...yet


    cheers

  17. #16

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    Last week I heard from time to time a crackling noise for a short instance. Then I cleaned the left power tube pins and put some contact spay on them. Problem was gone. Sound was there again without the deterioration of sound which was occurring from time to time. It may be that the amp sometimes runs on one output tube... I had this problem incidentally for some time now but I guess I found the solution. If the contact spray is not solving the problem on the long run, it might be a faulty tube socket, or the connections of the wires/resistors, like was suggested above.

    Tomorrow at a session I'll see of the amp stays OK. Otherwise, I'll have my tech have a look at the power tube sockets, and have them maybe replaced. Thanks a lot for thinking along!