1. #1

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    Hey.

    Quick question to maybe avoid visiting the technician.


    When the amp is getting warmed up, it starts to do a "shhhhhhh" sound with random pops. The guitar (plugged in or not) doesn't affect it at all.

    It only happens on the 1st channel. The vibrato channel sounds all good, no trouble whatsoever.
    When toying with the volume knob, it seems the issue appears from "3" and above. When reaching to "10", the issue goes away.
    Also, the 1st channel has a slight hum while the 2nd channel is very quiet even when turning the volume way up.

    I swapped the preamp tubes each way. Also used new ones - no difference. Other than the new tube in channel 1 sounded much better than the old one (probably just old and worn out). But the "shh" and popping sounds stayed the same.

    edit: I swapped the power tubes (6v6) with eachother but the issue remained exactly the same.

    So, that's the puzzle.
    ----------------------

    1. If the issue is one of the power tubes - is it safe to swap them to check out if one of them is the culprit? I mean, they do some biasing there but I don't even have a clue what it is...
    2. Could a faulty volume pot cause this? The amp was unused for many years and I suspect the pots are not NASA quality.
    3. What else could it be? And is it possible to pinpoint it with no knowledge of electronics?

    Thanks,
    Eman.
    Last edited by emanresu; 07-12-2020 at 05:59 AM.

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  3. #2

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    Not an amp expert here but following pure logic: if the problem only occurs in the first channel chances are it is to be found there and not in the power section...

  4. #3
    I thought it too.

    I swapped one new tube with the old one, each of those one by one to see which one would cause it. But didn't get rid of the problem.

    Last effort, I decided to change all those small tubes with new ones. And discovered I only had 1 new ax7 left. So, all I could do was to change 1st channel's at7 and one ax7. And now the issue is gone. Sounds good... so far.
    Can't see how this worked and one-by-one method didn't.
    Tubes are so weird.

  5. #4

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    Typically preamp tubes fail before power tubes. So knowing that going to swapping out the preamp tubes saves time. Also the preamp tubes have specific functions. Typically from the sounds described, which highly common, it is narrowed done to 1 or 2 of the preamp tubes.
    Once you owned a tube amp for a while you learn what to expect. I have enjoyed that part of owning them. You might want to do some research on this amp. I would recommend the Fender Guru site.

  6. #5

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    I have been, before retiring, after 39 years a industrial electronic technician.

    Some of the possible causes.

    Dirty or corroded tube socket.

    Bad solder joint on a tube socket or volume or other control.

    Bad solder joint anywhere else.

    Dirty or corroded control.

    I had a similar issue with a old Wurlitzer electric piano. The cure was to reflow all the solder joints.

  7. #6

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    Agree with the gentleman above.
    I noticed it’s a reissue, I’ve worked on them and as you know they are circuit board construction not point to point. They are prone to poor solder joints so getting in and reforming all joints is not a bad thing at all. Just be careful as those circuit board sockets are very sensitive (flimsy?). Also use as low temp an iron as will flow the solder, those boards burn easily.

  8. #7

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    Be careful about assuming - either channel can make noises and send those though the power stage, independent of which is plugged into or what the controls are set at (including volumes).

    The noise you describe a "shhhh" is often fixed by cleaning the tube pins of corrosion... which happens naturally when you insert and pull them; so often in the process of going through a series of diagnostic problem solving the pins get cleaned and the tubes resume working quietly.

    "And discovered I only had 1 new ax7 left. So, all I could do was to change 1st channel's at7 and one ax7."

    The Normal channel does not have a 12AT7

    The tube layout for the Deluxe Reverb, looking at the back from right to left...


    12AX7 gain stage for channel 1
    12AX7 gain stage for channel 2
    12AT7 Reverb driver
    12AX7 additional gain stage for channel 2 and reverb return amplifier
    12AX7 Tremolo
    12AT7 Phase Inverter

    You can use 12AT7 in place of 12AX7 if you want (some do this for the darker tone, some do this when trouble shooting and don't have a 12AX7). You can also remove one of the first two tubes from the right (the gain stages of Normal and Vibrato channels) and just play though the other.

    The 12AT7 is tougher than the 12AX7, both the reverb driver and the phase inverter work pretty hard... typically those are not good places for the 12AX7.
    Last edited by pauln; 07-12-2020 at 01:51 PM.

  9. #8

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    Quote Originally Posted by BBGuitar
    I have been, before retiring, after 39 years a industrial electronic technician.

    Some of the possible causes.

    Dirty or corroded tube socket.

    Bad solder joint on a tube socket or volume or other control.

    Bad solder joint anywhere else.

    Dirty or corroded control.

    I had a similar issue with a old Wurlitzer electric piano. The cure was to reflow all the solder joints.
    I had a similar issue with a tweed Blues Deluxe - eventually some part failed completely (I'd have to pull the receipt to tell you exactly what, sorry) and it took a trip to the amp tech. Using a good cleaner on all the tube sockets would be a safe and easy DIY possible preventative. If you have soldering skills, then you could try touching up the solder joints.

    I'd wait till the problem recurs consistently or something fails completely before going to the amp tech. Intermittent problems are the hardest ones to diagnose.

    HTH

    SJ

  10. #9
    Thank you so much, people!