Pre amp tubes a bias

Discussion in 'Marshall Amps' started by Chrome, Apr 14, 2021.

  1. Chrome

    Chrome Firebird Freak Gold Supporting Member

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    Got a new to me 1978 JMP 2204 last week. Practiced with it all week, done a show with it Saturday. No issues all week but during the show had a subtle high pitch feed back the hole day. Even if I dropped guitar volume. Only time it would completely disappear is when I put in stand by. Sound guy said sounded like the pre amp tubes ( may as well do all of them) need replaced and rebiased. I guess it’s time I learned to do this shit myself.
     
  2. South Park

    South Park Well-Known Member

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    Getting new tubes is not a bad idea. Might be the guitar getting some feed back try different cables and guitar first . In tube world guitars and cables can become radio antennas
     
  3. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Sounds like a microphonic pre amp tube. I woild start with one known good tube and change out one tube at a time and see which one is culprit. V1 or V2, as V3 is PI it can't go microphonic.
    No bias is required for that and dont change tubes unless they are bad. Just have a spare set and backup amp.
    But try the cable first cause maybe power supply at venue led to a big antennae!
     
  4. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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    Yes, most definitely a microphonic pre amp tube and most likely V1! Get a new high quality tube for V1 and I most certainly bet your issues will be solved!
     
  5. Chrome

    Chrome Firebird Freak Gold Supporting Member

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    thanks guys, I will definitely try that before anything else. I love this amp, it is probably the best sounding amp I have ever played through.
     
  6. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Also, as I think about it, micro tubes should sound worse as gain is jncreased with volume amd maybe top of amp is rapped on. If this does not occur, it could certainly be somethi g else. Does it occur now at rehearsal space too or was it only at venue? That would narrow it down a little.
     
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  7. Old Punker

    Old Punker Well-Known Member

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    I heard that you can do a 'tap test' where after you take any metal shields off the pre amp tubes, you just give each one a tap with a chopstick, pencil, whatever and the microphonic one should make noise when tapped.

    Like @Jethro Rocker said, no need to bias the small ones.
     
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  8. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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    yes, you can...I was just going to mention this test. I've done it before but my reco is dont tap too hard as yes, you will hear the tapping through the amp and thats OK. However, if the tube is bad and you tap on it, it will send the amp into a strange "mircophonic" output sound...and thats where they get the "microphonic" tube labeling from!
     
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  9. Old Punker

    Old Punker Well-Known Member

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    Hey guys, something I've noticed in the past with a few amps...occasionally if I take a high gain amp off standby while it is on the highest gain channel with the master volume on 2 or higher, I can get that microphonic feedback sound for a few seconds. Or if I dial a fair amount of gain on an amp and then hit a boost pedal with its level maxed (and guitar volume maxed), I can get the same micro feedback, and it goes away by just turning the pedal level a little lower. Other than these kinds of rare situations, the same amps do not exhibit any micro feedback tendencies during normal use.

    Is this fairly normal? Or should I be checking tubes?
     
  10. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Sounds normal to me, depending what you mean by high gain amp amd how loud it is at 2.
     
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  11. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    I mentioned rapping on top of the amp cause if it is really bad, that should be enough to get er going and wasieron tubes than tapping on them. I had a xouple micro nad enough I could get it ringing with a flat hand swat on top of the amp but that is with volume up.
    I wonder with OP if it is same issue, with volume right down amd all...would be a bad micro!
     
  12. Chrome

    Chrome Firebird Freak Gold Supporting Member

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    With volume on guitar turned all the way down I still had it, it only went away when I put the amp in standby. I have rehearsal tomorrow, I will give this a try.
     
  13. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Yes do. If it continues there then it must be micro preamp tube. One way you can test is take V3 the one closest to the power tubes out and swap with V1. A microphonic tube is ok on the PI slot, the V3. Then you might know!
     
  14. Max Gahne

    Max Gahne Active Member

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    skimming this thread I haven't seen you mention if you're using any pedals.
     
  15. Chrome

    Chrome Firebird Freak Gold Supporting Member

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    In regards to pedals, not really, I have a volume pedal, a tuner and a wah. That’s it.
     

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