opinions, do tube shields really matter?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pfrischmann, Jun 28, 2005.

  1. pfrischmann

    pfrischmann Member

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    Hi guys,
    Still working on the Plexi/bass amp with the oscillation.

    I just shortened the cathode wires,
    Added an additional ground to the tone pots
    moved the neg feedback wire to the 8 ohm tap (was on 4)
    converted amp from '69 tp 68' specs.


    I was playing and the squeal is still there , especially when I push the input with a graphic eq (frown face).

    I realized that the amp never had the tube shileds on it...
    could that cause phantom coupling?

    I jst assumed they were there to block RF.

    Thanks,
    Paul
     
  2. Blue Strat

    Blue Strat Member

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    It's certainly easy enough to try. Let us know.
     
  3. pfrischmann

    pfrischmann Member

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    Hi Mike,

    I actually don't have the shields. that's why I'm asking.
     
  4. Blue Strat

    Blue Strat Member

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    If nothing else seems to work, it's a relatively cheap experiement (less than $2).

    All the theories in the world can't prove what an experiment can.
     
  5. pfrischmann

    pfrischmann Member

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    very true..I guess i'll go get some. I'm just curious as to their original design.
     
  6. KLB

    KLB Member

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    Paul,
    A couple of things to try. Does your amp have the 47pf cap across the PI? If not, try it. If it already does, increase the value to 220pf or more.

    The squealing could be a microphonic cap or resistor.

    You are sure the preamp tubes are good?

    Does it squeal with the amp away from the cabinet?

    Ken Fischer and Greg Germino don't like tube shields, FWIW.

    By the way, I have the Ultimate Attenuator now. You are welcome to come check it out.

    Cheers,
    Ken
     
  7. pfrischmann

    pfrischmann Member

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  8. KLB

    KLB Member

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    Paul,
    I believe the 47pf is usually on the circuit board, but you can put it on the socket. Even with a 220pf, you aren't audibly affecting high frequencies compared to the 5K speaker rolloff. I vaguely recall that the 220pf rolloff is around 16K or so. The bigger the value, the more it cuts, and this could stop the squealing -- assuming of course, that the problem is in the power section.

    If the head squeals well away from the cab, then that pretty much eliminates vibration as a cause.

    Hang in there!

    - Ken
     
  9. pfrischmann

    pfrischmann Member

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    I will try moving the head. Never thought of that. I believ this is true phantom coupling as in Parascitic oscillation.

    I believe the 47p cap you are talking about is the one that feeds to pins 1 and 5 of the PI....It's there...I'll mess with it.

    I believe it has to be the layout or the iron at this point. Eveything else has been replaced... :D

    I should check the input wiring too...
     
  10. Blue Strat

    Blue Strat Member

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    If it's the transformer/output section, the amp will squeal without the PI in. Have you tried this? If it only does this with the PI installed, look at the preamp.
     
  11. pfrischmann

    pfrischmann Member

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    Mike,
    Freakin Genius!!

    I didn't think of that!! Thanks, BTW. there are lots of ghost notes, like another note riding along in the backround....It may be cone cry ('68 basket weave cab) . The cab has never done this before though.
     
  12. stratopastor

    stratopastor Member

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    valve shields - how hard would it be to improvise one, just to see if it improves matters? any small tin can, empty of contents, remove one end, place over valve and make sure the can connects well (electrically) to the chassis. Jab a couple of vent-holes in the can if you're going to leave it in place for more than a few minutes. This all assumes the chassis is well-grounded and you're careful with the can's sharp edges.....!
     
  13. Blue Strat

    Blue Strat Member

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    Ghost notes are most often caused by underfiltering of the power supply in either the preamp or power amp.

    Can you post a schematic. It won't reveal the actual wiring but it could help.
     
  14. tralfax19645

    tralfax19645 Member

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    Hi,
    Sounds like to me it could be component placement/lead dress issue.
    Just something to check any way is to make sure the wires from the PI to power tube grids are not any where near the preamp section of the amp, it will oscillate,and if you are using a post driver MV, keep it as far away from the tone controls as possible. and also make sure the input from you first preamp tube stage is nowhere near the output of the 2nd.
    If they are really close together the output of the 2nd stage will induce positive feedback into the input and sqeal like a pig. It also will give the "ghost note " sounding effect.
    Good luck.
    Rob
    ( if you cant get it, I love to debug homebrews !)
     
  15. pfrischmann

    pfrischmann Member

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    Thanks guys,
    I really appreciate you taking time to help me out.
    The schematic is a Marshall 1959 with split cathode on v1. I can dig one up but it's nothing special.

    Rob,
    I'm going to pull the amp apart in the next day or two and see what's what. I appreciate your suggestion. BTW your amps are really nice.

    Thanks,
    Paul
     

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