Input grid resistor?

Discussion in 'Amp Tech Center' started by dougstrum, May 5, 2021.

  1. dougstrum

    dougstrum Friend of Leo's

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    I have a Pro Jr first edition. Got when new and have done mods and maintenance.
    It's a great amp except~

    The one problem it has is picking up external noise. At home it makes scritchy noises when my wife is sending or receiving messages. At a gig if I am on same circuit as kitchen, it picks up momentary noise when blenders or other kitchen equipment is run.

    The input grid resistor is a 10k, on my home built amps I use a 47k and I never pick up any extraneous noises.
    I wonder if a larger resistor helps block that kind of interference?
     
  2. Jon Snell

    Jon Snell Tele-Afflicted

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    Mains borne interference is difficult to stop except at the source.
    Try your antiparasitic resistors and also fit a common mode choke, with a filter network, between the mains input and the transformer.
     
  3. DADGAD

    DADGAD Friend of Leo's

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    Do you plug the amp into a surge/noise suppressor power strip?

    Does it happen with no instrument plugged in?

    Just thinking of eliminating external sources first.
     
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  4. dougstrum

    dougstrum Friend of Leo's

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    Power strip we use has emi filtering. Funny thing is the Pro Jr is the only amp I've got
    that picks up emi; phones and small kitchen appliances are the things I've noticed.

    I know a larger input grid resistor helps block picking up radio stations, and wondered
    if it also helps reduce phone/appliance interferance.
     
  5. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Unshielded tube socket?
     
  6. dougstrum

    dougstrum Friend of Leo's

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    No shields on pro jr preamp tubes, but I've never had other amps give me trouble without shields.
     
  7. printer2

    printer2 Poster Extraordinaire

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    Won't hurt to go up.
     
  8. dougstrum

    dougstrum Friend of Leo's

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    I could clip a 68k or so parallel with the 10k resistor, then turn on the amp while my wife is texting, that would certainly give an idea of whether it snubs noise entering the circuit.
     
  9. printer2

    printer2 Poster Extraordinaire

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    In parallel it lowers the resistance and gives the amp less EMI rejection. You need to put the resistor in series with it or replace the 10k with something higher.
     
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  10. Paul G.

    Paul G. Friend of Leo's

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    In parallel you'd now have 8.7kohms. Swap or series.

    If you don't want to do the maths, https://www.omnicalculator.com/physics/parallel-resistor
     
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  11. dougstrum

    dougstrum Friend of Leo's

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    I know if equal value resistors are in parallel it halves the resistance.
    But with unequal values~the mathso_O

    Best to just swap in a larger value~
     
  12. tubedude

    tubedude Tele-Holic Silver Supporter

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    Upping Rg is good. But is the resistor at the tube pin, or is it on the jack or a circuit board. It's most effective at the tube pin.
    The input is one source of entry. Through the A.C. mains is another. Is there an input filter? If not, can you install one?
    Is the chassis screened/sheilded?
     
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  13. dougstrum

    dougstrum Friend of Leo's

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    Pro Jr is circuit board amp with ribbon connectors; so resistor not direct to tube.
    It's single input amp~jack with to 10k to grid and 1meg from grid to ground typical
    voltage divider except 10k seems small. I generally use 47k

    Diode rectifier, no choke just strait to filter caps. I doubled the first filter cap from 22/500 to 47/500.

    Chassis is shielded top, bottom, front and back, but the ends are open about 2"x 4" opening at each end~
     
  14. the fatch

    the fatch Tele-Meister

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    A 10k grid stopper, along with the Miller capacitance of approx. 200pF is letting frequencies under around 100kHz into your amp. Try a 470pF cap between grid and ground, which should ‘stop’ anything over 24kHz or so. A 680p will cut under about 18k.The 34k (two 68k in parallel) in a Fender high gain input cuts at around 23k.
     
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  15. LightningPhil

    LightningPhil Tele-Meister

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    24kHz! I can only dream of 12...

    But for the input to a guitar amp, is there much point allowing anything over about 6 through?
     
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  16. Mowgli

    Mowgli Tele-Meister

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    Something here seems inconsistent. I believe cellphone signals transmit on a different (much higher) frequency than line signal (60 Hz). Unless all the noise are exact or close harmonics of each other there may be two separate issues.

    Try this: 1st place your wife's cellphone close to the amp's input without a cord inserted and send a message. Repeat but with a cord inserted and stretched out with your guitar plugged in. Then move the cellphone about 6 inches from the pickup that is selected.

    Any change in the noise level?

    This may allow you to narrow down the source of one of the noises. If the noise increases with the latter two then the issue may be with cord shielding. If it's much louder when closer to your guitar, consider guitar shielding issues.

    If you recently changed to 5G service, it may be related to the change in signal strength and change in frequency range (bandwidth).

    Occasionally I will change TV channels with my remote while playing a guitar plugged in. If I hold the remote close to the guitar while changing channels it will make a scritchy noise (a great description of that sound, BTW!).
     
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  17. Mowgli

    Mowgli Tele-Meister

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    General rule (IIRC): When two resistors of different values are in parallel, their parallel value will never exceed the value of the lower resistor.

    I think I am remembering that rule correctly.
     
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  18. the fatch

    the fatch Tele-Meister

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    Or if more convenient, fit the cap between grid and cathode on tube socket.
     
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  19. the fatch

    the fatch Tele-Meister

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    With a calculator, clear the memory. Enter first resistance, take its reciprocal (1/x) and add it to memory. Enter second (and subsequent) resistance(s), take the reciprocal and add it to memory. When both (or all) of the resistances have been entered, recall the memory, take the reciprocal which is your answer.
     
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  20. dougstrum

    dougstrum Friend of Leo's

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    @the fatch you must be a maths genius!
    First level calculus was like a brick wall for me:eek:

    Does the cap tend to bleed off treble frequencies?
     
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