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Can Guitar Noise Be An Amp Issue?

Discussion in 'Amp Tech Center' started by gabasa, Jan 20, 2021.

  1. gabasa

    gabasa Tele-Meister

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    Hey Everyone,

    I built a wicked sounding Harvard recently and am very, very happy with it. I made a couple small changes to the circuit, including using a ClassicTone choke. I'm using a very warm speaker with the amp (too warm!) so I added in a small bright cap across the volume pot, which will stay there and get removed if ever I put in a better speaker match for the amp (like the 1028k). In short though, I'm very happy with the sound of this one and it's every bit as gorgeous sounding as the Champ, Deluxe and Bassmans I've built in the past, but with it's own special, Harvard character.

    I've noticed more guitar noise through this amp that I'm used to. It's very quiet with PAFs, louder with my Tele, and loudest with my Strat. It's not a typical single-coil hum though, it sounds like some kind of EMF noise is getting into the amp via my guitars or my cable. When I turn the guitar volume down, the noise disappears completely, so it's definitely guitar-related.

    However, I made the dumb decision to leave out the 1M grid resistor on the input jacks, as per the 5F10 schematic, knowing fully well that it's better to add one in, just to see how the stock input wiring behaves. I'm inclined to add it in regardless, because it's better design, but wanted to ask one question: Can leaving out this resistor induce guitar noise into the circuit? Is there some kind of noise filtering related to the 1M resistor that's not happening when it's absent?

    Thanks so much.
     
  2. Ricky D.

    Ricky D. Doctor of Teleocity

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    If it was my project, I would just alligator clip in test leads with that resistor and see what happens.
     
  3. schmee

    schmee Doctor of Teleocity

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    I would put the resistor in.
    IIRC the layout for the Harvard is wrong on the inputs jacks though...? maybe someone can confirm?
    Make sure your pre amp area is grounded near the input jacks, not a star ground with the power section.
     
  4. gabasa

    gabasa Tele-Meister

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    I'll do both... check with the alligator clip first, then solder it in afterwards regardless. Thanks.

    There's a preamp ground on the chassis near the input jacks, and a separate power amp ground, so only two chassis ground points.
    It still comes across as EMF in the air or something, it's just more noticeable than with my other amps. If I move around it gets louder and quieter, just like 60-cycle hum.

    I have to go to work ... teaching late classes tonight. I'll let you know if this resistor quiets anything down once I try.
     
  5. Diverted

    Diverted Tele-Meister

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    Following this one with interest as I’m going to start a 5F10 pretty soon.
     
  6. Middleman

    Middleman Friend of Leo's

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  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Not a Harvard guru, but maybe this is like the 5F1 drawing famously missing the cathode bypass cap on v1, even though they all seem to have it that came from the factory?
     
  8. gabasa

    gabasa Tele-Meister

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    Ok, update!

    That weird EMF noise wasn't there today. There must be something in my house that, when it runs, gets just enough EMF to my guitars that it goes into the amp via the pickups. Maybe it's the dishwasher when it's running, because the kitchen is right above me when I'm playing in the basement. For whatever reason, the noise levels and the type of noise were perfectly normal today. Everything was very quiet other than 60-cycle hum with single coils.

    I tried with/without the 1M grid resistor on the input jacks. It affects tone. Everything is clearer and brighter, in a nice way, without that resistor. It's a marginal difference but it's definitely there.

    Even though it's better form to use the resistor, I'm going to leave it out and just make sure that whenever I play the amp, there's always a guitar plugged into it to keep things safe for V1. Original Harvard amps did not use this resistor in the actual amps, and they've survived 60 years of use, so that's what I'm deciding for now. Maybe if V1 ever redplates on me because I forgot to plug in my guitar, I'll add it in.
     
    corliss1 likes this.
  9. gabasa

    gabasa Tele-Meister

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    Btw, the Harvard has a nice tonal balance for recording. It's a very good recording amp, no wonder it's Sheryl Crow's #1 at her home studio.
     
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