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Hot Rod Deluxe Hum

Discussion in 'Amp Tech Center' started by OneHeadlight, Nov 23, 2020.

  1. OneHeadlight

    OneHeadlight NEW MEMBER!

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    My Hot Rod has a hum in the clean channel only. I changed all 3 AX7 tubes thinking that it was a pre amp problem as the hum is not present on the saturated channel. But this did not help. It was time to change them anyway.
    I have the same problem no matter what outlet I plug the amp into, my house, drummers house, wherever, so I don't think it's an AC problem.
    I'm pretty technical, but since I retired all I have is a schematic and a multi meter. If someone has an idea, please let me know where to start checking.
    The hum happens after I strum a chord or note, not while the amp is just on. It's like an after hum. I hit a chord and it sounds fine, but a second later instead of sustain I get an awful sounding hum sustain.
    Maybe a capacitor? I've uploaded the schematic if that helps.
    Thanks,
    Ron
     

    Attached Files:

  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Do you have a different speaker you can try? Hook up Hot Rod to other speaker, and other amp to Hot Rod speaker, just to see if that is an issue.
     
  3. Wally

    Wally Telefied Ad Free Member

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    Welcome, One headlight.
    I am not going to be much help here, but I will share what little I can. First, as I see it, this is a s8ngle channel amp with gain opti9ns that are dropped into the signal path.
    Since your Gain/Lead mode does not exhibit the noise porblem, I would suggest loooking at those components and circuits that are in use only in the Clean mode. The first of these would be C18, R9, and R6. That cap could be bad, the pot might hAve gone bad, or there could be a connection problem there...a ground is unmade. This might be the best guess as your description could lead one to think that a ground problem is brought about by vibrations. When you switch to the lead mode, those three components are no longer ‘active’ as the circuit is diverted from those three components. If you don’t find a problem there, then search through that schematic for other areas that are exclusive to the Clean mode. I think you have a connection problem....cracked trace or a cold solder joint somewhere. Just a guess....
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Ah, it's clean channel only :D

    How old is the amp? 15 years old or more, and it's due for a service of all the electrolytic caps anyway.
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    To add to Wally's observation, the switch connections are in the path as well.
     
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  6. wabashslim

    wabashslim Friend of Leo's

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    A sound clip would help, I'm not sure what you're describing.

    As a not-helpful aside...looking at the schematic - there's no way in hell any guitar amp should be so complicated. Especially since the gain & more-gain channels suck so bad.
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    @wabashslim - a lot of the "extras" are the low voltage supply for the channel switching and reverb circuit. The actual amp stuff is pretty straightforward.

    If you really want to cry, check out any modern Mesa stuff :D
     
  8. Wally

    Wally Telefied Ad Free Member

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    and...if you want to play good amp, check out a boogie. (;^)...imho.
     
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  9. wabashslim

    wabashslim Friend of Leo's

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    I'm aware. Had one for awhile, did some work on it, got rid of it.
     
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  10. SamIV

    SamIV Tele-Holic

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    My brother has is worked on. Hummed on both channels. Tech said he took care of some bad solder connections, but did not mention where, and changed the power amp tube. Brother says no more hummmmmmm.
     
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