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Low Level Hum in My Fender Tube Amp

Discussion in 'Tube Audio' started by tube-a-lou, Jan 7, 2018.

  1. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Thank you for the info, here's something I just did, I just replaced the sixty year old tone pot
    with a newer old stock 1 meg cts pot with a switch and I just put it on for a few minutes and
    the hum is pretty much gone, I'm thinking it might be the old controls are on there way out.
     

     

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  2. Alan0354

    Alan0354 Super Member

    Messages:
    2,735
    Location:
    Bay Area
    Now, I did not expect that. A tone pot causing hum. You sure you have the wire exactly the same. But anyway, if it is gone, it's gone.
     
  3. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    No I rewired it right and it's dead quiet, I used to touch the back of the speaker and felt the cone
    vibrate now it's gone. I don't know who made the early pot's for them but the CTS are better to
    clean out. I did read here on this site or the Telecaster site I go on someone told me that the pot's
    with the switch causes hum and noise when the get old plus last night when I turned it on the
    the amp didn't go on I had to jiggle it a little then it went on. I did measure the old pot and it was
    about 8oo or so the new one is closer to 1 meg which is spec.
     
  4. Alan0354

    Alan0354 Super Member

    Messages:
    2,735
    Location:
    Bay Area
    Yes, it's the CTS pot I refer to. Actually it's my friend D. Allen of D Allen pickup that told me. CTS 250K pot is actually more like 220K to 230K. That shaves off a little sting from the Strat pickup to make is a little fatter. Like capacitor, you really need to measure the value and test before you can say this brand is better. CTS is so famous ONLY because it's lower resistance and make the strat sound smoother.

    Ha ha, for Gibson with HB pickup or P90, they use 500K to get back some highs, don't use CTS pot, it's it is 450K, that will muffle the sound for the HB and P90 pickups.

    No, pot with switch does not cause hum. Incorrect wiring that strung in the wrong place will cause hum. Vacuum tube circuit is very high impedance, it's easy to pickup hum with bad wiring. It is engineering to design the wiring. You move the wire, you can change the sound.

    I use pots with switch in my guitar amp because I have different sound option and I use those switch to kick in the effect ( mostly push pull pots). Gain of my amp is very high ( high gain channel for distortion), I absolutely do not have hum problem. It's all about the layout.

    Funny that I use an old Bassman 100 chassis as experimental amp. I ripped everything out, output transformer, choke, turret board, everything out except the power transformer. I even drill new tube socket holes closer to the volume and tone pot to shorten the wiring. I design the circuit and design the layout of the component and build into the amp. But then during the testing, I start changing the circuit, adding extra circuit. The wires of the new circuits start crossing and get messy. After complete the design, I use the same circuit and build into a brand new chassis, I designed the layout and placement to optimize the final circuit and built the amp. The two amps sounded different. The old Bassman 100 prototype amp always have a little bit of the nasal sound that I don't like. The new amp has none of that. It's more open and transparent. the only difference is the new amp was layout with all the additional circuits, layout is smooth, no crossing. The prototype amp has crossing from the added circuit.

    there goes to show wiring placement is very important. People might confuse changing a component change the sound and the truth might be the wiring is change along the way that change the sound.
     
    Last edited: Jan 13, 2018
  5. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    I don't know about that, the wiring was the same in the same location. I just changed the
    switch.
     
  6. PakProtector

    PakProtector AK member

    Messages:
    3,672
    Location:
    Ann Arbor, MI
    One of the gents that taught me a bit about guitar amps used to tune up stage gear for Hendrix. They'd plug in a set, and get a complaint and direction to try another piece. Lather rinse and repeat, until they put the first one back in, got approval and all was well. The direction I got from him was trust engineering for most of it, and just resign yourself to the fact that there is both stuff that is not well defined, and religious fanatics who believe in it.

    Pots with built in switches that have some miles can leak a bit between the pot and switch contacts. Glad you discovered the problem and fixed it.
    cheers,
    Douglas
     
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  7. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Yes I think your right, a pot from 1952 sure has seen a lot of use.
     
  8. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Are You kidding me, I went to turn on the amp tonight and nothing. So I took off the back and I found
    out that there's a thin wire that goes from pin7 of the 6V6 tube to the light above (see pic). I removed
    the broken wire and was able to remove the cloth covering and put it on a heavier wire and it worked
    again, but seriously that wire just hanging there like that powering the 6V6 tubes is a bit odd. :eek:

    P1060594.JPG
     
    Last edited: Jan 14, 2018
  9. drew_t

    drew_t Well-Known Member

    Messages:
    682
    That's the heater wire.
     
    Bill Ferris likes this.
  10. solderjunkie

    solderjunkie New Member

    Messages:
    49
    I think your loose filament wire further reinforces the need to carefully go through the amp and check all of your connections, including all chassis ground connections. It is also possible that the pot you swapped out was just fine and the real issue was related to it's ground to the chassis as Bill Ferris suggested. Pots that drift out of spec generally don't cause hum. Poorly grounded pots and jacks do. Sage advice here...

     
    tube-a-lou and Bill Ferris like this.
  11. Bill Ferris

    Bill Ferris Super Member

    Messages:
    1,667
    Location:
    NE. FL.

    Thank you solderjunkie, most kind of you Sir.
     

     

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  12. Alan0354

    Alan0354 Super Member

    Messages:
    2,735
    Location:
    Bay Area
    Old pot does not cause hum, period. There must be something else going on. Wires got moved, loose ground wires, too many possibilities. Have to follow the logic. I think the best is to have the assembly drawing, check all the wires to make sure they are in position as drawn.


    [Mod Edit] Look at the assembly drawing, wire by wire, pull on the wire to make sure it has good connection.

    I can assure you the old pot is NOT going to cause hum.
     
    Last edited by a moderator: Jan 14, 2018
  13. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    That was all done when I first received the amp I cleaned and tighten the control's. Actually the switch in the pot
    was no good when I would switch the amp on it would not come on, I had to jiggle it a bit and it would come on,
    not good I say.
     
  14. knockbill

    knockbill Addicted Member

    Messages:
    7,679
    Location:
    SE PA
    Lou, from the last picture, some of the solder joints look pretty rough,,, specially the one to the main CT on the side of the chassis.... If that little side panel is just bolted to the rest of the chassis, that could be a high resistance ground path if the bolts are dirty or loose...
    I would carefully re flow all the solder joints on that amp, specially the grounds, as its using the chassis for the ground buss...
     
  15. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Hi Bill, the solder joints I redid on the tube sockets and from the board to the grounds, I going reflow
    that spot and yes the chassis is spot welded together so I think that's okay.
     
  16. knockbill

    knockbill Addicted Member

    Messages:
    7,679
    Location:
    SE PA
    Good its hard to tell in the pic,,, I'm working on a pre amp that has grounds soldered to a little side panel like that, and it is bolted on,,, I moved the joint to the main chassis...
     

     

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  17. PakProtector

    PakProtector AK member

    Messages:
    3,672
    Location:
    Ann Arbor, MI
    Just what do you think happens to the internals as they move around? You think it is impossible for a conductive path to get established between the pot and the switch contacts?
    cheers,
    Douglas
     
    tube-a-lou likes this.
  18. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    Still quiet! it cut the noise level to half, before I would stand 5 feet away and here the hum
    now I pretty much have to stand on top of it or close to hear output which is good.
     
  19. solderjunkie

    solderjunkie New Member

    Messages:
    49
    That's great. Congratulatons, man.

    Something that came to mind to be aware of down the road is noise and hum emanating from the line voltage, particularly if there are lighting fixtures with florescent bulbs plugged into the same circuit. I have spent ages troubleshooting a seemingly noisy amp only to find out that it was a particular wall socket causing the issue.
     
  20. tube-a-lou

    tube-a-lou Super Member

    Messages:
    4,428
    I'll have to look into that as well, thank you for all the help. I did order a pair of new
    Ohmite 250ohm 8 watt resistors, there heavy duty like the original one that used
    to be in the amp.
     
    solderjunkie likes this.

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