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Old Fender tube amp question.....

Discussion in 'Amps [BG]' started by two fingers, Dec 28, 2012.

  1. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    Help me not do something stupid. I have an old Fender PA100 that I just love for practicing at home, taking to the occasional jam (and letting my kids sing along with YouTube). These days, it starts to hum after it has been on for 20 minutes or so. On the back is a "Hum Balance". It is down in what looks like a 1/4" jack, but it's a slot head like a screw. What is that? I don't want to just start turning it without knowing what it does.

    Please let me know if I haven't given you enough info. Thank you!
  2. Tvrtko

    Tvrtko

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    Give it to someone who can trace the problem. Might be corroded plug inside amp or bad solder, or something much worst.
  3. B-string

    B-string Supporting Member

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    The hum balance is used to obtain the least hum from the amp. It sounds like you have a tube going bad somewhere though if it happens after 20 minutes. The control references one side or leg of the tube heater supply to chassis ground.
    It is meant to be adjusted BTW.
  4. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    Thanks B. I was kind of hoping you would chime in. So I can tweek on that a bit without doing any damage? And if it doesn't work I can just test my tubes to see if there are any issues? From what I have read, channels 1 and 2 share a preamp 12AX7. I use those more than anything so possibly that's my bandit.

    I'm an electronics student but just a first year. So pretty much I know enough to know what I don't know. :D My semi-conductors teacher this coming semester is an old tube head. He already promised me I can derail him into a tube discussion with little or no effort. So hopefully by April or May I won't have to hit you up every five minutes. Cheers.
  5. T-Bird

    T-Bird

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    Hi.

    Yep, You can tweak the hum balance all you want.
    Actually, I've made it into a habit to give it a little twist every now and then, it sometimes becomes inoperable over time if not used

    Look up the schematic for that amp, You'll then understand how it works.

    With some caps, the ripple can increase when the operational temperature is reached, so those may also be the source of Your problems.

    The tubes however are the easiest to swap, so start with those.

    PA100 is a great, unbelievably under-appreciated offering from Fender, it's a small wonder how they're as cheap as they tend to be. Over there in the US at least ;).

    Regards
    Sam
  6. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    Thanks Sam. Yep. It sounds like BUTTUH! This thing with a P bass is pure magic. I already swapped out the input resistor so I could get a little more gain on the front end (a very common mod for this amp for those of us who use it for guitar/bass). Now it growls pretty nice but I can switch over to channel 2 for just clean and warm.
  7. B-string

    B-string Supporting Member

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    I agree with Sam. I used to work the hum balance stop to stop at least once a year. Rotate the control back and forth till the least hum is heard and leave it there till next time it is needed. 60 Hz hum will be heater circuit and some bias supplies. 120 Hz will point to the power supply filters.
  8. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    Noted. I'll try to get to it this weekend and report back.
  9. BawanaRik

    BawanaRik Supporting Member

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    In general the balance is used to adjust the current between the two half of a push pull output stage of an amp. The push pull can kinda work like a humbucker to keep noise down. I doubt that's your problem. To do n easy test remove that V1 and turn the amp on and wait. Time it with a clock so you don't get antsy. If the noise is gone it might well be V1.

    The only fender PA I ever had was a Solid State. This could be a real cool amp since even "bad" Fender are better than most other amps "good" amps.

    Do you have a gut shot so we can see if it's wired like a regular Fender guitar amp?
  10. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    I don't, but give me a sec to go get my thumb drive. I think I downloaded a schematic for it recently.
  11. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    EDIT: Wait! Wrong file! First one was wrong but I fixed it.

    If you can't blow this up enough I will be glad to email it to you. Just PM me.

    Attached Files:

  12. B-string

    B-string Supporting Member

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    The pdf link won't work two fingers :(

    Hum Balance and Balance controls are two different controls. Fender if equipped with a balance control are not on the rear panel. They locate them on the same plane of the chassis as the tube sockets. ;)
  13. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    And................ fixed.
  14. B-string

    B-string Supporting Member

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    Bingo! You have both two fingers! :) The hum balance is at the lower right side of the schematic on the heater winding of the power transformer. Do not adjust the output balance without test equipment handy BTW. Output balance control is not labeled usually and not easily accessible. :)
  15. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    If I have learned ANYTHING so far, it is to NOT touch ANYTHING unless you know EXACTLY what it does. Until then, I have YOU! Thanks!
  16. B-string

    B-string Supporting Member

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    No trouble at all! Learning is almost as fun as passing on the knowledge :D

    After looking at the schematic the 120 Hz hum rule will not apply to this amp BTW. This amp is using half wave rectification so it would also be 60 Hz from the HV supply. Can't use that short cut on this one :(
  17. walterw

    walterw Supportive Fender Gold Supporting Member

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    just make sure that this noise that shows up gradually isn't accompanied by one of the power tubes glowing a dull red in the center of the grey metal part!

    that's the current running away and Bad Things Happening to the tube (and the amp)!

    if the tubes aren't "red-plating", then yeah, tweak the hum balance to your heart's content.
  18. BassmanPaul

    BassmanPaul Gold Supporting Member

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    No it's full wave rectification Mr B. Two sets of tripled rectifier diodes to the B+ and ground from the HV secondary center tap. Ripple will be 120Hz.
  19. B-string

    B-string Supporting Member

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    You sure Paul? Only the positive half is rectified, note the diode orientation? It is an odd way of doing things and would reduce available B+. Ripple frequency I am not sure about with this configuration.
  20. B-string

    B-string Supporting Member

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