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Amp hum issue

Discussion in 'Amp Central Station' started by Rick-kcmo, Jul 21, 2017.

  1. Rick-kcmo

    Rick-kcmo TDPRI Member

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    I have a Hamstead Artist amp. Made in UK, not distributed in U.S. 20W EL34 head. Love the sound, but... There's a hum issue. It doesn't come on after warm up (no stand by), it is audible immediately upon power up. It comes through the speaker, not a mechanical thing. I'm not sure what to look for. It doesn't seem like it would be a tube issue since it does it before the tubes are working. There are setting for 240 and 115V and it is on the proper one. Any ideas? Thanks
     
  2. tubejockey

    tubejockey Tele-Holic

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    Not familiar with that amp, but you may have a filament issue with a tube. Some filament supplies have 2 100k resistors forming a virtual center tap. If one is open or shorted it will hum badly.
     
  3. Rick-kcmo

    Rick-kcmo TDPRI Member

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    Thanks. Even before the tube is warmed up enough to pass a signal?
     
  4. Rick-kcmo

    Rick-kcmo TDPRI Member

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    I should mention that it's not terribly loud but is noticeable.
     
  5. elmoscafeo

    elmoscafeo Tele-Afflicted

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    Just a stab in the dark, but if it's immediately then I would think power supply filter cap or coupling cap. Depends on the hum. Motor boat hum or steady, droning type of hum? Just a guess.
     
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  6. Rick-kcmo

    Rick-kcmo TDPRI Member

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    Steady. Kind of like a ground hum.
     
  7. elmoscafeo

    elmoscafeo Tele-Afflicted

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    Is this the amp?

    http://www.soundonsound.com/reviews/hamstead-soundworks-artist-20rt

    Does the hum change when you turn the reverb, voice, tremolo or master volume up? If this is the amp, it looks like it should be fairly easy to track down. The board looks minimal and laid out very well. Possibly a coupling cap. Pull your pre-amp tubes and see if the hum stops.
     
    Last edited: Jul 21, 2017
  8. elmoscafeo

    elmoscafeo Tele-Afflicted

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    .
     
  9. Ira7

    Ira7 Doctor of Teleocity

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    Remember the old days when you just reversed the prongs and stuck the power cord in the socket?

    Wasn't it always a polarity issue?
     
  10. Wally

    Wally Telefied Ad Free Member

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    Put all controls at zero before firing the amp up. When you switch the amp in this state, is the hum there? If so, I suspect an electrolytic has gone bad.
     
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  11. Rick-kcmo

    Rick-kcmo TDPRI Member

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    Mine is the no tremolo/reverb model but the same one otherwise. Volume has no effect, all the way down or full up. I'll try your suggestion. i just didn't know if it was likely since the hum starts instantly.
     
  12. elmoscafeo

    elmoscafeo Tele-Afflicted

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    It's probably not a tube, but removal of pre-amp tubes isolates or removes part of the associated circuitry. You should then be able to track down which of the caps may be causing the problem.
     
  13. Rick-kcmo

    Rick-kcmo TDPRI Member

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    I'll try it but I'm curious. Would the preamp tubes be passing a signal the instant it is turned on? Thanks
     
  14. Rick-kcmo

    Rick-kcmo TDPRI Member

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    Removed preamp tubes. No difference.
     
  15. elmoscafeo

    elmoscafeo Tele-Afflicted

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    You can try the same thing with the power tubes, but don't leave it on long. Just long enough to ascertain if the hum is still there or gone. You're getting close to opening up the amp once you can determine what section it's in.

    What kind of test equipment do you have? Do you have experience with vacuum tube high voltage circuits? I've searched for schematics, but can't find any on line. Do you have any?
     
  16. Rick-kcmo

    Rick-kcmo TDPRI Member

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    Changed back to original tubes, same thing. I don't have a schematic. I'm visually impaired. I've been around tubes forever since I do high end audio but I can no longer see well enough to get into a circuit, especially with high voltages. Mainly I was hoping it wasn't likely to be a transformer problem. It's probably time to think about taking it to a pro. Bought it 3 weeks ago and it didn't hum for the first 10 days.
     
  17. elmoscafeo

    elmoscafeo Tele-Afflicted

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    Unless you've abused the transformer by running it at high volume under a mismatched load (or no load for too long), then I think it's probably ok. As Wally pointed out earlier, it's probably an electrolytic capacitor. The next step would be to find another cap of the same value as your coupling caps and temporarily connect the new cap in parallel with the original cap and see if the hum changes. If you don't feel comfortable going any further, you might consider taking it in to a tech. It probably won't cost much to fix it.
     
  18. Dacious

    Dacious Poster Extraordinaire

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    Can you teach it the lyrics? Sorry - bad joke

    My vote is - you have a bad ground especially as it happens the instant you fire it,, tubes in or out. Either the 'fake' filament centre tap, or something else. Maybe a shield?
     
    Last edited: Jul 23, 2017
  19. Rick-kcmo

    Rick-kcmo TDPRI Member

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    Thanks to everyone. I'm sending it to Hamstead. They have been very good about it and will deal with it under warranty. I bought it almost new but used so it's very decent of them.
     
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