Trouble with tube amp.

drselect
drselect Posts: 664
edited February 2011 in Electronics
Yesterday, I turned on my tube amp and a short time later I heard a pop and saw a flash. I tried to turn the switch off but the little red light on the front stayed light so I unplugged it. I pulled the switch apart and as shown in the picture below something bad happened.
So I installed a new 20amp DPST toggle switch. However, when I turn on the amp as the tubes warm up it begins to start humming and the humming gets loader until I shut off the switch. I have replaced all of the tubes and still have the hum.
Could the new switch be causing a problem or do I have bigger problems?
Post edited by drselect on
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Comments

  • george daniel
    george daniel Posts: 12,096
    edited January 2011
    I'm thinking transformer or maybe a ps cap,, is there anyway that you can bypass the switch and power it up and evaluate the hum,,,carefully?

    On second thought,,check the big caps first.

    Let us know what you discover--good luck
    JC approves....he told me so. (F-1 nut)
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  • drselect
    drselect Posts: 664
    edited January 2011
    george daniel wrote: »
    I'm thinking transformer or maybe a ps cap,, is there anyway that you can bypass the switch and power it up and evaluate the hum,,,carefully?

    On second thought,,check the big caps first.

    Let us know what you discover--good luck

    George, I do not see anything visually wrong with the caps so I am not sure what you are meaning by checking the big caps first. Thanks for your help.
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  • drselect
    drselect Posts: 664
    edited January 2011
    Since I had the time I went ahead and tried removing the switch. The hum is still present. I also let the amp stay on for awhile longer and it appears the hum increases to a point and then stays about the same. In addition, when I removed the face plate of the amp I had to remove the volume knob and it went back together and moves more freely than before. I disassembled the volume pot and put it back together but it doesn't appear to be working. This is based on the fact that I went ahead and tried to play some music with the amp. The hum stayed constant and the volume of the music stayed constant even when I turned the volume knob. The scheme for the amp can be found here if that helps any: http://www.polkaudio.com/forums/showthread.php?t=113468
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  • drselect
    drselect Posts: 664
    edited January 2011
    Here is some pictures of the amp hoping someone might see something thats not right.
    Thanks
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  • drselect
    drselect Posts: 664
    edited January 2011
    And some more
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  • ben62670
    ben62670 Posts: 15,969
    edited January 2011
    I would check the resistors near the power supply. Actually I would test all of them. At first just make sure none are open. I have not had a resistor partially fail so the value was off. I am not saying it doesn't happen, but usually they just blow. Also the two little blue caps by the power supply should be removed and tested. Be careful. Those caps sound like they are holding a charge.
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • george daniel
    george daniel Posts: 12,096
    edited January 2011
    When I think "hum" in a tube amp usually it's the caps in/near the power supply. Like Ben said,, be carefull,but check the caps ,,usually they are the large ones(at least on mine),remember,,they may still be charged.Let us know what you find.
    JC approves....he told me so. (F-1 nut)
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  • sucks2beme
    sucks2beme Posts: 5,681
    edited January 2011
    drselect wrote: »
    George, I do not see anything visually wrong with the caps so I am not sure what you are meaning by checking the big caps first. Thanks for your help.

    If you aren't technical, please take it to a tech. Better to be out a few
    bucks then hurt yourself. If you don't have test equipment, and it's not a tube,
    it's the only way to find out what's wrong. Even when something goes
    in a big way, there's often little stuff you miss, like caps on the edge of
    going bad.
    "The legitimate powers of government extend to such acts only as are injurious to others. But it does me no injury for my neighbour to say there are twenty gods, or no god. It neither picks my pocket nor breaks my leg." --Thomas Jefferson
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  • zingo
    zingo Posts: 11,258
    edited January 2011
    george daniel wrote: »
    When I think "hum" in a tube amp usually it's the caps in/near the power supply. Like Ben said,, be carefull,but check the caps ,,usually they are the large ones(at least on mine),remember,,they may still be charged.Let us know what you find.

    Ditto. All of my hum problems with amps in the past have been caused by bad filter caps. The easy way to test for this without test equipment is to get a cap that has the same capacitance or high (and always equal or higher voltage), and touch it's leads to the leads of the filter cap in question. It should remove or decrease the hum if you are testing the good cap on the bad cap.
    Jake
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  • drselect
    drselect Posts: 664
    edited January 2011
    For starters thanks for the replies.
    To make sure that I am on the same page:
    ben62670 wrote: »
    Also the two little blue caps by the power supply should be removed and tested.
    george daniel wrote: »
    the caps in/near the power supply. ,usually they are the large ones(at least on mine),
    zingo wrote: »
    Ditto. All of my hum problems with amps in the past have been caused by bad filter caps.

    Are the blue caps next to the black diodes by the power supply considered filter caps and/or large ones?

    Also when Ben refers to big or large caps does that mean physically or voltage wise?
    ben62670 wrote: »
    Be careful. Those caps sound like they are holding a charge.
    george daniel wrote: »
    Like Ben said,, be carefull,....remember,,they may still be charged.
    sucks2beme wrote: »
    If you aren't technical, please take it to a tech. Better to be out a few bucks then hurt yourself.

    I have readily admitted that I am on a learning curve when it comes to working with amps, tubes, DC current, caps, ect. and appreciate the reminders about the dangers involved (which again just reinforces what a great site PF is).
    I would say that I do have a technical background and have learned from that if you don't know ask don't assume. And above all SAFETY FIRST. I am struggling greatly with the one hand behind the back method not because of the safety issue but because I am just not the graceful. Also working on the mental part of this is STILL ALIVE even if its not plugged in. I have done some AC work and always make sure the power is off by turning off the breaker or unplugging before start working but realize that is not true for DC.

    Again thanks for all of your help.
    I just wished I didn't get so hooked on this before my "source" went away. I think I went through some withdraws this weekend:biggrin:
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  • george daniel
    george daniel Posts: 12,096
    edited January 2011
    you might want to shoot fgtv a pm with the schematics,and a link to this thread,,he is a whizz at this kinda stuff,,,,after re-reading this thread,I am hoping that it's just a ps/filter cap,Good luck and keep us posted.
    JC approves....he told me so. (F-1 nut)
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  • drselect
    drselect Posts: 664
    edited January 2011
    george daniel wrote: »
    you might want to shoot fgtv a pm with the schematics,and a link to this thread,,he is a whizz at this kinda stuff,,,,after re-reading this thread,I am hoping that it's just a ps/filter cap,Good luck and keep us posted.

    So am I. I was going to wait for some new aligator clips to come in before testing those blue caps. I also can't find my magnifying glass to see if I can determine what the writing says on the side of them.
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  • dragon1952
    dragon1952 Posts: 4,907
    edited January 2011
    You might consider some remote help from an expert. I've used Scott Frankland before and was able to repair my tube amp after a similar incident.
    His email is audioeng@pacbell.net and he charges $165 per hr, but I think it ended up costing me around $100 total before we figured out the problem. You can Google the name but he designs and builds tube amps. As long as you have the basic electronics skills, which it sounds like you do, he can help you.
    2 channel - Willsenton R8 tube integrated, Holo Audio Spring 3 KTE DAC, audio optimized NUC7i5, Windows 10 Pro/JRiver MC29/Fidelizer Plus 8.7 w/LPS and external SSD drive, PS Audio PerfectWave P3 regenerator, KEF R3 speakers, Rythmik F12SE subwoofer, Audioquest Diamond USB cable, Gabriel Gold IC's, Morrow Audio SP5 speaker cables. Computer - Windows 10/JRiver, Schiit Magni 3+/Modi 3+, Fostex PMO.4n monitors, Sennheiser HD600 headphones
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  • drselect
    drselect Posts: 664
    edited February 2011
    Back in business.:smile::smile:

    Thanks George and FGTV for all of the help.

    I had a box from mouser containing new volume pot. After sum struggles and a new brand I got it installed. And no HUM.

    dragon1952- thanks for the reference-may come in handy latter.

    Now I can get back to burning in the new SDA IC from ben2670
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  • george daniel
    george daniel Posts: 12,096
    edited February 2011
    now thats a happy ending--congrats:smile:
    JC approves....he told me so. (F-1 nut)
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