Video: Discharging a Tube Amp

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Billm, Jan 22, 2012.

  1. Billm

    Billm Member

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    It's not perfect, but I made a video of discharging the caps on a Blues Junior. It seems like the question comes up periodically, so I figured a video would cut down on the questions I get from my customers.

    Comments welcome.

    http://billmaudio.com/wp/?page_id=1150

    And yeah, :cool: I got a new camcorder and had to test it!
     
  2. Tele Wacker

    Tele Wacker Member

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    Thanks. Great info Bill. I reviewed some of your other tips on you website. I'm going to use them on some of my old Fender amps.
     
  3. wall_of_sleep

    wall_of_sleep Member

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    Nice video, Bill. You have an easy to follow teaching style. Interesting website as well.
     
  4. shooto

    shooto Supporting Member

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    nice...thanks

    because I've never heard of this method before, just turning the amp power off after it's up to full operation (leaving stanby on), will completely discharge the caps safely?...is it because the normal (suggested) method of shutting off standby THEN power that current is left in the caps?
     
  5. BlameCanada

    BlameCanada Member

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    Great video. Thanks for that.
     
  6. Billm

    Billm Member

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    Some amps, like the Deluxe Reverb, isolate the main filter cap from the rest of the circuit when you put the amp in standby -- it remains charged.

    That's why you want to leave the amp in play position and just turn it off. That way, you're sure that all of the caps are discharged.
     
  7. shooto

    shooto Supporting Member

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    ^ is that just in Fender amps?...all Fender amps? (I have a Super Sonic 22)....Marshall?

    thanks!
     
  8. Billm

    Billm Member

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    I don't know about the Super Sonic 22, but the standby on the big one cuts off everything.

    But it doesn't matter. Just leave whatever amp in play position and let it discharge.
     
  9. '58Bassman

    '58Bassman Member

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    That was your site with the LED discharge tool on FDP a few years ago, right? I made one and it works great- a good tool to use for verification if a meter isn't handy.
     
  10. Hwoltage

    Hwoltage Member

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    Once you have most of the voltage out of the amp, it's never a bad idea to go around and touch the + end of all the big caps just to be sure. Procedural redundancy is never really a bad thing.

    My 2 cents.

    This might also be a good one to sticky.
     
  11. MCK

    MCK Member

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    Great video but I would probably make this point clearer. Given the Blues Junior does not have a StandBy button wired in the "let conducting tubes drain the caps" method work well but the statement that this will work on all amps might potentially mislead. Also what if you have a failed tube in there etc. Suggest a write-over as the video is great otherwise.

    On a related safety note, the amp still plugged in has live wires around the power switch. For a video aimed at newbies it might be a good idea to suggest that they totally unplug before sticking their hand in there. One hand in pocket rule could also be mentioned...

    Just a few humble suggestions. All the best.
     
  12. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Hi Bill -

    That's a nice, easily-understood tutorial.

    In the interest of idiot-proofing... You might consider explaining where the other end of the yellow jumper wire (with 10Kohm resistor) is connected, when discharging with that method.

    Cheers!

    - Thom
     
  13. Billm

    Billm Member

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    Hi Thom, I put a caption on the screen that says "the other end of the yellow wire is grounded to the chassis" or some such.

    And yeah, if you had a Champ with a dead filament in the 6V6, it might hold some charge because the discharge rate through the 12AX7 is much slower. But one output tube out of two will still discharge the amp very quickly.

    And yes, that was me with the "ultimate discharge tool." It was just a Sunday afternoon fun project--a pen body with some resistors, an LED and a momentary contact switch to give you resistance/no resistance.
     
  14. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Easy fix! :aok Very cool!

    - T
     
  15. Soothsayer

    Soothsayer (Paul) Supporting Member

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    Excellent Bill, thanks!
     
  16. mcdes

    mcdes Member of no importance

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    Great educational vid!
     
  17. ECistheBest

    ECistheBest Member

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    we can see that he uses a pretty awesome multimeter. 4.5 digits! :D
     
  18. Billm

    Billm Member

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    And an infrared thermometer with a laser pointer!
     
  19. wagdog

    wagdog Member

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    Well...

    I ALWAYS unplug an amp and discharge the caps even if I "know" the tubes were warm when I turned it off, or I left it on standby and played a few chords. I also have an amp (a twin) w/bleeder resistors and it's yet to hold any charge for more then a few seconds however I still go through the motions with that one as well.

    I also make a point to discharge at all points on the B+ rail that might hold a charge.

    IOW's, my routine is to unplug the amp and hit all of the caps on the power rail every time no matter what, and then check w/a voltmeter that it's safe.

    It only takes a minute, max, and I think it's better to be safe.
     
  20. Hwoltage

    Hwoltage Member

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    Agreed. My process is nearly identical.
     

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