1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

AES Six Pac project assist

Discussion in 'Tube Audio' started by cwh, Dec 2, 2018.

  1. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    Hey gang, picked up a tossed out pair awhile back and I'm hoping to get them working. I've included a schematic that was drawn up the venerable Paul Carlson.

    Goal is to not toast the new tubes/cause additional issues upon initial power up.

    Visible issues:

    Both amps have one burnt resistor. #1 EL 84 PIn 3 and #2 EL 34 Pin 5. Each have a 0.1uf bypass that show signs of heat.

    Broken impedance switch, broken fuse holder, damaged filter cap, no tubes.

    Where I'm at:

    Replaced broken mechanical bits, main filter caps and resistors.

    New Jim McShane tubes at the ready.


    Thanks for any hand holding you may provide.


    IMG_2256.jpeg IMG_2333.jpg IMG_2334.jpg
     
    Last edited: Dec 2, 2018

     

    Please register to disable this ad.

  2. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,030
    Location:
    Southern NJ
    thats a bit of an odd way to do a constant current source on a diff pair inverter. I see an SS device on the plate side, so not sure why one wouldn't be used on the tail side as well instead of the EL84 tube.

    Pin 3 on the EL34 has a resistor connecting the plate to the screen at pin 4. About the only reason that should torch is a failure in the EL34. Possibly it could happen if the bias supply failed, which might have something to do with the EL84 resistor failure.

    Could be the other way round though, a failure in the EL84 caused a loss of bias that drew excess current in the output stage.
     
  3. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    Gadget all I see is a NPN transistor handling the B+ regulation with a EL84 acting as a current sink off of the phase inverter. What that all means and does you'll have to educate my naivete.

    I also see a fried resistor that's in the bias circuit and on the other amp a fried resistor in the power tube cathode circuit. Guess that's part of the bias circuit as well.

    These amps have a bias pot that goes zero to max. My stupid guess would involve over biasing or improper power tubes doing something. Oscillation?

    Your words are valuable to me so don't hold back.
     
  4. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    Going a order up some resistors.

    AES/Cary used Dale RN65Ds. For appearance sake I'll order the newest Vishay RN iteration. Believe that series is a 1/2w 500v resistor.

    Does 1/2w 500v sound correct for those circuits?
     
  5. kward

    kward AK Subscriber Subscriber

    Messages:
    3,512
    Location:
    Utah
    I like to isolate as much as I can so you test the smallest bits first and then add to them. What I mean in this case is you could remove the EL84 from the socket and clip in an 18K 3 watt resistor in between the plate pin and the junction of the 680 Ohm and 51 Ohm resistor in the tail of the EL84 constant current sink. That way you can test the rest of the amp firstm w/o the EL84 in the circuit. Then when the rest of the amp is verified, come back and add in the EL84 circuit.

    Looks to me like the transistor in the HV circuit of the inverter acts as a pseudo constant voltage source, while the EL84 in the cathode circuit acts as a constant current source. I suppose that's not such a bad idea of using both voltage and current regulation on the inverter stage. A pentode makes a pretty good current source/sink. A good current sink in this position will force balanced drive to the output stage. Not the way I would have done it either. I would have used a pair of depletion mode mosfets probably, but hey...the designer wanted to use an EL84, and obvously it works pretty well.

    Maybe what the designer missed is a deeper analysis of the failure mode(s) this amp is susceptible to. It would be nice to build a bullet proof circuit that won't fry stuff when tubes fail. I know that's hard to do sometimes.

    Anyway, I've always wondered what that EL84 did in this amp--now I know.
     
  6. vaathi

    vaathi baski Subscriber

    Messages:
    994
    Location:
    SF Bay Area, CA
    I assume you watched wonderful restoration video by Mr. Carlson.
     

     

    Please register to disable this ad.

  7. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    Kward thanks for the ideas. I believe you are correct about the HV circuit. I'd favor owner abuse over design failure in this case. Haven't seen chatter on such but you could be right. I do see (physically) an amp that wasn't cared for.

    Is the 18K resistor a general value for testing a circuit sans tube? Or specific to the EL84?
     
  8. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    I did, both videos. That's where the schematic was borrowed.
     
  9. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    Here's a bottom side if folks are interested.


    IMG_2335.jpg
     
  10. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,030
    Location:
    Southern NJ
    I think the idea is that it will sub well enough to make the inverter work without the tube being present in this particular case. It should flow about 7.2 ma, by the numbers in stock form it runs at 6.8 ma. Close enough for testing purposes.
     
  11. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    Cool. Well I ordered the 1/2w replacement resistors and tossed in a 18k 3w as well.

    Hope 1/2w is correct.

    The apparent heat stressed 0.1uf bypass caps; with limited test, tested ok. Should those be a concern? How self healing are PP caps?
     

     

    Please register to disable this ad.

  12. kward

    kward AK Subscriber Subscriber

    Messages:
    3,512
    Location:
    Utah
    Right...it was specific to that circuit given the published voltages on the schematic you posted.
     
  13. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,663
    Location:
    Middletown,New York
    How about a topside pic? Cool amp!
    Interesting links:




    Edit:
    vaathi I missed your reference, sorry

     
    Last edited: Dec 3, 2018
  14. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    Sure topside. As mentioned they weren't handled well. Broken fuse holder, broken switch and dented filter cap.

    IMG_2339.jpg

    If with gracious AK help I'm able to get them up and running here's the McShane tube compliment. JM had NOS 12BZ7s for the phase inverted circuit.


    IMG_2340.jpg
    Also I'm assuming as I go forward with other tube projects (this one too) I'll need a variac, so I'm trying to get this old Staco mounted today.


    IMG_2341.jpg
     
  15. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,030
    Location:
    Southern NJ
    I'd probably change them. Heat is about the only thing I have managed to kill yellow caps with. Possible those are fine but for how cheap they are why take the chance?
     
  16. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    Agreed. Of paramount importance is I don't mess up the tubes. The amps cost me a few dollars, tubes much more.

    For the time being I can pop in some IC polyesters.
     

     

    Please register to disable this ad.

  17. BinaryMike

    BinaryMike Pelagic EE Subscriber

    "Self-healing" is a reference to their ability to survive multiple internal arc breakdown events. Metallized film construction allows the conductive layer to vaporize and clear a short, where film/foil construction would stay shorted after such an event. As far as I'm aware, neither type can recover from failure due to excessive operating temperature.
     
  18. cwh

    cwh AK Subscriber Subscriber

    Messages:
    805
    Location:
    Oakland, Ca.
    Genuine question: Where would one encounter such an event?
     
  19. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,663
    Location:
    Middletown,New York
    Usually it happens during an over voltage evevt
     
  20. BinaryMike

    BinaryMike Pelagic EE Subscriber

    It's common in applications where the cap is connected to the AC line. Caps in powerline noise filters, for instance. They can eventually 'wear out' due to loss of capacitance from numerous arc events.
     

Share This Page