Any Theories on 6U8A Hum?

Discussion in 'Tube Audio' started by OBMG74, Feb 18, 2018.

  1. OBMG74

    OBMG74 AK Subscriber Subscriber

    Messages:
    152
    Location:
    Rockford, IL
    Ok, I'm back and I have assembled the parts for the modification.
    I just got done replacing a half dozen or so resistors that were way out of spec in the original circuit. (the unit had been re-capped when I purchased it several years ago.)
    Tested the amp and I am now back to ground zero and ready to do the circuit modification.
    Replacing these resisistors forced me to study the schematic in depth and now I have a better understanding what the circuit mod is going to accomplish.
    I ordered enough components to make a voltage divider for each channel.
    Should I proceed in that direction or just make one voltage divider and run both hum pots off of that?
    I have no problem going in either direction, I just want to do it properly.
    Thanks,
    Jim
     

     

    Please register to disable this ad.

  2. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    34,880
    Location:
    Southern NJ
    If you're unhooking the hum pots from the cathodes, you can tie both of them to a single voltage divider.
     
  3. OBMG74

    OBMG74 AK Subscriber Subscriber

    Messages:
    152
    Location:
    Rockford, IL
    Ok, single voltage divider it is, thanks for responding.
     
  4. OBMG74

    OBMG74 AK Subscriber Subscriber

    Messages:
    152
    Location:
    Rockford, IL
    Success!
    Gordon's calculation on values for the voltage divider are perfect.
    I now have 37 volts of positive DC bias on the heaters. (The CL-90 drops the line voltage to 117 VAC which is what is spec'd as input voltage on the schematic)
    I put in 2 fresh 6U8A's and am now ready to re-install the amp back into my component rack.
    Have a 2nd ASR-433 which is going to get the same treatment.
    Thank you to all for your responses and explanations. It really helps to understand the "why" for the fix.
    Jim
     
    Pio1980 likes this.
  5. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    2,672
    Location:
    New York City, NY
    But the most important question remains unanswered: did this reduce the hum?
     
  6. OBMG74

    OBMG74 AK Subscriber Subscriber

    Messages:
    152
    Location:
    Rockford, IL
    Not suprisingly, the old tube still hummed (damaged, I believe)
    Put in 2 fresh 6U8A's, the hum is gone!
    Looking forward to many hours of hum-free performance.
     
    Retrovert likes this.
  7. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    2,672
    Location:
    New York City, NY
    Yes, as I've written once the alumina insulation becomes damaged the leakage will be unstoppable, and ever increasing, and performance is irrecoverably reduced and rapidly degrades. But tube amplifiers have many sources of hum, so it was important to identify non-elevated heaters as the cause. For the want of two resistors, far more expensive tubes were lost.

    Elevated heaters are an important design factor which is honored more in the breach.
     
    Pio1980 likes this.
  8. OBMG74

    OBMG74 AK Subscriber Subscriber

    Messages:
    152
    Location:
    Rockford, IL
    Yes, I've gone through at least 3 tubes, if memory serves me correctly, with a very limited number of operating hours.
    A couple of resistors, a cap and a little labor is cheap in comparison. And that doesn't even count the frustration level...)
    Great to put this problem to rest.
     
  9. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    34,880
    Location:
    Southern NJ
    If I ever managed to get mine back together, a voltage divider is on the list of things to do for it. I really have no good excuse for it being in pieces as long as it has been, other than simple laziness on my part.
     

     

    Please register to disable this ad.

  10. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    2,672
    Location:
    New York City, NY
    Nobody has a good excuse for omitting the elevated heaters, since it's no longer 1960, and you're not saving $0.25 per unit ($5 in today's money) times fifty thousand units.
     
  11. OBMG74

    OBMG74 AK Subscriber Subscriber

    Messages:
    152
    Location:
    Rockford, IL
    Thought that I would add a side-note:
    I used NOS Sylvania JAN 6U8A tubes for my initial testing.
    Both hummed, the hum worsened if I had my hand near them and were very microphonic when tapped.
    Replaced with NOS RCA's, problem solved
     
  12. triode17

    triode17 Well-Known Member

    Messages:
    696
    It's not the tubes themselves, it the circuit design. The great design engineers of yesteryear didn't know everything. Plus, this is a problem that usually creeps up slowly, insidiously. This is one reason I don't build Split Load Phase Inverters into my own designs anymore, I now use Differential Amps (long-tailed pairs).
    The lower half cathode of a split load always has to be elevated because it's driven directly by the previous plate. You should float up a voltage on a voltage divider from the B+ that gives you about 75 Vdc. Place a 0.1uF film cap across it and connect that voltage to one side of the heater on the 6U8 and the problem should be alleviated. Note: The current 6U8 may be damaged now so try a new one.
     
  13. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    34,880
    Location:
    Southern NJ
    The dicey part about elevating the heater too much is that all the other tubes are on the heater circuit too. The output tube cathodes are about +12v, the inverter some +75v, the rest are pretty nearly zero. It ends up being a balancing act to minimize things as much as possible. Probably about half the cathode voltage is as good as you're likely to get. That should get everything within 40v.
     
    GordonW and Pio1980 like this.
  14. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    2,062
    Location:
    Montréal, Canada
    Good thread for my ASR-433: I've bought several JAN Philips 6U8's a few years ago and it didn't take a while before half of them began to hum... The two remaining good ones now resides in my 222D.
     
  15. OBMG74

    OBMG74 AK Subscriber Subscriber

    Messages:
    152
    Location:
    Rockford, IL
    I just completed the heater circuit modification on my 2nd ASR-433. (and replaced out of spec resistors, re-cap was done around 6 months ago)
    Another success, they are great amps!
     
  16. 6DZ7

    6DZ7 Super Member

    Messages:
    1,871
    So was modifying the heater circuit the solution or just getting better tubes than what you first had?
     
    Last edited: Apr 14, 2018
  17. OBMG74

    OBMG74 AK Subscriber Subscriber

    Messages:
    152
    Location:
    Rockford, IL
    Modifying the circuit was the solution.
    I have complete confidence that this is the long-term solution but given that these amps get light usage my report on the effect of the circuit change will take a while.
    However, I will report back at that time.
     

     

    Please register to disable this ad.

  18. 6DZ7

    6DZ7 Super Member

    Messages:
    1,871
    So the original problem with NOS Sylvania's has been dimminished and they sound OK, now?
     
  19. OBMG74

    OBMG74 AK Subscriber Subscriber

    Messages:
    152
    Location:
    Rockford, IL
    No, the Sylvania JAN's were microphonic, I replaced them with RCA's and all is good now.
     
  20. 6DZ7

    6DZ7 Super Member

    Messages:
    1,871
    Ok, but hum from microphonics is different from hum from AC leakage. Microphonic hum is from a chassis vibration source usually the PT. So I'm just wondering how you know the change to the heater circuit was the actual solution to the hum problem.
     

Share This Page