Any Theories on 6U8A Hum?

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

  1. OBMG74

    OBMG74 AK Subscriber Subscriber

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    After searching AK on this subject, I have learned that a fair amount of people have experienced this problem
    and some brands seem to hum, others don't.

    My personal experience on the voltage amp/phase splitter 6u8 tubes in my ASR-433 has been that Westinghouse and RCA's were ok for a while then developed a hum. I tried a set NOS GE's that hummed right out of the box. When a hum in a tube develops, the hum follows that tube to the other channel when swapping L & R channel tubes.

    Apparently, some people have tried 6GH8A, 6BL8, and 6EA8's as substitutes with some success.

    My question is, does anyone have a theory as to what makes the 6U8 susceptible to hum or develop a hum?(not in my amp necessarily, just in general) Bad design of the tube?, the tube not being used in circuits for the purpose it was designed for? or?

    Would running DC on the heater make any difference?

    Thanks in advance for any discussion on this topic.
     

     

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  2. s-petersen

    s-petersen Scott Subscriber

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    Adjusting the hum pot dosen't fix it? DC on the heater would certainly fix it. I just built a ASR 444 amp section and I tried 6EA8's which hummed, but the 6u8 didn't
     
  3. OBMG74

    OBMG74 AK Subscriber Subscriber

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    Adjusting the hum pot doesn't affect the hum, finding the right 6U8 does, at least for a while. The latest RCA lasted a couple years with very moderate use.
     
  4. gadget73

    gadget73 junk junkie Subscriber

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    make sure the hum pot is good. Both of the ones in my asr-433 were bad.

    If they are good, DC on the heater would probably fix it.
     
  5. Pio1980

    Pio1980 AK Member Subscriber

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    First best guess is heater to cathode leakage. Certain modes of use, such as phase splitter, are stressful to the electrical isolation between these elements.
     
    Last edited: Feb 20, 2018
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  6. OBMG74

    OBMG74 AK Subscriber Subscriber

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    Pio,
    That might explain then why tubes used in a phase splitter mode work for a while and then due to continued stress develop heater to cathode leakage?
    Do you think that using DC on the heater even with leakage to the cathode would alleviate the hum?
     
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  7. Pio1980

    Pio1980 AK Member Subscriber

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    In another thread, it was mentioned that some amps "float" the heater bus independent of ground to allow it to find it's own somewhat elevated level, a small detail worth noting. Other amps may use a resistor network to establish a DC level compromise for all tubes on that heater bus.
     
  8. BinaryMike

    BinaryMike Pelagic EE Subscriber

    The 6U8, like almost all similar types, was designed principally for TV service, and it appears that Stromberg didn't fully appreciate the potential for heater-cathode leakage trouble in the voltage gain stage. The presence of a large unbypassed cathode resistance means that very little H-K leakage can cause excessive hum. A DC heater supply for the 6U8s could be an excellent solution if you can find a way to implement it without overloading the power transformer. Is there room for an auxiliary xfmr?
     
  9. Retrovert

    Retrovert AK Subscriber Subscriber

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    The reason for the hum is the insulation begins to break down above 90 V and current flows between heater and cathode.

    Elevated heaters is the solution.
     
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  10. OBMG74

    OBMG74 AK Subscriber Subscriber

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    I was toying with the idea of implementing an external DC power supply for the 6U8 heaters only.
    Would welcome any ideas or sources for an external 6.3VDC to supply (2) 6U8's.
    I will definitely test those hum pots.
     
  11. OBMG74

    OBMG74 AK Subscriber Subscriber

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    Not familiar with how to elevate heaters.
    Please explain?
     

     

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  12. Pio1980

    Pio1980 AK Member Subscriber

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    Last edited: Feb 18, 2018
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  13. BinaryMike

    BinaryMike Pelagic EE Subscriber

    I would try something like a 9VDC 2A or better switch-mode PSU with a 3.0 ohm 5W minimum resistor in series. The resulting soft start will maximize heater life and should keep the PSU from limiting out at start-up time. You could simply ground one side of the 6U8 heater circuit, but there are more elegant ways to do this --- especially if you hear any switching noise artifacts, which isn't likely.
     
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  14. Retrovert

    Retrovert AK Subscriber Subscriber

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    Here's some more details, culled from notes and postings I've made.

    Briefly, the heater wire is coated with an insulator, alumina, having a certain breakdown voltage. If the cathode-to-heater voltage exceeds this breakdown voltage then current flows from the cathode (higher potential) to the (lower potential) heater, degrading the heater's insulation (which flakes off for further degredation) and ruining the tube. That current flow causes the hum. Once the insulation begins to degrade nothing can save the tube. Consider why 7199 tubes are in such short supply. In televisions, for example, the 7199 failed from heater-to-cathode arcs when the insulation failed. Circa 1995 Matt Kamna reported the the 7199 leakage issue as being caused by heater-to-cathode limits, so this was a well-known issue. This explains why the 7199 fails with alarming regularity in audio equipment.

    While the tubes often specify 100 VDC, this is unrealistic as it exceeds the insulation's limits. By the time the tube fails it would be hundreds or thousands of hours and the consumable would be expected to fail, even though it could last longer without the abuse.

    Leakage occurs in every single tube with elevated heater-to-cathode potential, and the amount and rate depends upon the thickness of the insulation, both in general and in specific instances, such as where the heater was bent. Current flow gradually erodes the insulation until the leakage becomes an arc.

    The solution is moving the relative potential of the heater up so it is lower than the cathode-to-heater breakdown voltage point. This is published for the tubes. A number of amplifiers do not properly elevate the heater voltage. A resistor ensures you don't pull too much B+ current into the heater supply. A few mA is all you need, just for stabilized dividing, since it is a potential not a current.

    The published limits are about 90 V, but RCA and others published studies showing that any difference above a few volts difference begins to migrate metal. Either nickel from the cathode sleeve into the tungsten filament or tungsten and alumina into the nickel cathode sleeve. This is why the tube specifications list a different value for the positive potential difference vs the negative potential difference; the polarity determines the direction of migration. The higher the voltage the faster the migration. Metal migration is why two different values are specified in the datasheets, depending upon whether the heater is positive or negative.

    Elevation moves the range from the normal range of +/- 3.15 VAC (the voltage moves above and below the 0 V baseline) to that same range but with a different baseline. So, if the heaters were elevated to, say, 200 VAC, the range would now be 200 VAC +/- 3.15 VAC. This just changes the baseline from which the voltage fluctuates.

    Elevation process.
    AC heater: elevation uses the transformer's center tap so the voltage is above/below a baseline. Neither side is elevated. (range is baseline - 3.15 VAC <= VH <= baseline + 3.15 VAC)

    DC heater: the elevation could use only one side (range is baseline <= VH <= baseline + 6.3 VDC) or be split as a fluctuation above/below a baseline (range is baseline - 3.15 VDC <= VH <= baseline + 3.15 VDC).​

    A Warning
    In DC it matters if the heater is positive to the cathode or negative to the cathode, because the issue is pulling metal ions from the cathode sleeve into the alumina and filament, or pulling tungsten and alumina ions into the cathode sleeve.

    In AC the change in voltage pulls metal one way then the other. This does not balance out because the movement of metal ions varies with polarity.

    Regardless of the type (DC or AC), the voltage difference should be as small as possible to minimize metal ion migration.
     
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  15. OBMG74

    OBMG74 AK Subscriber Subscriber

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    Thank you to all of you for your responses so far, you've given me some great explanations and potential solutions
     
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  16. s-petersen

    s-petersen Scott Subscriber

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    Last edited: Feb 18, 2018
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  17. Retrovert

    Retrovert AK Subscriber Subscriber

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    Ummm, that configuration is not properly elevated because the voltage is not at the same potential as the cathode. It's at whatever potential it floats to from leakage. Still stresses the insulation, just maybe not as badly.

    A lot of these amplifiers mutzed resistors to save money. Resistors are expensive if it is 1960 and one is making tens or hundreds of thousands of something, because the multiple dollars per unit (component cost, acquisition and stocking cost, assembly cost, etc.) add up.

    We can fix that with a voltage divider. Why use an unknown floating voltage when the precise one can be dialed in?
     
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  18. gadget73

    gadget73 junk junkie Subscriber

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    Its elevated to about +12v from the output tube cathode. The specific value depends on the output tubes but its reasonably predictable. The cathode of the phase inverter runs about +75v, so 63 odd volts of difference. A divider off B+ would sort it. You would need two due to the heater supply design in this amp.

    They didn't cut too many corners in this amp really. Its got a split power supply, each channel has it's own heater string and the B+ splits after the voltage divider. It also has a ground buss system and its loaded with shielded wire. Most other makers just weren't doing that sort of stuff. They could have very easily cut down on component count. I suspect they simply went by the datasheets and determined that what they were doing was sufficient for the task.
     
  19. GordonW

    GordonW Speakerfixer Subscriber

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    Elevated, but not necessarily enough, for some 6U8 tubes.

    The original design gives about 12v of elevation on the ASR433 (the bias voltage of the output tubes)- but, it would be safer if that was more like 35v or so (like it is in the ASR444- it's like 35v or so in that one, due to the 7027 biasing at about 35v as opposed to 12v bias for the EL84s in the ASR433). This can be done with a new tap from the power supply- I would recommend coming off the main 340v tap of the power supply caps (that's what runs the output transformers), and make a voltage divider, from a 330K 1w resistor and a 39K 1/2w resistor connected in series, with the 330K connected to the power supply and the 39K connected to ground. Bypass the 39K resistor with about a 220uf 63v cap. Remove the hum pot heater "ground" connection from the output tube cathodes, and run them to the junction between the 330K and 39K resistors. That should provide between 35 and 40v positive DC bias to the heaters.

    Regards,
    Gordon.
     
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  20. gadget73

    gadget73 junk junkie Subscriber

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    and again, make sure the hum pots are good.
     

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