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Resistor (to act like a fuse) for heaters center tap grounding

Discussion in 'DIY' started by mroboto, Oct 12, 2018.

  1. mroboto

    mroboto Super Member

    Messages:
    1,465
    Hello,

    Situation: This is for a pair of 6SN7 heaters, the center tap of the 3.15V supply

    First, the heaters center tap was elevated, I removed the elevation and I connected it to ground via a 120 ohms 2 watts resistor.

    But I got a slight hum, so I ordered lower values resistors to have a better grounding for the heaters center tap.

    I also grounded the chassis with the 3rd prong of AC power socket, I don't know if this is or could be the culprit for the hum, but first, I think about my bad grounded heaters center tap.

    I will soon receive:

    4* 0.01 ohm 1 watt Vishay Dale
    2* 75 ohms 2 watts Panasonic Metal oxide
    and a lot of 0.12 ohm 1 watt Generic Metal oxide

    What is the best choice of one or a combinaison of those resistors for this situation ?

    Or any other value ?

    Thanks for your help.
     
    Last edited: Oct 12, 2018

     

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  2. avionic

    avionic " Black Knights " Subscriber

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    Location:
    Fort Dodge, Iowa
    Temporarily lift the ground.
     
  3. mroboto

    mroboto Super Member

    Messages:
    1,465
    I lifted the ground from the AC power cord 3rd prong and I still have a slight hum that seems to be the same, when I put my ear close to the woofer (90dB sensitivity woofer).

    I hope to be able to keep it grounded by the 3rd prong, because otherwise, the chassis is only grounded by the RCA input jacks.
     
  4. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,062
    Location:
    Southern NJ
    The heater supply should have no current flow to ground, so the value of the resistor is not going to be critical. Honestly you could tie it direct to ground and it won't matter.

    Why did you un-elevate the heater supply? If it was quieter that way, I'd put it back the way it was. Often heater supplies are elevated to control heater-cathode voltage differences. The greater the difference, the more chance for hum. It also tends to cause damage in the heater insulation if the voltage difference is too great.
     
  5. mroboto

    mroboto Super Member

    Messages:
    1,465
  6. mroboto

    mroboto Super Member

    Messages:
    1,465
    I tied the transformer heaters center tap directly to ground, and I still have that slight hum.

    I will try to elevate it as it was first..

    It was elevated from the B+ cap with a 120K resistor and a 46K as bleeder for the 22uf elevation cap.
     
    Last edited: Oct 13, 2018

     

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  7. mroboto

    mroboto Super Member

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    1,465
    I re elevated it, and I still got that small level of hum.

    Appart of removing the cathode feedback, my other mod is to remove the 100 ohm resistor and connect the 2.7K cathode resistor directly to ground, and an addition of a 4.7uf 100V CBB film bipolar cap in parallel with the first stage cathode cap and resistor, don't know if my hum could be related to that film bipolar bypass cap, I think I must remove it to see.

    ps: I also added bypass caps to 2st stage cathode cap and other power caps.
     
    Last edited: Oct 13, 2018
  8. mroboto

    mroboto Super Member

    Messages:
    1,465
    I removed them, still have little hum, and I am about to give up.

    I don't remember hearing that hum before, but it is very slight, maybe it was there at some point and I didn't hear it..
     
  9. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,062
    Location:
    Southern NJ
    If you removed the negative feedback, the gain of the amplifier will increase significantly. It may just be picking up noise from imperfect lead dress or a lot of other things that it wouldn't have with feedback just because of all the gain.

    Why remove the feedback? Its a good thing in terms of amplifier performance.
     
  10. mroboto

    mroboto Super Member

    Messages:
    1,465
    Yes, the gain increased a lot, before, I turned volume to 10 o'clock , now at 8 o'clock it's enough.

    There is sometimes benifits to remove the feedback from what I read.

    From what I think, I was better to not add a bypass film cap in parallel with the cathode capacitor without discarding the feedback circuit, and I wanted to add this film cap.

    Maybe it's one faultly KT88 tube ? The last faultly Shuguang KT88 for the first hours was working OK, and after some time was caused ticking noises in both channels, replacing them with Genalex Gold Lion reissues fixed the ticking noise issue, I didn't notice hum in the beginning, but now...

    Like every time I do a mod on my new amp, I have tubes problems that makes me undo my work only to realise after that it was a tube related issue.

    Maybe tommorow I will try the Shuguangs to see if the ticking noise come back and the hum is gone.

    The AC cabling is well made, twisted by the factory.
     
    Last edited: Oct 13, 2018
  11. mroboto

    mroboto Super Member

    Messages:
    1,465
    I did try the Shuguangs and I still got hum, so, the Genalex reissues tubes are OK.

    As previously said, the gain increased and he probably pick up more noise.

    Would replacing the 1st stage cathode resistor for a higher value be a good idea ?

    An approximately twice the value resistor like 5.6K can reduce the bias, the gain, and the hum considerably ?
     
    Last edited: Oct 15, 2018

     

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  12. triode17

    triode17 Super Member

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    Looking at the schematic, my concern is the 120v on the second cathode. This violates the 100VDC max. spec. for H-K voltage and can cause a steadily increasing hum. I would rig a voltage divider off the B+ to float the CT on about 60 Vdc. But add a 0.1uF film cap across the 60v. leg to Ground. This should shunt any residual hum to Ground. I've done this and it worked.
     
  13. mroboto

    mroboto Super Member

    Messages:
    1,465
    The capacitor in place was a 100V , instead of 120V or more , so I assume there was less than 100V, seeing the schematic, I replaced it for a 200V, I already added a decoupling cap , a 4.7uf 450V CBB, thanks for your suggestion.
     
  14. wdecho

    wdecho New Member

    Messages:
    12
    Tubes can be noisy. If your heater taps are elevated doubtful if that center tap is the problem. Having a scope would be helpful to determine if transformer or power supply hum. Transformer hum is 60hz and power supply hum 120hz. There are youtube videos that one can hear the difference. Many times the hum is caused by input wires and their placement. Be sure input wires or cables are no where near AC sources. With 90db speakers you should not have hum from say 3 ft. Ground input to see if that makes a difference. Persevere and you will find a cure. I have moved input wires a tiny bit and cured hum on many builds of mine. Shielded input wires are desirable.
     
  15. rothwellaudio

    rothwellaudio Forums Sponsor Sponsor

    Messages:
    1,508
    Hum can be caused in many different ways - I'm not convinced it's the 3.15V heater supply which is causing it here.
    Yes, I agree, the hum could be coming from anywhere and the extra gain has just made it easier to hear.
     
  16. mroboto

    mroboto Super Member

    Messages:
    1,465
    Here is a picture I taken prior to I remove the cathode feedback and replace the cathode 2.6K (That appears like a 2.7K on schematic) + 100 ohms by a single 2.7K , and add a 4.7uf 100V CBB.

    https://www.cjoint.com/doc/18_09/HIbmLRdbNys_IMG-4.jpg

    I ordered 5.6K resistors to put instead of the 2.7K , will see.
     

     

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