5e3 HV center tap ground location

Discussion in 'Amp Tech Center' started by JamesAM, May 18, 2020.

  1. JamesAM

    JamesAM TDPRI Member

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    Hi all, I’m currently rehabbing a 5e3 I picked up and I’m trying to track down the root cause of a buzz. I’ve got another thread about my troubleshooting here: https://www.tdpri.com/threads/5e3-expert-opinions.1023925/

    but my question here, which I think warrants a separate thread, is about the standby switch. Mine is currently being fed directly from the HV center tap, to the switch, to the star ground in the power section with the safety ground and one half of my filament (no heater center tap).

    robrob’s layout has the HT going directly from the PT to the negative lead of the first filter cap.

    is this due to the possibility of excessive noise from current running to the chassis? The result should be 60 cycle hum, right? If the first filter cap is grounded using the brass plate, would it make more noise?

    What’s the safest way to test this- should I clip a meter lead to the switch tab on the 200v ac setting and touch it to the negative filter cap lead to see if it quiets the amp down, or Should I drain the caps, route a double sided clip lead from the standby switch to the cap (paying strict attention to polarity) and power the amp on?

    if the filter cap is grounded to the brass plate, would the HT there create a ground loop?

    thanks, here are pics:

    D8D6E389-C066-4488-A9D8-16A30075535D.jpeg C68C7C7E-92AE-4A4C-8DB1-EF3ABC3FBED0.jpeg

    6E7FF987-3AE5-43B1-909A-AA6E2083B5A1.jpeg
    956026C9-9244-4B70-B541-BFCE5BA64BC4.jpeg
     
  2. Nickfl

    Nickfl Tele-Afflicted

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    I'd look at your filaments before worrying about the HT center tap ground reference location.

    If I recall correctly power supply noise would be 120 hertz and heater hum would be 60hz.

    If you have the old style heater wiring with one side of the filament supply run through the chassis that is by far your most likely source of hum. Replacing that with twisted pairs and an artificial center tap will help and elevating that artificial center tap on the power tube cathodes will help even more.
     
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  3. JamesAM

    JamesAM TDPRI Member

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    Thanks! I’m fairly certain I’ve got a combination of both 60 cycle hum (very quiet and only with volume on 0, acceptable level to me) and 120 cycle hum that’s either from the HT or from a bad ground. My guess is it’s a bad ground in the preamp, as the buzz increases with volume and goes away when v1 is pulled. I want to delete the standby switch anyway and was hoping rerouting the HT would help with the noise.

    I’m going to sort out the filaments next as they needed to be done anyway with an artificial CT. After that, I’m likely taking the whole board out and replacing it as time allows.
     
  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You can run the HV center tap from transformer to standby switch, then to the first filter cap negative lead and then add a "Power amp ground" wire from the first filter cap negative lead to ground.
     
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  5. JamesAM

    JamesAM TDPRI Member

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    Thanks! Will it be acceptable if the existing ground is from the cathode resistor to the brass plate on the control panel, or is that too close to the preamp?
     
  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yea, that's fine.
     
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  7. schmee

    schmee Doctor of Teleocity Silver Supporter

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    It may be. I do know on the 5E3 I built that separating the preamp and inputs from the power section grounds made a huge decrease in hum.
     
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