PYE Stereophonic Black Box Hum

Discussion in 'Shock Brother's DIY Amps' started by tvas22, Dec 9, 2019.

  1. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Hi everyone.

    Still slowly working on my Princeton-esque build, been waiting on parts for a long time for that.

    My friend has an old PYE Stereophonic Black Box with some bad 50hz hum, and he asked me to take a look at it. I am pretty much an amateur, but he doesn't want to pay to have it sorted and, bar the hum (which doesn't get louder as it is turned up, and so can be drowned out when loud) it works fine. I won't be doing this for another couple of weeks, but I figured I'd start having a think.

    The plug needs replacing anyway, so I thought I'd start by putting in a modern 3 prong and giving it a proper earth (which it currently doesn't have).

    I found a schematic online, which included some general tips on how to reduce hum - how would these suggestions help with the hum? The first (written at the bottom of the attached file) seems to suggest reducing a filter capacitor? Or, is it suggesting adding an additional one in at that point? Also, I can't figure out how the 100k resistors could help?

    I was also curious about the heater wiring - one hot wire feeding the heaters and one side grounded rather than a twisted pair. A little reading about this suggests that this is relatively normal in older radio circuits, but some suggest it could be a source of hum - would it be worth wiring the heaters as a twisted pair and adding an artificial centre tap?

    Thanks!

    (Apologies if this isn't the right place to post this..)
     

    Attached Files:

  2. Wally

    Wally Telefied Ad Free Member

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    It is of an age that one immediately suspects the electrolytics. Even if the hum is not coming from the e-caps, ime, they are of an age that mandates replacements.....or so I am guessing since it is a tube stereo amp. I could be mistaken...but the lack of a 3-wire Ac supply also dates it.
    Yes, that type of heater filament wiring can lead to noise, but that type of noise would have been there since the amp was built, it seems to me.
    Decreasing the value of that power supply filter to the input preamp stages in order to decrease hum is a new one for me.
    interesting....
     
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  3. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Yes, it’s a new one on me too. I’m not sure that is definitely what they meant - I will start by swapping it out and replace with single caps.

    Thanks for your advice, I will start with the electrolytic caps and then look at the heater wiring if that doesn’t help.
     
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