scott 299a hum from phase switch?

Discussion in 'Tube Audio' started by muscmp, Apr 15, 2018.

  1. muscmp

    muscmp Active Member

    Messages:
    128
    Location:
    socal
    i need to order a slide switch but, could a bad phase switch cause a hum in one channel? when i flip the switch, i lose one channel. i swapped the speaker wires and the hum did change to the other speaker. the switch does seem a little sloppy.

    i recapped it a while back, except for the bias cap can and it has been working quite well.
    thanks,
    mikeb
     

     

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  2. century tek

    century tek Super Member

    Messages:
    3,272
    Location:
    The Evergreen State
    Its possible that the hum is being caused by something else like a capacitor or tube or bad connection etc.

    I think your speakers have to be connected properly in order for the phase switch to actually work. All it does is reverse the + and - speaker wires on one speaker. I would assume that if the speakers were not connected properly to begin with that you might get the loss of one speaker. :dunno:

    Since it is connected directly to output of one channel, the phase switch really wouldn't be able to cause any hum. :idea:
     
    Last edited: Apr 15, 2018
  3. muscmp

    muscmp Active Member

    Messages:
    128
    Location:
    socal
    thanks, i'll start with the tubes. i didn't replace the bias cap can due to lack of space for more caps, but, may have to do it.
     
  4. Dandy

    Dandy Super Member

    Messages:
    4,471
    Location:
    North West, UK
    In my 222C, one of the four bias caps was bad. There is room in the 222c to replace all four under the chassis and just leave the original disconnected on top.
     
  5. triode17

    triode17 Super Member

    Messages:
    1,185
    As a side note, whenever I find a Phase Switch, I remove it and its wiring. The switch is a (relatively) high current device. I'm sure you can properly connect red + to red + and - minus to -minus, so you don't need the switch. The contacts will eventually go bad and if that happens while you are playing the amp, it could end up with no load on one channel which could blow your output transformer. About a $100 failure, if you can fix it yourself.
     
  6. muscmp

    muscmp Active Member

    Messages:
    128
    Location:
    socal
    i've pulled the unit and most likely will just replace the bias cap(s) and the switch also. the switch looks harder to access and replace than the cap tho. ha! or, i'll unhook it and leave it there for looks.
    thanks,
     

     

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  7. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,447
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    I leave them, but I just don't use them. Most things have a flying lead that you connect to the proper impedance tap, and then you get two terminals for the speaker to connect to. The other channel just has a common and the impedance screw. What I do with them is connect both speaker - terminals to the common, and the + leads to the proper impedance. That leaves the unit un-modified but you don't have to deal with the switch getting stupid.

    My Sherwood S-7000 has an odd reverse switch. Instead of the usual switcheroo at the speaker terminals, it actually changes the connection off of one of the tone amp tubes from plate to cathode, reversing the phase of the signal before it ever gets to the power amp. I left that one alone. My S-5500 and the Fisher 600 use the speaker switch arrangement.
     
  8. primosounds

    primosounds SE KT120 w/ 6J5G drivers. LCLC ps + 2 x 5ar4 Subscriber

    Messages:
    2,574
    Location:
    Terra, 21st century CE
    An odd hum can be from many different things but i would not suspect the switch. You can easily measure the resistance between terminals to find a difference but you might have to lift the leads if you try.
    The reason phase reversal switches were almost universal in integrated amps from back during this era was that most speaker cord was just "lamp cord" which is not marked for polarity. And so the user could strip the end of the cord and hook up whatever lead and make it all in phase with the switch. Of course nowadays all speaker cord is marked and a phase reversal switch is not needed. And sending the speaker signal thru another switch is also another layer of contacts which can reduce signal integrity or add anomalies.
     
  9. century tek

    century tek Super Member

    Messages:
    3,272
    Location:
    The Evergreen State
    Stereo was also pretty new back then and mono systems didn't need phasing unless you connected another speaker and in which case, it needed to be phased with the other one even if both were mono.
     
  10. muscmp

    muscmp Active Member

    Messages:
    128
    Location:
    socal
    thanks to all. i think i will leave the switch but change the wiring as gadget mentioned. and, i may as well do the bias cap in the process. the funny thing is that this is the only receiver/amp i have that can handle my 4 ohm infinity 2000 speakers.
     

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