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Elevated Heater Polarity

Discussion in 'Tube Audio' started by triode17, Jul 12, 2018.

  1. triode17

    triode17 Super Member

    In a phono preamp I built, there is an upper-lower triode pair, similar to a White Cath. Follower. The upper cathode has 87 Vdc to ground. So I connected a voltage divider from the HV supply to float the heater supply up on 60 Vdc. Positive, to the heater Common. The odd thing is, when measuring from Heater (positive meter lead) to cathode (neg. lead), I get -27 Vdc. Negative ! Why? Should I simply float the +12.3v heater supply Positive instead of the Common? I'm puzzled here.


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  2. gadget73

    gadget73 junk junkie Subscriber

    Southern NJ
    Thats proper. The heater supply is 27v below the cathode.
    GordonW likes this.
  3. GordonW

    GordonW Speakerfixer Subscriber

    Marietta/Moultrie GA USA
    Yep. And that differential is well small enough, to never cause heater-cathode shorting problems.

    My question is- what's the voltage on the lower triode cathode? It may be better to adjust that 60v, to where it's half-way between the voltage of the lower triode cathode and the voltage of the upper triode cathode. That would reduce the net risk to the cathodes of both parts, to a minimal level.

    I'm guessing the lower cathode is less than 10v DC above ground? If it's 10v, then the average of 10v and 87v, would be about 48.5v. So, in that case, something like 45v or 50v would be more practical than 60v, as a heater DC float voltage. That would put both cathodes at less than 40v differential from the heater, in opposite directions. That should be completely OK for any common triode I can think of...

  4. triode17

    triode17 Super Member

    The lower triode cath. is at 1.9v. I just drew the heater to cathode circuit and its makes sense now. The cath. is at 87v, the heater is at 60 volts, so the difference between them is -27, the heater being at a lower potential. The reason I balked is I was thinking that +87v and -25 v would equal 112v, which is the mean.

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