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DC heater question (Randall RG50)

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  • DC heater question (Randall RG50)

    Looking at a Randall RH50T with weak output. The caps. for the V1 & V2 heater supply are all bulging, one is leaky and the DC heater voltage is low.
    I will replace them with 16V rather than the stock 10V but my question is about the circuit. With 6.3VAC feeding the bridge won't the DC out be too high (about 7.5VDC) going to V1 and V2?
    I've never paid much attention to DC heater supply circuits, is this a common way of doing it?
    Second question about this schematic: what is IC4 (shown to the right of the power tubes) doing? Shouldn't it's output be going somewhere?
    Attached Files
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."



  • #2
    Yes thats the common way of doing it although I have seen a schematic with a 1 ohm
    (presume 5 watt) resistor after the first filter cap.
    Please see attached examples.
    Hmmm that Ic looks like it needs to go somewhere perhaps a line out ? which never eventuated.
    That schematic looked so familiar I had to look at the Ashton 50w supposedly designed in Aust and made in China. Couple of minor differences I spotted but the date is the same ????
    Attached Files

    Comment


    • #3
      Thanks. I will check the loaded voltage coming out of the bridge once the new caps. are in. The mesa drawing has the 39R resistors which maybe drops the voltage a bit. That is more what I expected to be the norm. I will check maybe the Randall does have them somewhere but not shown on schematic.
      Funny, the Ashton drawing does look the same, even has the IC to nowhere.
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


      Comment


      • #4
        Just to update, with new caps and bridge rectifier for the DC heaters, the voltage is a bit higher than the AC heaters but not as bad as I thought. With 123VAC line the AC heaters are 6.2VAC and the DC heaters are 6.6VDC.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment

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