[AMRadio] 75A-2 power consumption


Jim Wilhite w5jo at brightok.net
Thu Feb 19 18:25:10 EST 2009


It takes right at 90 volts for that rectifier to conduct.  When it starts conducting the caps will start forming.   If you are going to form the caps in this fashion, I would pull all other tubes and put the variac at 90 volts.  wait about 5 mins and raise it to 100 volts.  Put your finger on the cap to see if it is heating.  If so, back it down about 5 volts and wait for the cap to cool.  When it cools then start up with voltage again monitoring the temp of the cap.  If it gets hot, wait for it to cool.  That cap should, if it is good, form in about 20 minutes like this.  If it doesn't, get a new one.  If it does, check it in about a year because it probably will go bad by then.  

I wouldn't replace the tubes wholesale, you probably will throw away some of good tubes.  If you have a signal generator you can tell pretty well how good they are by the sensitivity of the radio.   If I remember right the fuse in that thing is 2 amp.  With the volume turned up loud it probably will pull near 1.5 amps through the fuse, so you variac meter should show about that amount.

Jim/W5JO
  ----- Original Message ----- 


  On Thu, Feb 19, 2009 at 3:11 PM, John Carrington, M.D. <jmc05 at charterinternet.com> wrote:

    I'm wondering if I don't see any filament glow with that tube... I'm going to be tempted to  bite on the Ebay ads that just replace all tubes... the set for this receiver is 69 bucks.

  It's real hard to see the filaments in that tube John.  Wait until the room is dark and take a look... they are pretty faint.
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