[AMRadio] 75A-2 power consumption

John Carrington, M.D. jmc05 at charterinternet.com
Thu Feb 19 18:36:54 EST 2009

Great info Jim... thanks so much.  I'll give it a try.


From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim Wilhite
Sent: Thursday, February 19, 2009 3:25 PM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] 75A-2 power consumption

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.

----- 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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