[AMRadio] ] Lifting center tap to kill B+


Jim Wilhite w5jo at brightok.net
Sun Dec 14 21:12:53 EST 2014


In the installation you used did you have trouble with arcing across the 
contacts of the relay when it opened or did you use a vacuum relay?  Were 
there any problems associated with breaking the HV AC?

Jim
W5JO

-----Original Message----- 
I used to break the CT connection to cut off the HV to my class B driver
stage during stand-by, since I use a  type 83 MV tube type rectifier, and
the filament has to stay on all the time, so there is no way to kill the HV
transformer without killing the rectifier filament as well. After several
years of service, I  lost the power transformer. Fortunately, I had an
identical replacement. But after installing it, I got to thinking that maybe
an internal insulation breakdown,  caused by repeatedly lifting the midtap
during stand-by, gradually crapped out the insulation. I changed from using
a SPST relay from CT to ground to using a DPST relay, with one set of
contacts opening the circuit at each end of the HV winding, breaking the
connection  between the transformer winding and each plate of the rectifier
tube.

Don k4kyv




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