[AMRadio] ] Lifting center tap to kill B+


Donald Chester k4kyv at charter.net
Sun Dec 14 20:46:13 EST 2014


Something to consider is  how well the transformer manufacturer insulated
the midtap connection to the HV winding. A problem with using a transformer
designed for standard full wave rectification is that the midtap may have
little or no insulation from the core or frame of the transformer, because
the tap connection normally is directly grounded or very close to ground
potential. I have seen some transformers, mostly old TV power transformers,
in which the CT was grounded directly with an internal connection. In the
full wave bridge circuit, substantial voltage appears at the midtap and may
break through the insulation to ground, probably ruining the transformer.

Something else to be careful about is the common practice of placing the
filter choke in the negative lead, from CT to ground. This keeps the tap at
or near DC ground potential, but under load the full a.c. ripple voltage
still appears across the choke and thus at the midtap. This ripple voltage
may peak nearly as high as the DC output voltage, causing the same type of
failure with a poorly insulated midtap as running the transformer in full
wave bridge configuration.

Lifting the midtap from ground to cut off the B+, you once again have the
same problem. While the switch is open, you have zero current flowing
through the  circuit and thus zero voltage drop across the transformer
winding, and likewise, zero voltage drop across the bleeder resistor, filter
cap leakage and any other conductivity there may be  to ground at the B+
terminal. This effectively shorts the +HV line to ground, albeit through a
high resistance, but it still leaves full HV at the midtap and the
possibility of insulation breakdown.

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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