Joe wrote:
>> However if the amplifier is underloaded, voltage in the
>> output
>> "tank" will increase. The "limit" for underloading is
>> operating
>> into an open circuit (T/R relay not closed, etc.). If
>> the bandswitch
>> is operating close to the limit already (as is the case
>> in some
>> amplifiers), overdriving an underloaded amplifier can
>> damage the
>> bandswitch.
Rich replied:
> Depends on whether the Tune-C or the bandswitch has the
> lower BDV.
> If the Tune-C arcs at 3600v and the bandswitch arcs at
> 5000v
> (SB-220), the Tune-C protects the bandswitch.
Another technical misstatement.
The SB-220 switch is from the same Centralab family used in
the lower voltage Ameritron amps, Dentron amps, and many
other places. Like any switch, it has a variety of breakdown
voltages depending on what contact to what other contact and
even how the soldering is done on the switch. Like any
system with high voltages, the shape of conductor surface
where it extends and everything around it is nearly as
important as the spacing for determining breakdown voltage.
The safe reliable upper limit in that switch, even when the
switch is optimally designed and connections have careful
lead dress and soldering, is just over 3000 volts. I know
that from months of actual testing in a lab with new
switches. It's actually a race to see if the tune cap or
bandswitch fails first with excessive voltage.
Tank voltage, when the amplifier load faults, a relay
doesn't cycle in proper time, or an exciter overshoots can
easily be several times dc anode voltage in an amp running
lower angle AB, class B or other conduction angles near or
below 180 degrees.
73 Tom
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