Hi Bob and all,
You wrote:
\
> Late versions of the SB-220 and, I suppose all of the SB-221 runs, had a
> different capacitof for C1 which had just a little bit wider plate
> spacing in the design. This was an attempt to stop arcing and spitting
> when the parasitics might occur.
That's utter nonsense.
The spitting and sputtering is not due to parasitics at all.
The plate variable/ tank arcing problem most often is incorrect
tuning, and secondarily incorrect relay sequencing in the relay
contacts.
What happen in ANY less than 360 degree plate current conduction PA
is the tube "tugs" on the tank (remember the tube is a varying
resistance, not a source). The tank flywheels (or more correctly acts
like a pendulum) and swings past zero volts negative from it's Q.
If you underload the PA, or remove the load through ANY type of
problem (it can be an output relay contact that fails to close, or
an antenna or tuner that arcs or faults), the PA has nothing to
dampen the swing of the tank voltage. In a few cycles, the voltage
can build up to several times the anode voltage. This voltage will
continue to build up, until the weakest point in the system fails or
until the voltage becomes so high power is transferred to the
load.
Normal fully loaded PEAK voltage in the tank of an AB class amplifier
is about equal to (only slightly less than) the supply voltage. look
at the dc voltage, and you can expect about TWICE that dc voltage on
the tube anode during peaks and about the same PEAK voltage as the dc
supply voltage on the tank cap (in the case of the tank it swings +
and - because of the blocking cap).
But if you underload the tank for ANY reason, the voltage can easily
reach three times the dc voltage.
The same is true if your exciter has a power transient (common with
many rigs), because overdrive is like underloading so far as the
anode system is concerned.
All of this is well documented in engineering texts, and not that
difficult to follow through with logical thought. Choke type fly-back
or resonant switching supplies work on the same principle.
73, Tom W8JI
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