[AMRadio] WARNING: Overload protection is not 100% reliable even when relays are functioning properly

Donald Chester k4kyv at charter.net
Mon Oct 26 02:31:19 EDT 2015

With both my converted Gates broadcast transmitter and homebrew HF-300  rig,
I have experienced the following scenario more than once: some kind of
anomaly occurs somewhere in the antenna system, maybe an arc-over at the
tuned circuit at the base of the tower, a bug crawls between the plates of
an air variable, the antenna falls down or shorts out in high wind, or maybe
a corroded connection fails. This throws the system off resonance and
de-couples the load from the final, so that manually re-adjusting the main
plate tuning control would dip the plate current to near zero, but under
the conditions of the malfunction, just enough reactance is thrown into the
system to de-tune the final to where it isn't  dipped all the way to zero.
Thus the final may be detuned from resonance just enough that the plate
current remains near normal -- but all the DC input is being dissipated in
the plate of the tube with no RF delivered to the output.  You could
simulate the effect by disconnecting the feed line from the transmitter,
dipping the final to minimum plate current, then adjusting the plate tuning
away from the resonant dip just far enough to bring the plate current back
to its normal value. The tube plates then dissipate well beyond their rated
maximum, but the final plate current  is not high enough or far enough from
normal to kick the overload relay.  Under such conditions, the tube(s) may
quickly become damaged or destroyed without the overload protection sensing
anything abnormal. This has occurred a few times while I was transmitting,
but I noticed nothing out of the ordinary until I happened to glance over at
the final amplifier plates and saw they were running bright red or orange,
or I noticed the monitor scope was displaying no output from  the
transmitter even though the final plate current meter showed a normal

This could easily occur at stations where the transmitter is remotely
located away from the operating position and controlled from another room or
building, or even right in the shack with transmitters  that have no viewing
window to allow the  operator to see the final tube plate(s).

Don k4kyv

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