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

C.L. Mitchell rsq14adam1 at hotmail.com
Wed Oct 28 15:07:47 EDT 2015

It seems that you could rectify a little RF from your monitor scope and use it to drive a relay [through appropriate DC amplification and filtering] in series with the overcurrent relay in the final.  I have not done this; it just seems logical that it would solve the possibly expensive problem.

Mitch,  K9PNP

> From: k4kyv at charter.net
> To: amradio at mailman.qth.net
> Date: Mon, 26 Oct 2015 01:31:19 -0500
> Subject: [AMRadio] WARNING: Overload protection is not 100% reliable even	when relays are functioning properly
> 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
> indication. 
> 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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