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

Steve WA1QIX wa1qix at piesky.com
Mon Oct 26 08:09:24 EDT 2015

This is a definite overload scenario, and there really should be a 
detection element for this condition.

In the overload protection system I use for the solid state 
transmitters, one of the inputs is SWR.  This is very easy to detect 
- I modified an existing SWR meter and fed the forward and reflected 
power outputs back to the overload shutdown system.  A comparator 
compares the reflected power to the forward power, and if the 
reflected power is too high, that's an overload and the system shuts down.

SWR shutdown is part of virtually every "rice box", and it's not too 
hard to implement.

Regards,  Steve

At 01:31 AM 10/26/2015, you wrote:
>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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