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

Rob Atkinson ranchorobbo at gmail.com
Mon Oct 26 05:21:16 EDT 2015

This is great advice.  Another thing to consider is whether or not
your final tubes require some amount of forced air cooling.  Let's say
your input power to the final is 350 watts and you have a 4-400A in
the final.  It has some small amount of forced air but not much so the
rig doesn't make a lot of noise.  You might think you are okay in
Don's scenario because the tube can dissipate 400 watts.  But if the
tube doesn't have the cooling it needs to dissipate its rated maximum
you may still have seal failures.


On Mon, Oct 26, 2015 at 1:31 AM, Donald Chester <k4kyv at charter.net> wrote:

> 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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