Current sensing overload shutdown for tube transmitters - Hall effect

[W2PFY]

Ok, so that answers the question. You  need to use a configuration where the screen voltage is supplied via a dropping resistor for a clamp tube to work. I guess the only problem with that circuit is the waste heat and the possibility of too much screen voltage under a light load. Thanks guys! 

[KA2DZT]

Terry,

I'm using a clamp tube circuit with my single 813 with screen dropping resistors.  Screen voltage does not vary much with lighter loads.  I have a screen voltage meter in the rig.  The clamp tube is a 6Y6.

With a 4-1000 probably a single 6Y6 may not be big enough to handle the current.

Fred

[K1JJ]

Unfortunately, a screen dropping resistor is impractical for a plate modulated 4-1000A. If the plate voltage is 4KV, we need to drop 3500 V at 140ma of screen current.   This is 490 watts of heat.


Solution:

Use a fixed 500 volt screen supply with a self-modulation choke. If loss RF drive protection is desired, it would be easier to sense the RF drive, rectify it and drive a relay to open up the screen circuit. Using a SPDT relay, connect the tube screen to ground during fault.   Also, using a Variac, an adjustable screen supply allows for fine-tuning effects of different plate loading and other variations on the fly that a fixed dropping resistor cannot. (unless a slider on the resistor is used)

I am also considering a Hall effect to sense for grid overdrive. This can easily happen if the exciter can supply more power than needed.   If the plate current and screen current are turned off by error, these overloads will not work, thus the grid protection too.

Because I'm using a small fixed bias, it will not require a loss-of-drive circuit. And hopefully the plate and screen trips will sense if the fixed bias fails.

T