The AM Forum

It is a pitiful thing to lose an expensive power tetrode by applying screen voltage BEFORE the plate voltage is there .... here is an idea for using a old technology tube (6BK4) as a HV detector (to 27kV)

                        -------------|------------------- B+
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                                      ------

            6BK4                  - - - ----|
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                                      |-----|---
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                                              |
                                             \          /------------relay coil----- +lv
                                              /       |/
                                 2k         \<----|    npn
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                                          gnd       gnd

well it just doesnt get too much simpler than this .... nothing fancy for the 6BK4 fil ...6.3v @ .2A near ground... just adjust the pot for desired minimum acceptable HV and use relay contacts to switch on the screen voltage

n.b. (7 sept)  of course you could just use a resistor(s) instead of the tube but would have a more limited detection range .... look at the tube manual curves .... also a differential amplifier would give a sharper go/no go decision

Better yet, detect the screen grid current.

G3SEK sells a board and kit to do just that, however it is quite elaborate.   I took the essence of his design and installed into my NCL 2000 Linear amp.   

This pic shows the detector, and you simply test with a LV power supply to determine resistor value needed for the Opto-coupler to turn on at your desired current level.

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/NCL-2000/Bias%20Bd%203/1%20640_zpspcuplfp1.jpg)

The opto-coupler in the previous pic is then followed by a emitter follower (Q1) and low current/voltage self latching relay.   The second set of contacts are used to actuate the amplifier's own shut down circuitry.
(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/NCL-2000/Bias%20Bd%203/2%20640_zpscemdkgwa.jpg)

If the amp is a multi-tube affair you can simply "OR" the opto-couplers as in the pic below.
(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/NCL-2000/Bias%20Bd%203/4%20640_zpsmxgzcb1x.jpg)

My brief article can be found HERE (http://www.northland-drifters.net/), scroll down to restoration/repair projects to "Screen grid protection"

    Ya,

        Gotta lash it up better. E scr should be
disabled until Ep is present.  The only thing I
worry about loss of Airflow.  Also easy to alarm.
Just need a "Round To It"...

         Sri about the jug....

/Dan

Even simpler is to use a series R off the HV and feed a high resistance relay, 5000 or 10000 ohm coil resistance, Use the contacts to turn on the screen V. No need for silicon or tubes even.

 
Darn tootin'. A sensitive latching relay is connected across a rheostat in the screen circuit of the 4-1000 here and it does indeed trip without delay, and illuminates a lamp on the front panel. It's unlatched by the front panel push button. The rheostat gives a wide variety of trip points. I think it's set about 150mA.

If there is a time delay or integration wanted so that a momentary pulse or excursion won't trip it, a small oil cap can be put across the rheostat.

If it is wanted to trip more hastily upon the onset of a very large current, an additional voltage dropping resistor can be put in series with the rheostat, and a small oil cap can be put across that.

I have not permanently added either 'enhancement' only played with this, and never blown a 4-1000 screen using the simple no-C circuit shown despite several major incidents.  I like the R off the HV supply to a relay but what's already there is OK.

look ma, no silicon!  is the point

If the high impedance relay is sensitive enough, just place it in the bottom of your high voltage meter reading string in proximity with the meter.   A couple of resistors ought to get it about right in that area yet keep the meter reasonably accurate.

E.g., A "Y" split at the bottom of the HV resistor string, one for the relay and one for the meter.