plate HV detector for tetrode / pentode power tubes

[w4bfs]

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+
                                         |
                                         |
                                      ------

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

[MikeKE0ZUinkcmo]

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.



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.


If the amp is a multi-tube affair you can simply "OR" the opto-couplers as in the pic below.


My brief article can be found HERE, scroll down to restoration/repair projects to "Screen grid protection"

[N8ETQ]

    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

[John K5PRO]

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.

[Opcom]

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

[W3RSW]

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.