[AMPS] YC-156 grid current at zero B+

[Dave]

We tend to get accustomed to the zero-bias 3-500Z triode and get
surprised at the grid voltages required in other tubes.

I recommend using the 3CX15,000B7 grounded grid curves for your design
work with the YC-156.  The two share the same cathode, grid and anode
core structure, the YC-156 merely has a smaller cooler.  And the
3CS15,000B7 is rated for CW, so the curves include more "normal" plate
currents that we might use - i.e. less than 10 amps.

The curves do not show what grid currents will be at zero grid voltage,
but Rich's words have merit: when the cathode's hot, it emits
electrons.  Even though this is a radial-beam tube, grid interception
can still happen, and I would expect grid current - maybe even "oh no"
grid current, with no grid voltage.  The curves don't, of course, show
the variability of grid bias versus tube, but I seem to remember some of
our YC-156's in production required about -80 for cutoff at 6kV.

This is a oxide-cathode tube with an indirect heater.  The cathode
connection is internally tied to on of the heater connections.  As you
look at the underside of the tube, one of the heater connections is the
center contact - a 10-32 threaded hole, I think (8-32?), the next outer
contact is the other heater contact and is also the cathode.  The ring
that bolts it to the chassis is the grid.  Use your voltmeter to confirm
that the bias voltage appears positive on the cathode with respect to
ground.  If it's negative, then you've basically turned the grid into
the anode.  By the way, for short (minutes) testing, don't worry about
killing that grid.  Eimac's internal test processes run the tube for a
few minutes at 2kV anode and 1 Amp on the grid for a few minutes.

Confirm this for us: does the heater need to be warmed up for you to get
this grid current or do you get it immediately on application of power?
If it requires the heater to be warmed up, then it's clearly associated
with the tube.  If it happens without a warmed up heater, I'd say it's
external.

Curious to watch your progress with this one...

Regards,

Dave W8NF


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