Hi Jon, George & all...
The underlying issue with respect to applying positive voltage to vacuum
tube plates and/or screens prior to completion of cathode warm-up is this:
until the cathode is uniformly hot and emissive, any strong electric field
at the surface of the cathode may draw current from, and create sufficient
current density at, one or more "hot spots" (i.e., small areas that get
"hottest soonest") sufficient to damage the cathode. I've heard that
particles of oxide may be pulled off the cathode, damaging it and
contaminating other elements of the tube. E.g., presumably cathode oxide on
a gold-plated grid is bad news for secondary emission.
However, if the tube is grid-biased beyond cutoff, no cathode current can
be drawn because the cathode is "shielded" (by the control grid) from
positive fields due to plate and/or screen voltage. This is phrased rather
seat-of-the-pants-like and others probably can refine the details and
underlying physics.
But the bottom line is: All ALPHAs apply standby potentials simultaneously
to heater, grid, plate, and screen grid if applicable, at turn-on - most
importantly, grid bias beyond cutoff. As would be expected, there's been
absolutely zero evidence of any negative consequences. This applies to
3CX1000A/3CV1500A7, 8877/3CX1500A7, 8874, 3CX800A7, and 4CX800A tubes, by
the many thousands, over the past 29 years.
-----Original Message-----
From: George T. Daughters [SMTP:gdaught6@leland.Stanford.EDU]
Sent: Monday, March 29, 1999 2:37 AM
To: amps@contesting.com
Subject: Re: [AMPS] Re:HV on tube w/o filament voltage
Mike wrote...
> But you have raised an interesting question. I would like to add another
> one: Is it allowed to apply HV along with filament voltage at start-up,
> provided that G1 is biased at cutoff and G2 voltage is zero? The tube in
> question is a 4CX800A. Svetlana says no way.
They may have misunderstood your question, or they are just "covering
their posteriors." (Their spec sheet says no operating potentials
until the warm up time is complete)
I do exactly this all the time with my 4CX800A: my G1 bias and HV
are applied immediately, but my circuit WILL NOT apply G2 voltage
until the warmup is complete. What does Alpha Power do in the
91-beta?
73,
George T. Daughters, K6GT
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