[Amps] warm up time for TX tubes

[Ron Youvan]

    John K4DLX Flood wrote:

 > Hope someone can help me understand tube warm up requirements.

   Slightly complex subject.

> Specifically, What are the necessary conditions for voltages at plate ,
> screen and control grid during Warm up for indirectly heated tetrode tubes.

> I have seen directions that say no plate voltage during warm up, but some
> amps (MLA2500) apply plate voltage and limit cathode current to a few mA by
> self-bias during warm up.

   Usually a few mA of plate current is such little power than the
heater, I don't get the point.

> What I have in mind for my amp with gu74b  (4cx800) is to apply fil voltage
> for 3 minutes while screen is at 0v, plate is full on and
> G1 is biased to cutoff (approx. -75v).

   Having plate Voltage without plate current seems to be a do nothing
to me.  (changing the electrostatic fields does not do much)

   Two things occur when we warm up a tube.  1, We elevate the emissive
surface of the cathode/s to an efficient emissive temperature plus
that heats the "getter disks" (or washers) spot welded above the pins
if any, 2, we also bring the envelope and tube pins (and their seals)
up closer to the final operating temperature.  (to minimize metal to
glass or ceramic seals cracking, admitting air)

   The nature of the emissive surface is one controlling factor.
   Some tubes need no preheating, plate Voltage can be applied all of
the time (or applied simultaneously) and they make power as soon as the
directly heated cathode starts emitting.  (4-500 and a 6146 replacement)
-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.