Bob Marston k1ta@contesting.com
Wed, 24 Nov 1999 13:48:12 -0800

Hi Gang

Been gone for a spell while I moved up to the Pacific Northwest. I've been
following the conversation here and found a growing percentage of it to be
nontheoretical. I've have a number of questions to throw out for the group.
Here is the first.

Tube filaments as most of us are aware of they come in 2 flavors: Oxide
Coated and Thoriated Tungsten. I'm only going to restate the merits and
demerits of each briefly here. The subject is given adequate treatment in
Eimac's Care and Feeding of Power Grid Tubes. The particular section
dealing with this is up on there Web at the end of Chapter 2. Svetlana also
addresses the subject on their Web on the subpage entitled How a Tube Works.

To restate the basics Oxi Coated filaments are best known for their use in
small signal applications. Oxi Coated filaments higher emission and require
less power than Thoriated Tungsten to obtain a given plate current. They do
have draw backs though. 1) They require warm up time, anywhere from 1.5 to
6 minutes. Thoriated Tungsten is Instant On. 2) In large tube applications
they are NOT rebuildable. When they go soft it's Paperweight Time.
Tungstens can be rebuilt. 3) In High Voltage applications they are less
resistant to Ion bombardment than Tungsten which will cause premature
aging. 4) They have a shorter lifetime than Tungsten.

It's the last point that I want to address here. Both the The Care and Feed
and Svetlana's How a tube works treat this matter in a cursory fashion.
While it is clear that OXI's due have a shorter useful life  How much
shorter   is yet to be proven conclusively. But of these write ups present
their data in an annecdotal fashion. As this one particular Tungsten
transmitting tube lasted 80,000 hours in commercial service. But is that
typical? What can the average user expect? In  CCS and in ICAS?

The amplifier constuction community has taken interest in the YC-156.
(3CPX-5000A7)  What I would like to see discussed is a comparison of this
tube against the 8170 (4CX-5000)  These 2 tubes are very simlar in many
respects but they have different filaments.  Will the guys who have built
amps around the YC-156 be dissappointed in a couple of years when these
bottles go soft? Will they quit that quickly?

Finally what factors influence cathode life? Is the number of Power Ups a
major or minor factor? Does the overall burning time play a major role or
is the amount of tube conduction time the primary factor?

Can anyone quantify these?

Bob k1ta@contesting.com

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