This is an old message as you can see from the 1999 date. I hope Dick
doesn't mind me forwarding this to the group.
73..de John/K4WJ
>From: "Richard W. Ehrhorn" <w4eto@rmi.net>
>To: "'jtml@lanl.gov'" <jtml@lanl.gov>,
> "'amps@contesting.com'"
> <amps@contesting.com>
>Subject: RE: [AMPS] filament life again
To: <amps@contesting.com>
>Date: Tue, 30 Nov 1999 21:36:55 -0700
>X-MIME-Autoconverted: from quoted-printable to 8bit by contesting.com id
>MAA05982
>Sender: owner-amps@contesting.com
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>
>
>Hello John...
>
>ETO had some life data on the 8877s used in our MRI amplifiers by GE
>Medical, Philips Medical and others. It was presented as a curve as number
>of tube failures due to all causes (per month or some such time unit)
>versus heater-on hours. After a smattering of infant mortalities in the
>first 100-300 hours or so, the failure rate stayed very close to zero
>until approaching 15,000 hours. As I recall there was a large "bell
>shaped" hump between 15 and 20k hours. And presumably not many left after that.
>
>The hump was presumed to encompass normal end-of-life, usually due to low
>emission, although the failure curve included all catastrophic failures
>due to broken filament welds, inter-element shorts/leaks, and so forth.
>Not many of the latter group at any time however.
>
>Notable exception to all of the above, of course, was the 12-18 month 8877
>production catastrophe that resulted from the ill-considered cathode
>support design "improvement" in the 1986-88 time frame. For about 12
>months' production, the failure "curve" would have shown the usual few
>infant mortalities, a large spike between about 200 and 400 hours, and not
>very many left after that.
>
>I think data on the 3CX800A7, which is of similar design but smaller, is
>quite similar.
>
>73, Dick W0ID
>
>
>(John Lyles had written the following in response to previous discussion
>of thoriated tungsten vs. oxide cathode power grid tube lifetimes):
>-----Original Message-----
>From: John Lyles [SMTP:jtml@lanl.gov]
>Sent: Monday, November 29, 1999 7:06 PM
>To: amps@contesting.com
>Subject: [AMPS] filament life again
>
>
>Thoriated Tungsten vs oxide cathodes
>
>At 11:46 PM -0500 11/24/99, Amps Digest wrote:
> >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?
>
>
>Both. Although the mechanical changes in TT filaments in larger tubes
>with handles are more prone to failure than the lower power heaters
>of oxide cathodes. For instance, we have found that the number of
>power ups/downs of the filament has led to problems in X2159
>tetrodes. These are also called the 8974 I think. What happened was
>that the cutoff bias was changing. As tubes aged, there began a
>trickle of DC plate current, which got higher and higher, as the tube
>was cycled. It came to the point where the tubes were in bad shape,
>with a lot of Xrays being generated along with probably internal
>cutting of the anode due to the DC beam. It is believed that this was
>due to the filament sag over time, changing the spacing of the grid
>to filament. We have since gone to a program of leaving the filaments
>lit as long as we can.
>
>The high power filaments in TT tubes move around a lot when they
>transition from cold to hot. This requires good mechanical
>engineering to develop the supports to compensate, and not allow the
>filaments to bow radially into the grid cage.
>
>We see failures in Burle 4616 tetrodes usually after raising the
>filament voltage. These are oxide cathode, 400 kW peak tubes at 200
>MHz. We only run them at 120 kW peak for 10% duty. The filament is
>low power, about 500 watts. Oxide cathodes are nice here as they do
>offer high peak emission for very short pulses. A lot of it relates
>to aging of the filament bars, which are painted with oxide in these
>tubes. Failed ones show signs of metal fatigue and cracking.
>Even though they are ramped up, when they get brittle enough, they
>fail, again when turned on. So in this case, the failure is going to
>happen, but the turnon is what causes it to finally occur.
>
>
>
> >
> >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?
> >
>
>80000 hours is not typical in my experience. A properly designed
>tube, when used within ratings, may reach this. However, it is my
>experience that 20,000 to 30,000 hours for a very carefully handled
>tetrode such as 4CW250,000, TH555, 4CW100,000 size tube. 15,000 for
>the Burle 4616 oxide cathode tetrode.
>
>As for the smaller tubes like 3-500Z, 4CX800, 1500B etc, I am not
>sure what to expect. Broadcasters like to see about 20,000 hours or
>more from their 1000 to 30000 watt tubes, which is just over 2 years
>of continuous operation. Any more is wonderful, a bonus, but not a
>guarantee.
>
>An extreme example, we are getting over 100,000 hours on some of our
>1.2 MW 805 MHz klystrons, so they have been in the sockets since
>1980s.
>
>John
>K5PRO
>
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>--
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>
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73..de John/K4WJ
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K8PXG from 18 Jun 59 to 11 Feb 97
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