----- Original Message -----
From: "Karl-Arne Markstr?m" <sm0aom@telia.com>
To: "Steve Thompson" <g8gsq@qsl.net>; <Amps@contesting.com>
Sent: Sunday, June 22, 2003 3:20 PM
Subject: Re: [Amps] Longetivity of 4CX250B
> Most interesting information.
>
> >From my point of view, Steve/G8IZY appears to have had an amount of
luck.
>
> Having managed Telefunken 20 kW HF transmitters at work using
> parallel 4CX250B's as drivers, the average filament hours we got
> usually were around 2 years (16000 h) before stage gain decreased and
IMD
> increased to a point where the tubes had to be replaced (the PA servo
tuning required a quite high stage gain to
> work properly). These transmitters were used for voice SSB services,
with a VOX relay reducing
> plate current to about 5 % of the normal idling value. (for a photo,
take a look at the end of this page:
> http://home.t-online.de/home/Hbusch/sdj.htm)
>
> On the other hand, the filament hours obtained on 4CX250B's in the
Standard Radio ST1610 ship's
> transmitters usually were less than 8000 when the transmitters were
used for automatic radiotelex service.
> A lot of effort was spent to find the causes, and poor filament
voltage regulation together with long intervals
> of stand-by without plate current was deemed to be the culprit.
>
> However, it seems that some oxide cathode transmitting tubes can have
exceptional life spans
> if properly handled. The otherwise infamous Eimac P290A comes to my
mind
> where a few tubes surviving "infant mortality" sometimes would perform
according to
> specifications for more than 70000 h. It appears that filament
regulation, proper cooling
> and flash-over current limiting can be of influence for prolonging the
oxide cathode life.
>
> I would find it interesting to know the date code on the "retired"
4CX250B
>
> 73/
>
> Karl-Arne Markstrom
> SM0AOM
>
>
>
>
> ----- Original Message -----
> From: "Steve Thompson" <g8gsq@qsl.net>
> To: <Amps@contesting.com>
> Sent: Friday, June 20, 2003 3:03 PM
> Subject: Re: [Amps] Longetivity of 4CX250B
>
>
> >
> > ----- Original Message -----
> > From: Steve Eldridge <g8izy@eurobell.co.uk>
> > To: <Amps@contesting.com>
> > Sent: 19 June 2003 15:28
> > Subject: [Amps] Longetivity of 4CX250B
> >
> >
> > > I am just about to retire a 4CX250B that was second-hand when I
put it
> > > into service on 144MHz in a homebrew amp. That was in mid 1998,
since
> > > then it has been powered up continuously- ready to go at a
second's
> > > notice, with the odd power outage being the only break .i.e
99.9999% of
> > > the time as it restarts automatically.
> > >
> > > In that timespan this valve has worked 41 DXCC and 194 grids on
2metres
> > > and always provided the RF (250W) when needed, despite long
periods of
> > > inactivity on standby and also some severe abuse.
> > >
> > > I calculate that this valve has been running (with heaters and HT)
for 5
> > > years or 43800 hours plus any other use it had in it's previous
life.
> > >
> > > I find this amazing. Is this normal or was I just lucky?
> > I'd guess it's not out of the ordinary in situations where the valve
is
> > properly cooled, and has proper heater voltage regulation. I've seen
15 year
> > old PMR gear with original valves that still delivered full
performance.
> >
> > Steve
> >
>
Thanks Steve and Karl-Arne,
The heater is regulated DC with a soft start circuit (317 + pass
transistor). This is set to 5.75V. There is reasonable airflow with
blower and sucker fans.
I'm sorry I can't tell you the date code as I have not taken it out yet,
but I have fairly good idea that it dates back to the 1970's.
73 Steve G8IZY.
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