[Amps] Longetivity of 4CX250B

[Karl-Arne Markström]

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 at qsl.net>
To: <Amps at contesting.com>
Sent: Friday, June 20, 2003 3:03 PM
Subject: Re: [Amps] Longetivity of 4CX250B


> 
> ----- Original Message -----
> From: Steve Eldridge <g8izy at eurobell.co.uk>
> To: <Amps at 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
> 
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