[Amps] 8877 filament

[MU 4CX250B]

What is the mechanism by which an indirectly heated cathode is
"poisoned" if the filament is run at a reduced voltage, with no other
voltage on the tube? I know the cathode can be ruined if the tube is
operated (drawing cathode current) when the cathode is underheated,
but that's a different circumstance than we're talking about here.
73,
Jim w8zr

Sent from my iPhone

> On May 5, 2017, at 10:38 AM, Jim Thomson <jim.thom at telus.net> wrote:
>
> Date: Thu, 4 May 2017 18:17:48 -0600
> From: Thomas Walsh <w2co at comcast.net>
> To: "Randall, Randy" <Randy.Randall at UCHealth.com>
> Cc: MU 4CX250B <4cx250b at miamioh.edu>, Jim Thomson
> <jim.thom at telus.net>, "amps at contesting.com" <amps at contesting.com>
> Subject: Re: [Amps] 8877 filament.
>
> ##  8877 is 5 V  at 10.5 Amps.   That’s a hot resistance of  .476 ohms.
> At just 3 volts, the current draw would likely be aprx  6.3 Amps =  18.9 watts
> And even less, if  2.5 V was used.   Dunno if a blower would be required or not
> with just 19 watts...vs the normal 53 watts.   I suspect the reduced fil V would
> poision  the cathode, esp after thousands of hrs.   Then when you do want to use the
> tube, can you switch from 3 Volts...to  5 Volts.... and apply drive asap ?   It might
> require some additional time to get the cathode up to temp, like 15-60 secs ?
>
> ##  Run it past Eimac, and see what they say.   The best you can do for now is just
> turn the amp on standby, then turn on the xcvr etc.   Even if you know you will be playing
> radio  for say 2-6 hrs, mainly tuning about on RX, 2-6 hrs with the 8877 fil on, is really
> not that big of an issue.   What is the fil life on a 8877 ?   If its 2-5 years, that’s still
> 16-40k hrs...which is a bunch.   Even at 6 hrs per day, the tube should be good for
> 8-20 years.
>
> Jim   VE7RF
>
>
>
>> And the blower would still need to be on as well, to keep from
> heat damaging the tube, it will still get hot. Never power a
> filament with no forced air! Even reduced filament voltage..
>
> Sent from my iPad
>
>> On May 4, 2017, at 12:46, Randall, Randy <Randy.Randall at UCHealth.com> wrote:
>>
>> I wonder how much the warmup delay would be shortened if the filament was run at, say, 3 V on a 24/7 basis, and then increased to 5V when the amplifier was powered up
>
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