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@telus.net> wrote:
>
> Date: Thu, 4 May 2017 18:17:48 -0600
> From: Thomas Walsh <w2co@comcast.net>
> To: "Randall, Randy" <Randy.Randall@UCHealth.com>
> Cc: MU 4CX250B <4cx250b@miamioh.edu>, Jim Thomson
> <jim.thom@telus.net>, "amps@contesting.com" <amps@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@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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