[Amps] 8877 filament

[Jim Thomson]

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