Date: Fri, 5 May 2017 10:52:14 -0600
From: MU 4CX250B <4cx250b@miamioh.edu>
To: Jim Thomson <jim.thom@telus.net>
Cc: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] 8877 filament
<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
## Dunno what the exact mechanism is. I found some old notes by W8JI
who stated not to run the 8877 fil at reduced fil voltage, under any
circumstances,
but dunno if black heat is included.
##Then found another note by W2WIK, who stated that he has been running 8877s,
with full rated fil V ON.....and NO blower..when in listening RX mode, for
hours on end.
Been doing that for 30-40 years now, zero issues. Apparently the 5 v at
10.5A
fil doesnt require air through it.... if no anode diss present.
## Tube data sheet sez the 8877 only needs 7.5 cfm for 500 Watts CCS anode
diss,
and they have also factored in 25 w CCS grid diss, and another 50 w CCS for
the heater.
IE: total of 575 watts CCS only requires 7.5 cfm.
## So with zero watts of grid diss, and zero watts of anode diss, leaving
only the 50 w of
fil diss, he is probably correct, no air required for just 50 w ccs fil.
## Bill sez you can run reduced fil v..aka black heat on TV 7 + 9 pin
tubes...dunno about the
main picture tube.... back in the 50-60-70s TV days. And those were all
indirectly heated.
But regular TVs without the black heat feature did not have a 3 min warm up
either.
## see what eimac sez. Even with black heat, like 3-4 volts, you may well
require XXX
amount of seconds, before applying B+ and drive...... right afer you increase
fil V to
the normal 5V.
## be glad you dont have a YC-179 or YC-156..with its 8 min warm up.
Although the
spec sheet sez 6 mins, eimac told me to increase it to 8 mins. And yes, you
can the full
B+ applied, while tube is warming up...as long as plate current is fully cut
off. IE:
normal stand by ...bypass mode...so no drive applied to cathode.
Jim VE7RF
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