[Amps] 8877 filament

[Jim Thomson]

Date: Fri, 5 May 2017 10:52:14 -0600
From: MU 4CX250B <4cx250b at miamioh.edu>
To: Jim Thomson <jim.thom at telus.net>
Cc: "amps at contesting.com" <amps at 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