Thoriated Tungsten tubes were used in a lot of broadcast radio transmitters
from the 50's to the present age, as they had reasonably long life, and
could be termed 'instant on'. In a line of FM transmitters using the Eimac
line of power tetrodes, Broadcast Electronics set the filament warm up
timer to be adjustable for 10-60 seconds I think, before HV could be
applied. I remember the discussion with the engineers at Eimac, and we
could have shortened it a tad, but we were conservative. When there is a
power outage (brownout), the thing is set to ignore the filament timer and
slam the HV back on as soon as bias is up, then follow with screen voltage
-assuming that this happens infrequently (during an electrical storm for
instance) and that the owner doesn't want to loose air time waiting for the
silly filament timer again. If the power was out for over a certain
duration, then the filament timer would reinitiate. Thats how we handled
that. Indirectly heated - oxide- cathodes are rarely used in those types of
rigs. (did use the 8877 at 1500 watts FM, as it was such a good part for
100 MHz).
My SB220 is about as fast a warm up as I care to see. Nothing like a pair
of 3-500Z or 3-1000 for fast heat.
Speaking of which, I read something about painting the insides of the SB220
RF compartment with black shoe polish. Is this some kind of cult that I am
missing out on? I love these. (I'm sure there must be a thermal reason that
one of you will explain to me, although I have been very satisfied with my
stock SB220 cooling, except for the day that the fan froze up while I was
on standby).
I remember when CD's were being challenged by the purist audiophiles,
someone came up with the idea that if you cleaned them or rubbed them with
some magic goo, then they 'digital grundge' would be remedied. So various
charlatans sold CD polish and CD solutions, that would dampen the
vibrations of even the cheapest CD players, and improve the sound. One of
them was to be smeared around the edge of the compact disc. I better not
continue on this, because I'll start talking about the Kinergetics company,
and their little black modules which would cancel out the 'hysterisis'
effects of transistors, capacitors, solder, and even chassis wiring. They
had an inverse-sounding block for all of those horrible sounding electronic
components in their stereo componentry. You should see the letter I sent
them in the 1980s and their presidents reply to me!
Ta Ta,
John
K5PRO
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|