[AMPS] fil info

[John T. M. Lyles]

At 11:52 PM -0500 2/22/00, owner-amps-digest@contesting.com (RF 
Amplifier Discussion Dige wrote:
><<
>  Better, but even more expensive, is a regulated filament supply that is
>  current limited so the inrush current can't exceed twice the normal current,
>  and doesn't vary with line volts. You can afford to build that sort of thing
>  into a home brew amp.......but commercially it's a no-no.
>

The commercial FM B'dcast xmitters that I designed in the period of 
1980-85 and those of several competitors used special filament 
transformers, which would limit inrush to 2 X nominal current. They 
had a gap in the EI core (near the center laminations I believe) 
which caused leakage flux. This took care of inrush, to the approval 
of Eimac and RCA. The second thing was a rheostat, simple 50- 100 
watt device, in series with the 220VAC line to the iron. It was 
chosen to have a small range of adjustment of the line voltage. The 
third thing was a "Sola" type constant voltage transformer, harmonic 
neutralized to 3%. These three items, commonly available, took care 
of filament voltage, without a complicated electronic regulator. It 
was in the style of keeping it simple.

These days I also use several tubes (with handles, or more 
appropiately, eye hooks) from Thomson of Thonon, France. They will 
specify the filament operating voltage for the particular load line 
and cathode current requirement. The data sheet doesn't even have a 
fixed value of suggested filament voltage - only a current inrush 
maximum. I use a special saturable reactor in series with the AC line 
(480 VAC) to the filament transformer, to limit inrush, and to allow 
a slow ramp up of filament over 10 minutes. Would publish my circuit 
for this, when I get it completely documented (correct the drawings 
that I am using), for academic reasons - It would be overkill for 
most of us, unless we use the 8973 sized bottles for our 'rigs'.

Thesedays we (me and the team of  technicians at work) always turn 
back the filament power, to where cathode emission is starting to 
slump, and then increase a slight amount. This gives a longer 
lifetime, especially important for tubes that cost $20 to 200 
thousand dollars a pop.

73
John
K5PRO


--
FAQ on WWW:               http://www.contesting.com/ampsfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm