Date: Sat, 16 Jun 2012 10:38:47 +0930
From: "Leigh Turner" <invertech@frontierisp.net.au>
Subject: Re: [Amps] filament instant heating query / simple remedy
All good points you make here Jim.
When it comes to the small desktop amps like the SB220 and TL922 having
separate filament transformers I find that inserting a pair of GE CL-60 NTC
thermistors in each leg of the filament transformer primary winding provides
a simple remedy for implementing a soft-start. The observed glow of the two
3-500Z filaments comes up nice and slow as the thermistors warm up...no big
surge in brightness at initial turn on as it was with the stock design.
This is in addition to a simple short-duration resistor / AC relay coil
style step-start to both the HV and filament xfmrs timed predominantly from
the HV electrolytic charge up time constant. This relieves the inrush
current surge stress on the front panel ON/OFF switch contacts and the
electrolytic capacitor bank.
Cheers,
Leigh
VK5KLT
## I also looked into the thermistors... since RF parts sells em just for
this tube
fil application. In normal operation, they run hot all the time though. You
cant
cool em with any air either, or the resistance will increase on em. Since the
HV supply
needs some step start anyway.... you can kill 2 x birds with one stone ! A
simple
25 ohm resistor in one leg of the 240 line works wonders. Fil brilliance is
aprx
.66 of normal. Once the resistor is shorted out, tube brilliance comes up to
normal..albeit instantly. I like your thermistor idea though, since it
brings
the tubes up slower. I already have step start for the hv and fil xfmr... so à
adding just one thermistor into one leg of the fil xfmr is no big deal. I will
get a bunch, and implement it into the 3 x L4B’s I have. Do you need
one thermistor per leg... or just one period. How long is the slow ramp up,
3 secs, 10 secs or what.
## I see no reason why thernistors couldn’t be used on my 3CX-3000A7 fil
xfmr...
provided they are sized correctly of course. I use a variac anyway, used in
conjunction
with a sola constant v xfmr... to precisely set the fil V..and to slowly ramp
up the fil.
Thermistors would allow me to leave the variac alone, and just use it to set
fil V.
tnx...... Jim VE7RF
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