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[Amps] filament instant heating query / simple remedy

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Subject: [Amps] filament instant heating query / simple remedy
From: "Jim Thomson" <jim.thom@telus.net>
Date: Sat, 16 Jun 2012 06:02:32 -0700
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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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