That's right Pete, the SB220 had a well designed filament transformer that
self-limited the cold inrush current to a safer value close to what the tube
manufacturer Eimac specified. What the NTC thermistors do in both the SB220
and TL922 is bring the 3-500Z filaments up in a nice slow and controlled
manner.
Eliminating the inrush current surge completely like this just seems so
intuitively a good thing to do to mitigate thermal stress on the cold
filament structure.
Leigh
VK5KLT
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Pete Smith N4ZR
Sent: Saturday, 16 June 2012 10:43 PM
To: amps@contesting.com
Subject: Re: [Amps] filament instant heating query / simple remedy
Don't I remember that one of the few things Rich Measures said, that
others agreed with was, that the SB-220 filament transformer inherently
limits inrush current to a safe level?
73, Pete N4ZR
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On 6/16/2012 9:02 AM, Jim Thomson wrote:
> 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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