[Amps] filament instant heating query / simple remedy

Leigh Turner invertech at frontierisp.net.au
Sat Jun 16 21:09:40 PDT 2012


 

Hi Jim,

 

With the TL922 filament xfmr I use one CL-60 in each leg of the primary
winding. The second one is probably not necessary but I put it there because
it allowed me to fine-tune the filament voltage slightly to achieve
precisely 5.0 Volts at the tube pins in standby mode.

 

Their quiescent body temperature is only warm...you can touch them in static
operation, so there's no excessive heat generated.

 

The filament voltage drops to only 4.95V when amp is keydown delivering full
whiskies....I have a pretty stiff AC mains supply feed to the shack.

 

I'm lucky at my QTH as the AC mains supply voltage remains a pretty constant
245 Volts all year round / throughout the day.

 

Ramp up is relatively quick, but you can see the filament glow rise up nice
and slow compared to the thermistors being absent with the associated inrush
surge in brightness. This is also evident when observing the AC current
waveform on a storage scope with a clip-on current probe.

 

The CL-60 thermistors seem nicely sized for the TL922.

 

Leigh

VK5KLT

 

 

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Jim Thomson
Sent: Saturday, 16 June 2012 10:33 PM
To: amps at contesting.com
Subject: [Amps] filament instant heating query / simple remedy

 

Date: Sat, 16 Jun 2012 10:38:47 +0930

From: "Leigh Turner" <invertech at 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 thermistors 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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