Absolutely true and it goes back even further in a Heath letter to a
customer asking about the need for a S-S.
I also think it was one of the very few things that Rich and JI agreed
upon....
Carl
KM1H
----- Original Message -----
From: "Pete Smith N4ZR" <n4zr@contesting.com>
To: <amps@contesting.com>
Sent: Saturday, June 16, 2012 9:13 AM
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
> The World Contest Station Database, updated daily at
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> arcluster.reversebeacon.net, port 7000
>
> 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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>
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