[Amps] Miller-Larson effect on thoriated tungsten filaments

[Carl]

Most commercial ham gear limits inrush with the all purpose power 
transformer. The only part possibly in danger is the On-Off switch but 
blowing one of those is pretty rare.

Carl
KM1H


> Thanks, Leigh. If nothing else, those books emphasize the importance of a
> soft-start circuit.
>
> Evidently, it's the *change* in temperature that affects the life. If
> holding the temp constant at those critical temperatures would actually
> rapidly destroy the filament, I can't say.
>
> A good friend of mine, WW8N, uses CL-60 current limiters in the 240 vac
> PRIMARY circuit of the filament xfmr of his 3-500Z amp. These limiters are
> 10 ohms cold and a fraction of an ohm at rated current.
> http://www.rfparts.com/inrush.html
>
> 73, Mike
> www.w0btu.com
>
> On Tue, Jun 25, 2013 at 12:48 AM, Leigh Turner 
> <invertech at frontierisp.net.au
>> wrote:
>
>>
>> Yes indeed Mike, that's the reality with these tubes.
>>
>> I like to think we can mitigate at least some of this tube life 
>> degradation
>> through soft-start circuitry. My favourite means that has great merit is 
>> an
>> automated slow-ramp up of the filament voltage over a time period of
>> several
>> seconds implemented with a phase-controlled Triac on the AC primary of 
>> the
>> filament transformer.
>>
>> I have such electronic slow 4 second soft-start circuits implemented on 
>> my
>> twin 3-500Z tube based TL-922 amp, as well as my other ceramic-metal
>> tetrode
>> based amps that run oxide-coated heaters.
>>
>> Now whether it's just a pointless feel-good measure or actually yields a
>> practical benefit in extending filament/heater service life I cannot say.
>>
>> However, gently controlling cold filament inrush current in this way
>> certainly seems a sensible thing to do.
>>
>> Of course the plate HV xfmr also benefits from the slow ramp-up in AC 
>> mains
>> to prevent the large turn-on current surge of the initially uncharged HV
>> electrolytic capacitors over taxing the amp's ON/OFF switch contacts.
>>
>> Leigh
>> VK5KLT
>>
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