[Amps] Miller-Larson effect on thoriated tungsten filaments

[Mike Waters]

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
>