[Amps] Miller-Larson effect on thoriated tungsten filaments

[Leigh Turner]

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


-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Mike Waters
Sent: Tuesday, 25 June 2013 2:02 PM
To: amps at contesting.com
Subject: [Amps] Miller-Larson effect on thoriated tungsten filaments

I discovered some new (to me) information on maximizing the life of
amplifier tubes with thoriated-tungsten filaments. I just uploaded it to:

http://www.w0btu.com/miller-larson_effect.html

Basically, every time the tube filament passes through a certain
temperature range as we turn it on (or power it down?), we shorten the life
of the filament, due to the Miller-Larson effect.

I would appreciate any comments on it, good or bad.

73, Mike
www.w0btu.com
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