[Amps] Miller-Larson effect on thoriated tungsten filaments

[Gary Smith]

I'd heard about this long ago and never had read anything to formally
substantiate it. I remember learning that hams were far harder on 
tubes in this sense than commercial services that left the tubes on 
for extended times while not pushing them. Remember reading a 
slightly reduced filament voltage is also of great benefit as regards
longevity.

I haven't heard how this fares for indirectly heated tubes like the 
8877 I use. I try to minimize radical changes in heat by keeping the
fan on high at all times and after I have used the amp, I let the 
tube cool down for more than a minute before turning off the amp, 
before turning the amp off.

Gary
KA1J

> 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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> 
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