[Amps] General question on filament life

[Phil Clements]

> Subject: Re: [Amps] General question on filament life
 
> I have been thinking about this subject a lot myself.  This is what I
> have
> concluded.  Since ham radio is an occasional thing we do with little
> transmit
> time compared to off time or listening time.  Seems like it would  be
> senseless to leave your filaments running 24/7.  On the other hand if  you
> cycled them
> a lot that could be costly too, so for hams as Bob says it  probably makes
> little difference.  But I think the ideal situation would be  to turn off
> the
> filament when not in use and to bring the filament up slowly  with a
> variac to
> proper operating voltage every time you use the amp.  It  would be a pain
> to do that every time, but all that hassle aside, it might be  "the best"
>thing.

It depends upon the amplifier that you are using. The stock filament
transformer on the SB-220 will not put out excessive current at turn-on due
to the way it is wound; i.e. "tailor-made for the load of a pair of
3-500Z's. Putting in a "standby switch" is a waste of time and money; it
gets you nothing in return. Peter Dahl and others can provide filament
transformers custom-made for every application so that in-rush issues are
eliminated. Most of the older gear has no way of adjusting the filament
voltage and were designed for 115/230 volt mains. Most of us have mains
voltages of 242 volts and up. In this case, it is simple to install a power
resistor in the primary of the transformer to reduce the voltage at the tube
pins to the recommended value, or slightly less. This WILL prolong tube
life, and the resistor provides some in-rush limiting. The variac is fine,
but as you say, a pain in the posterior trying to use it as a step-start


> You all notice that USUALLY  when a light bulb burns out it is when you
>first turn it on.  Only my 2  cents.  Lou

A light bulb has no in-rush limiting. I would say the bulb will fail 99.5%
of the time at turn-on. I do not recall seeing a bulb burn out after it is
illuminated; it is usually when the switch is moved to "on."

My 2 and your 2 make 4 cents worth! 

(((73)))
Phil, K5PC