[Amps] Advice on Filament Protection Sought

[R.Measures]

On Aug 26, 2004, at 5:01 PM, Harold B. Mandel wrote:

> Sirs,
>
> The new amplifier will be running a pair of 3CPX1500A7 triodes
> with floating filaments through bifilar chokes, powered by 5 volts
> Direct Current through a floating bridge rectifier and filter.
>
> In order to reduce the thermal shock to the filaments in the tubes
> a method of reducing the inrush current is sought.

Harold -- According to Eimac, No inrush current protection is needed 
with heater type cathodes.  Thoriated-tungsten is the only type of 
cathode that occasionally needs inrush-I protection.  The reason is 
that a directly-heated cathode has a much higher hot/cold R-ratio than 
an indirectly heated one.  In other words, a heater still has quite a 
bit or R when it is cold, so the inrush current is not large.    Eimac 
says to limit inrush current on certain types of thoriated tungsten 
cathodes, but says nothing about doing so on 3cx1500A7 and other heater 
type tubes.

> > ...
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org