[Amps] AC or DC better for indirectly heated cathodes?

[Ian White, G3SEK]

R. Measures wrote:
>
>
>>On Wednesday 19 November 2003 13:09, A.J Moss wrote:
>>snip
>>>     If I were to make a floating adjustable regulated DC supply, I could
>>> use an ordinary pot to control the heater volts, meter it, and have a fine
>>> adjustment to within ca. 1V either way to set optimum heater volts. I could
>>> also then add a current limiting option on the regulator to limit inrush at
>>> switch-on
>>>
>>>     Are there benefits, advantages, or just risks of using such a scheme?
>>> Constant unvarying power at the heaters seems good, but is 'less' 'more' to
>>> use the cliche.
>
>>Repays itself in no time at all if you're ever going to operate portable.
>>
>**  True, but there is no reason to limit inrush-current to heater-type
>tubes.

Not with a normal small mains transformer, which has some degree of 
built-in current limiting anyway.

But a voltage regulated power supply will do the opposite - it will try 
to push *extra* current through the lower resistance of a cold heater, 
unless there is some kind of current limiting or a slow ramp-up in 
voltage at startup.

A ramp start can usually be achieved by finding the potential divider 
for the voltage regulator (often a TL431) and connecting a high-value 
capacitor in parallel with the resistor on the hot side. SMPS circuits 
are all pretty similar in basic principles, but the details vary a lot, 
so you'd have to work out the details for yourself.

Bill is absolutely right to recommend experimenting with a dummy load 
resistor or a light bulb, before you come anywhere near connecting the 
SMPS to a tube.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek