[Amps] Filament transformer needed

[Ian White GM3SEK]

Roger (sub1) wrote:
>On 5/25/2011 12:03 AM, Jeff Blaine wrote:
>> Glen,
>>
>> You may want to rethink that headroom.  The transformer will tend to 
>>have a bit higher voltage if running at less than rated load.
>> And higher than optimal fil voltages equate to shorter tube life.  In 
>>addition, there is less inrush trama to the filament if you
>> have a matched transformer.  I think the latter is subject to some 
>>debate however.
>>
>> In any event, you would want to trim the actual fil voltage with a 
>>series resistor for max life.
>>
>
>I look at it as you need the extra headroom allowing for trimming the
>voltage to one that is ideal. Of course with the line variations we run
>into today, ideal might be right at the bottom of the permissible range
>to keep the top within the permissible range.
>

Another option is a cheap packaged switch-mode supply, eg one of the 
'Mean Well' series from eBay suppliers. This particular brand are very 
solidly built with good filtering and shielding, and a straightforward 
electronic design using a TL494.

I just took delivery of a 12V 12A SMPS for a 1296MHz triode PA. At that 
frequency the heater will need to be operated at the low end of its 
voltage range to minimize back-bombardment, leaving no tolerance for any 
further reduction due to line voltage variations.

A stabilized SMPS might try to drive a lot of current through the cold 
heater at switch-on, so it may be necessary to 'slug' the voltage 
control loop with a capacitor. (I'll report back in due course.)


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek