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
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