ORIGINAL MESSAGE:
On Fri, 10 Sep 2010 00:03:26 +0200, Marius Hauki
<rezycle.bin@gmail.com> wrote:
>-I have read that running DC instead of AC on the filament is OK.
>However I have read somewhere else that it is not OK. I have an AC
>trafo, but if the primary voltage varies, the filament voltage could
>be too high or too low and this may hurt the tube. Instead I would run
>a regulated DC supply, since the filament current is not that high.
>Any comments on that? Alternatively, any clever way of regulating the
>trafo by a small series drop resistor or something?
REPLY:
I would use AC power on the heater. Install a pair of banana jacks on
the front panel and run them to the filament connections right at the
tube base. Place some small RF chokes (1-2.5 mHy) in the leads and
bypass the banana jacks to ground with a .01 uF cap at each jack. Then
use a small variac to set the filament voltage exactly. Used variacs
in the 100-200 watt range are cheap and readily available on eBay. You
could mount the variac either in the amp itself or in an external
enclosure.
I don't like regulate heater supplies unless you provide some kind of
step start protection or current limiting. With a regulated supply,
the initial inrush current can be quite high. With a transformer,
there is some current limiting due to the internal resistance of the
transformer. Even so, if you are really fussy, you might want to use
step start there, too. I don't and have never had any trouble.
Bill, W6WRT
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