Here's what I do.
Use a transformer with higher filament voltage than needed. Then put a 100
to 150 Ohm 12.5 watt rheostat (pot) in series with the primary. Set the
desired filament voltage with the rheostat. This puts a series resistance in
series with the primary of the transformer which makes an automatic current
limiter. And with the rheostat you can adjust your filament voltage.
I used a 125 ohm 12.5 watt in series with the 240 volt primary on my 8877
transformer. The transformer is from Ameritron and is used in the AL-1500.
73
Jim W7RY
On Thu, Jan 14, 2010 at 6:06 PM, <texasrf@aol.com> wrote:
>
> Roy, even though no inrush protection is called for, it can't hurt
> anything.
>
> Over a 35 year period of using an 8877 on 2m EME I have lost two tubes;
> both had an open heater. I have to believe a surge limiter might have saved
> one if not both of these tubes.
>
> 73,
> Gerald K5GW
>
> In a message dated 1/14/2010 7:37:32 P.M. Central Standard Time,
> royanjoy@ncn.net writes:
>
> FYI all,
>
> RFP part #F-41X for $9.95 each, is rated 26.5 volts @ 2 amps with 120 VAC
> on
> primary. I purchased two and connected primaries in series with
> secondaries
> in parallel. My 247 VAC line yields 26.96 volts for the pair with a 4 amp
> resistor load. I'm planning to convert my Titan-2 to a GU-84B. Any
> thoughts
> on whether or not an in-rush circuit needs to be used with this indirectly
> heated cathode tube? I read somewhere that the original 4CX1600B specs did
> not call for any in-rush protection. Thanks.
>
> 73, Roy K6XK
>
>
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