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

[Steven Grant W4IIV]

Good Morning
For directly heated cathodes, AC is almost a must for bias reasons.
For indirectly heated cathodes with a separate connection from the filament (unless grid driven) I have hear that DC with a good voltage regulator is an advantage.
Also, the lower filament voltage to where the emission falls off is for directly heated cathodes......for indirectly heated cathodes, I suggest sticking to the manufacturer's ratings
Steven Grant W4IIV

"A.J Moss" <linear.amplifier at ntlworld.com> wrote:
Hello amps group,

I'm in the final throws of getting my 'does all' PSU ready for my first new amp. It's a VHF stripline parallel pair of triodes from the GI-7 family, to a design by YU1AW.

Now I'm in a fix over this, and thought I'd cast this one out to get some opinion. I want to run the heater (I was almost tempted to say filament then :-) ) at the optimal voltage for the tubes (from what I gather, it's got to be experimentally found by backing off volts until Po starts to drop, and going from there). 

The original design uses AC for the heaters and connects both the tubes in parallel with copper strapping for 12.6V AC. The original design also uses a 10 Ohm, 10W pot to adjust heater voltage in the primary of the heater transformer. A 10R 10W part like that isn't easily found here.

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.

73 and thanks for any input

Adam



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STEVEN GRANT    W4IIV


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