[Amps] Plate Impedance, ARRL
jkearman at att.net
Wed Apr 12 12:38:44 EDT 2006
> I read this over again last night, and it didn't mention anything about triodes
> only. The main reason I posted this was to show the differences, plus find where
> the illusive 1.8 came from in print. So far, the only book I've seen 1.8 listed
> in was Bill Orrs Handbook. I don't have any newer ARRL handbooks past the 90's
> as they seemed to be the same old thing, over and over, with not that much new.
Amen, brother! I got my insight into the tetrode situation from Rich Measures' website, which I should have mentioned in my OP. Until they published (mangled) George Daughters' 4CX1600B amplifier, recent ARRL Handbook legal-limit amps have used triodes. The 8877 was king and I think they based their writeup on that. Rich wrote a chapter for them that was never published for political reasons, but it's on his site and worth a look.
Frank Carcia gave me the insight I needed to help me pick the values of tuning and loading caps when he reminded me of the relationship between Q and impedance as you vary them. Assuming you're using a tapped coil, not a variable inductor, you can pick a value of L for each band that will let you cover a range of Z and Q that are still in the park. Regardless of the actual value of Z, the Q will be high enough, but not too high. It takes several iterations of the program to figure it out, but it was a real relief to get numbers that should work without a lot of reworking.
> I would like to find out though where the factor of 1.8 was
> first mentioned.
It would be interesting to see how the rule of thumb came about, for sure. In designing my pair of 4CX800As I've planned to make the loading cap large enough to cover my butt for a range of load impedances, by padding it on 80 and 160. It'll be easy enough to remove unneeded caps, not so easy to install them afterward.
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