>
>Hi there
>
>We have recently purchased a second-hand Ameritron AL-811X amplifier. It
>works fine on all bands except 10/12m where it is difficult to output more
>than 300W without turning the anode parasitic resistors into smoke!
>
>I have tried replacing the original carbon composition resistors with carbon
>film types but the problem persists. It looks like this is a design flaw.
28MHz poses a difficult balancing act for R-supp. in VHF suppressors. If
L-supp is increased to improve suppressor performance, the dissipation in
R-supp increases exponentially. If R-supp burns out - leaving only
L-supp - a parasitic oscillation is likely. One improvement is to
replace carbon-composition resistors with Metal Oxide Film (MOF)
resistors. The 100-ohm, 3w Matsushita MOF resistors we use in our
suppressor retrofit kits will dissipate 12w each for 1 hour in open air
at 75 deg. F. One way of improving suppressor performance without
increasing L-supp (and stressing R-supp) is to replace a copper or silver
L-supp with resistance-wire. In N7WS' comparison tests, resistance-wire
suppressors exhibited a 41% Q reduction at 100MHz compared to copper
wire. suppressors. .(see "Calculating Power Dissipation in Parasitic
Suppressor Resistors", p. 25, March, 1989 *QST*)
// It is my opinion that tuning up key-down on 10m puts R-supp at
unnecessary risk. . With 811As (which are rated at 65w anode
dissipation) key-down tuneup on any band is bananas. It is better to
tune with 50wpm cw dits, or with a tuning pulser.
>
cheers, Brian
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