It is a mistake to assume that the Tune-C arced because the previous owner switched bands while transmitting since as soon as a bandswitch is moved the tank immediately goes out of resonance and there is little V in the tank. A more likely reason that a Tune-C arcs is the presence of VHF energy. If you are curious to what's going on here, couple a dipmeter to either side of the DC blocker cap and observe the sharpness of the dip c. 95MHz. This is the AL-811's anode's parasitic resonance. - note - G-G amps are only self-neutralizing below their grid-resonant freq. 4, 811As have 800Ωs of feedback XC at 95MHz and their grids resonate c. 75MHz (KM1H) so there's enough feedback to allow them to oscillate at their parasitic anode resonance of 95MHz. To reduce the chances of VHF oscillation, reduce the VHF-Q of the VHF suppressors. This lowers the VHF RL on the anodes of the 811As and that reduces their VHF gain. You can expect 28MHz power out to drop c. 2%. . . . An arced Tune-C is a good thing because it saved your bandswitch. . . . Parasitic osc. puts high EMF stress on filaments due to the unloaded condition. I would add a glitch R to the HV+ circuit that will limit peak I to 150A or less. I have seen an AL-811 with shattered 811A filaments because it did not have a glitch-R when it oscillated at 95MHz.
Originally Posted by KJ4AQU
Murphy was right.
• Rich, AG6K, [url]www.somis.org[/url] 805-386-3734