>Hi All
>
>The ZL6QH team is using an FL2100B amplifier that recently failed. Both of
>the Svetlana 572B tubes
>had developed grid-to-filament shorts.
>
>Closer inspection showed that the soldered connection to the anode ceramic
>blocking capacitor had
>broken loose. The parasitic resistors were also discoloured and starting
>to crack. I am guessing
>that some sort of high energy VHF oscillation occurred after the blocking
>capacitor went open
>circuit.
>
>Anyhow, we have now resoldered the capacitor connection, replaced the
>parasitic resistors and
>installed a new set of Taylor 572B tubes.
>
>I have just started testing using a reduced voltage HT tap on the power
>transformer (425V instead of
>the usual 850V). There is no obvious evidence of VHF parasitic
>oscillations but the amplifier can be
>made to oscillate in the 20, 15 and 10m bands when the input and output
>tank circuits are tuned to
>resonance. The oscillations are only evident when the input circuit is
>UNTERMINATED.
>
>Two questions
>
>1. Have other owners of FL2100B amplifiers noticed that the amplifier has
>a tendency to oscillate on
>the higher bands when the input circuit is unterminated? Is this "normal"
>for the FL2100B?
>
>2. Have any other owners of FL2100B amplifiers experienced the same
>failure mechanism - blocking
>capacitor open along with grid-to-filament shorts? If so, then is there
>anything else that we should
>be watching out for?
>
** grid-filament shorts are rare in 572Bs. Broken/shattered filaments
are not.
>Note. I HAVE read the earlier threads about parasitic suppression -
>including Richard's web pages.
>No need to repeat this information please.
>
>Thanks and 73
>
>Brian Miller ZL1AZE
>Registered Electrical Engineer
>67 Hornsey Rd
>Melrose
>Wellington
>New Zealand
>
>Tel 027 4446614
>Email: brianmiller@xtra.co.nz
>
>
>
>
>
>
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