On Oct 1, 2005, at 3:59 AM, almacottage wrote:
> i have the AL811 with 3 tubes , i directly swapped the 811a tubes for
> 572B tubes and run the high voltage at 1900v [ie in the red segment of
> HT meter] .
>
> i found that on these amps the original anode parasitic chokes
In a HF/MF amplifier, R//L VHF parasitic suppressors are not "chokes".
They are a device that reduces the VHF Rp* presented to the anode.
Less Rp means less VHF amplification, which in turn means more
stability at the anode's unavoidable parasitic resonance. (typically
from 43MHz to 160MHz)
* Rp = Parallel-equivalent R. Achieving less VHF Rp requires less VHF
suppressor Q.
> overheat and burn out when being used on 10m if amp is used for any
> period of time other than for very short dx pileup calling .
>
> to cure this i made new chokes using 2x 100ohm [2 watt] carbon
> compound resistors in parallel and a u shaped inductor made from
> 16swg enamelled copper wire, 85mm total length of wire and bent around
> body of an n-plug .
>
> i find that the cooling of this amp is inadequate and id say that this
> is the real problem in AL811 failures more than anything else .
>
Inadequate tube cooling does not cause parasitic suppressor resistors
to toast or filaments to shatter. .
> ...
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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