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 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 .
the fan should be at least a 120mm diameter one as opposed to the 80mm or so
presently fitted , also id put one fan blowing into rear of case and one in the
exhaust grille drawing air out , or even a hole cut in top of case directly
above the tubes and a 120mm papst type fan fitted [with grille] that draws hot
air out of the case .
as per the above fan modification to top of case, with the addition of
convection , would prove a more efficient way of removing heat rapidly from
within the amp case than fitting a fan in the side of it .
personally i think more amp failures are down to inadequate cooling per
operating conditions as opposed top actual component failure .
i know the 811A tubes dont last long in the AL811 amps if used at high output
levels , but direct swapping to 572B tubes seems to improve matters, however
the 572b swap does make higher output from the amp and of course draws more off
the transformer .
i know that perhaps fitting 572b tubes in place of the 811a extends tube life ,
but i guess operators need to keep the drive down so they dont kill off the
transformer .
for interest , when fitted with 3x 572b tubes on 80m with 60w drive i see
900w output from my AL811 amp , into dummy load using a daiwa CN801 power
meter .
higher freq where i have bird 43 pep meter there is around 800w output on 10m
with 60w drive .
on 17m input swr is a little higher [3-1] but this could be the fact that
bandswitch covers 17/15m in one , so possibly its actually tuned for 15m as
opposed to 17m .
i dont use the amp myself anymore and have leant it to a freind to use , but i
have owned it for 10years and its still running ok .
any contesting i have done on 10m though allways involved burning out one or
more of the anode parasitic chokes .
i learned about the choke problems on my SB200 and SB220 amps and fit similar
chokes in those and the AL811 , but the heathkits are are used at 50mhz where
obviously current drawn through anodes is probably higher than down on HF .
afraid im not well versed on amplifiers , but this is just a few things ive
found that can improve the AL811 range and also heathkits .
73 ian m0bcg
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