[Amps] AL-811H etc

[almacottage]

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