|[AMRadio] Globe Champion 350 & HQ-170A|
petef at sprynet.com
Mon May 14 10:14:40 EDT 2007
Kim et al:
First I've heard of AX9909's being fragile but for me the issue is moot as they were just way too expensive. I don't know why Leo used it but he was always interested in a good deal on surplus. When I got my 350A ten years ago I happened to ask a PA land ham about it and he sent me a copy of the Philips data sheet, and the tubes, if they met specs and worked as indicated, would indeed be appropriate tubes. He said they were used in a very few industrial applications but not available there either.
Studying everything at the time led to a couple conclusions. I wasn't able to get a reasonable deal on a 5894 or even 829B's, and everyone who had one said someone had given it to them. One of the finals in the rig was cracked when I got it, it was one of those rigs that had been sitting in a barn for years.
More importantly, none of the tubes were spec'd for the kind of service used in the 350A although clearly they did work for others. Just about only common tube I found that had published specifications for AM operation at 1000 volts on the anode was the 4CX250B, which I had a few of. The interelectrode capacitance and so on are also extremely close. So that's what I did and it's worked ever since, even after I dropped the rig on a dirt floor in a friend's barn when the steps I was carrying it up disintegrated under me :(
It's a simple mod on the A, harder on the others. The fan is removed and the tube socket sits in a tuna can mounted vertically over the fan hole with the flat squirrel fan mounted atop one of the transformers feeding a plastic tube plenum. Electrically the filament has to be dropped by .3 volts and the drive increased since it's a tetrode, but as I recall that's all that's needed. A no-holes mod.
I should write this up but it's been a tough ten years...
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