[AMRadio] GK500 OTA almost


Jim Candela jcandela at prodigy.net
Tue Jan 3 16:46:44 EST 2017


Collin,
    One thing I remember on my GK-500B was the single screen bypass capacitor for the 4-400 tube. It was something like a 0.002 uf Mica-mold condenser. I replaced mine with four 500PF HV caps, two on each G2 pin, and then going to each grounding to a socket hold down screw using a solder type terminal lug. I also bypassed the filament leads in a similar way, but the exact details escape me now. I do recall that those changes made the King load up closer to ideal....max out and MIN plate current dip, and when the screen current was at a peak...In fact I liked to use the loading control to get no more than ~ 40ma screen current. 

   The GK 500 has a high impedance Screen supply, therefore the screen voltage rises as the screen current drops. It might be something like 600V at 20ma screen, and 400v at 100ma screen. The screen current on a tetrode drops as the tube is heavily loaded, and therefore the screen voltage rises. Perhaps your in a situation where loading to > 300ma plate current causes the screen voltage to go too high, and then the power sensitivity will be so high that a parasitic is more likely?? The screen bypassing, and a whole host of issues might be at play here, but the point however is that low screen current from a heavily loaded plate current might make parasitic oscillation more possible. It just depends how much regeneration you got going on..You don't want to solid state that screen supply.

JimWd5JKO 

    On Tuesday, January 3, 2017 2:39 PM, Collin <n4tua at aol.com> wrote:
 

 I will check out the buffer amp parasitic choke when I get home. The RF plate choke looks good and I see no burn or scorched areas. I will pull the HV bypass capacitors and check them. 
Good Day, Collin
"You can live to be a hundred if you give up all the things that make you want to live to be a hundred."  Woody Allen


On Jan 3, 2017, at 12:23 PM, Jim Candela <jcandela at prodigy.net> wrote:


Jim
W5JO


   


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