I have finally completed the breadboard of my 5B255M amps. I am getting 112WRMS from four of them in PP triode with 2% distortion. Nice, it sounds good too.
Using an IRF820 source follower for each output valve into a 2.2K load. I know its not ideal but that is the trannys I have. Increasing the secondary load to 12R, giving a reflected impedence somewhere near the ideal actualy results in more distortion and much the same power.
Anyway my question is: how about screen drive? I have searched the archives and cannot find the old threads relating to screen drive. Is it worth trying?
I may as well try it whilst I have it in the lash up stage and I guess with the followers and more than enough drive voltage, I may as well give it a go.
Cheers Matt.
Using an IRF820 source follower for each output valve into a 2.2K load. I know its not ideal but that is the trannys I have. Increasing the secondary load to 12R, giving a reflected impedence somewhere near the ideal actualy results in more distortion and much the same power.
Anyway my question is: how about screen drive? I have searched the archives and cannot find the old threads relating to screen drive. Is it worth trying?
I may as well try it whilst I have it in the lash up stage and I guess with the followers and more than enough drive voltage, I may as well give it a go.
Cheers Matt.
Screen drive is a great technique- just not with that tube. It's best with very high perveance tubes, usually TV sweep tubes. You can run them very close to class B and get great results.
Ok I seem to remember it also had something to do with tubes with a low screen voltage rating? ie designed to be efficient in a class B setup, much like line output in TV sets. Perhaps even class C, maybe good for RF finals.
I know the 5Bs I have dont mind their screen grid being abused big time. I have run them at 500V UL and 500V anode 400V screen just to see. Nothing happened, infact it apeared to me that the 300V limit the datasheet quotes was actually the ideal as they worked best there. Mind you they sound dreadful in tetrode mode. Maybe some Schade FB would sort them out in that setup.
Cheers Matt.
I know the 5Bs I have dont mind their screen grid being abused big time. I have run them at 500V UL and 500V anode 400V screen just to see. Nothing happened, infact it apeared to me that the 300V limit the datasheet quotes was actually the ideal as they worked best there. Mind you they sound dreadful in tetrode mode. Maybe some Schade FB would sort them out in that setup.
Cheers Matt.
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