Although not a circlotron, I wanted to share a prototype circuit I built this past week. The output is simular to what has been bouncing around this thread, tube controlling a bjt with its cathode. I used triode strapped 6aq5's driven by 6j6 in long tail pair configuration. I used adjustable fixed bias on the 6aq5's to set idle current through the output bjt's.
The output transformer is easy to wind yourself. Primary is 200mH center tapped and the secondary is 100mH winding. Simply a 2:1 step down. Be sure to use heavy enough wire to hold at least 3 amps.
The biggest shortcomming is low gain. Adding a grounded cathode stage after the long tail pair to increase gain made the amp unstable (motorboating). Since i am using fixed bias on the 6aq5's, this leaves me using 100k max grid resistors, so using something like a 12ax7 for the long tail pair is not a good option. So now I am working on a preamp for it.
The output stage can muster about 35 watts before grid current of the 6aq5's sets in. It is biased to run in AB with 100ma idle current through the bjt's.
The output transformer is easy to wind yourself. Primary is 200mH center tapped and the secondary is 100mH winding. Simply a 2:1 step down. Be sure to use heavy enough wire to hold at least 3 amps.
The biggest shortcomming is low gain. Adding a grounded cathode stage after the long tail pair to increase gain made the amp unstable (motorboating). Since i am using fixed bias on the 6aq5's, this leaves me using 100k max grid resistors, so using something like a 12ax7 for the long tail pair is not a good option. So now I am working on a preamp for it.
The output stage can muster about 35 watts before grid current of the 6aq5's sets in. It is biased to run in AB with 100ma idle current through the bjt's.
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