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Old 11th June 2009, 03:49 AM   #1
Bitrex is offline Bitrex  United States
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Default I think I understand CCSes now...time for application

I was trying to grok constant current sources as I was looking over some schematics, and I think I finally have it, I wondered "How can the output voltage across the tube change, if the current through the tube doesn't change?" Then it struck me, it's the current through the series combination that stays constant! If the tube conducts harder, the active device (tube, transistor, etc.) conducts less and vice versa, keeping the total current through both devices constant - the two devices fight each other in essence. If the current through the current source decreases when the tube draws more its effective resistance increases, and again vice versa, a mirror image of whatever the tube does. So this is how it behaves like a dynamic resistor with a high effective impedance, causing the Rl term in u/1+rp/rl to drop out and make the gain essentially mu.

I'm working towards building my first tube power amp project and I have been interested in the single ended 6EM7-based projects since they seem pretty straightforward and relatively inexpensive, and I've also been reading about current sources both solid state and based on high transconductance pentodes. I'd like to modify the basic single ended design to make it "my own" by using constant current sources, and I've been thinking about adding a tube mu follower or solid state CCS load for the 6EM7 triode, and something based on the LM317 to the cathode of the output stage - if I used a pentode for the signal triode's plate load I could bias all the tube's heaters up to prevent heater to cathode breakdown on the pentode, and also direct couple the two 6EM7 sections while I'm at it, right? Maybe put a voltage regulator tube or two in there for the screen of the constant current source if it's a pentode (someone gave me 10 0D3s and several 0A3 (please help me put them to some use!) Are there any high transconductance dual pentodes to keep parts count down? Any comments or suggestions on a design like this would be welcome.
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Old 11th June 2009, 04:38 AM   #2
Stuey is offline Stuey  Australia
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You'd probably be best trying in the 'Tubes' forum.
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Old 11th June 2009, 06:22 AM   #3
Bitrex is offline Bitrex  United States
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OOPS...Sorry about the mispost! Perhaps a moderator can move it over?
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Old 11th June 2009, 09:08 PM   #4
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