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In Reply to: RE: AC Current Path (Diagrams) posted by Triode_Kingdom on May 26, 2017 at 23:26:52
What is the current path for the upper output of this circuit?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Well, the mu follower isn't quite so clear-cut. In a circuit using typical values, about 80% of the AC load current derives from the upper triode. Roughly 18% flows into the load from the lower triode through its anode resistor, R1. The remainder, one or two percent, reaches the load through Cg2 and Rg2.
This all changes if a high impedance choke or CCS is substituted for R1. That breaks the AC current path from V1 anode to the load. Now, nearly all the AC current will be delivered by V2, with a small contribution through Cg2 and Rg2.
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Buy Chinese. Bury freedom.
How about these stages?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I had to model the first stage of that circuit to be sure what was happening. It appears to be a follower, not a CCS as I first supposed. Nearly all the output signal current is supplied by the MOSFETs. Seems pointless to have a tube in the circuit at all. Why not just build a MOSFET amplifier and save the cost and trouble of a HV supply? Definitely not something I would ever do... :)
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Buy Chinese. Bury freedom.
"It appears to be a follower, not a CCS as I first supposed. Nearly all the output signal current is supplied by the MOSFETs. Seems pointless to have a tube in the circuit at all."
I believe the tube provides gain.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"I believe the tube provides gain."
It does, but so would a FET in that position. You know, it's only a small step from believing that a MOSFET used as a follower or CCS won't affect sonics to replacing all the tubes in the circuit with solid state. When a group consists of individuals who will readily argue which resistor, capacitor, diode, fuse or wire type sounds best, I think this is extremely hypocritical. Not referring to you, just the forum in general.
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Buy Chinese. Bury freedom.
I see the sonics being affected more by a FET as a follower than as a CCS.
One more question. Is the output impedance value of the FET follower affected by the plate resistance of the tube below it?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The more important question ought to be, 'what about SS sounds so different'? Their input capacitance varies depending on the voltage across them for one. But that does not seem to be of any interest; 'SS is bad', or some such and then complete dismissal...LOL
Tubes are not without significant limitations, which when crossed leave them sounding like crap. I suspect combinations that take advantage of the strengths are much more capable than either alone.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
The Rp of the tube has almost no effect when it's driving a MOSFET follower. Nearly all the AC signal current to the load is supplied by the MOSFET. That's what sets the output impedance.
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Buy Chinese. Bury freedom.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
This exercise gave me a chance to review the current paths for these various configurations. I had forgotten that signal current in the cathode follower flows through the power supply. That means it's important to have a good quality cap in the last position, and to be sure the cap's full potential isn't degraded by semiconductors or poor quality components upstream. Maybe this is another reason some CFs have an unpleasant sound. A common cathode stage with a CCS or anode choke wouldn't be vulnerable to this.
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Buy Chinese. Bury freedom.
Do the MosFets represent a CCS if the signal were taken off the plate of the 6c45?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"Do the MosFets represent a CCS if the signal were taken off the plate of the 6c45?"
Yes, that configuration impresses an AC voltage differential across the gate and source, creating a CCS. The tube will provide nearly all the current to the load.
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Buy Chinese. Bury freedom.
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