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In Reply to: RE: First Bench Test posted by vinnie2 on May 20, 2017 at 09:11:00
That will take more time. I'm confident this will prove to be exceptionally transparent, with very little sound of its own.
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Buy Chinese. Bury freedom.
Follow Ups:
Well, if you are going to share the design when you are done, I will be looking forward to seeing it.
Yes, I'll put it into the public domain, so to speak. I'm debating exactly how much circuitry to include in this project. Much as I like the simplicity of separates, the functionality of a full-featured preamp can be very convenient.Incidentally, I didn't have any 12B4a 9-pins on hand, so all the tests of the cathode follower circuit were with octals that I thought would bias correctly with the DCR of this choke. I was surprised to discover that none of the tubes required socket wiring changes. They were all plug-and-play except for appropriate changes to filament voltage. Here are the tubes I tested. Note that all the pentodes were wired in triode mode:
6K6 RCA
6V6 RCA
6N6C (Russian 6V6)
6P3S (Russian 6L6GB)
6P3S-E (Russian 5881)
6L6GC Sylvania
1626 Ken-RadThat's right - the little ST-bottle 1626 often used in "Darling" amps plugs right in. Performance differences between these tubes were minimal, mostly having to do with idle current and output impedance. The 6K6 idled at the highest current, but the 6P3S-E and 6L6GC exhibited the least drop in output voltage when heavily loaded (substituting 1.5K for the 12K load). I also tested every tube with a load consisting of the 12K resistor and a 1500pF capacitor in parallel. In every case, 10kHz square wave response was only slightly degraded by the capacitor, and there were no indications of instability or visible waveform distortion.
I have to say, the 1626 would look pretty cool in this application, and a pair of large, shiny 6P3S-Es wouldn't be bad, either. Strictly from the standpoint of functionality though, the 6K6 is my favorite. It idles at roughly half its rated dissipation, and it draws very little filament current for a power pentode in this class.
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Buy Chinese. Bury freedom.
Edits: 05/22/17
mine is about as neutral and 'not there' as I have seen...and by your standards, it is *LOADED* with silicon.
With a triode cathode follower, running it cascode, with a MOSFET on top serving as the screen grid is quite useful. It leaves the triode with a nearly constant voltage across it( if you make the circuit properly), and the very high effective plate resistance leaves it quite immune to noise( like PS ripple ). Changing the voltage the gate is at, is comparable to changing the g2 voltage on a pentode, and is quite useful for setting idle current in a system where the cathode resistance is not a parameter you can change.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Douglas, can I see a schematic?
Thanks.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
send me an email and I can get one for you...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
"the very high effective plate resistance leaves it quite immune to noise( like PS ripple )"
Don't think I've seen that technique. Won't a high plate resistance degrade the output impedance of the follower?
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Buy Chinese. Bury freedom.
It is the gm of the lower element that is of concern. The usual triode-on-triode cascode eats up plate voltage like a fiend...and the MOSFET nearly totally eliminates this.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I'm not sure we're on the same page. A cathode follower won't work if a CCS is supplying its anode voltage.
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Buy Chinese. Bury freedom.
"A cathode follower won't work if a CCS is supplying its anode voltage."
Why not?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
A CCS at the anode will block the AC current path through the tube. Very little will reach the cathode.
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Buy Chinese. Bury freedom.
So then how does a common cathode stage feed by a CCS work?BTW. A CCS (or a choke) at the cathode will block the AC current path through the tube in the same way as a CCS (or choke) at the anode will.
But isn't the AC current path through the load what we're interested in?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/26/17 05/26/17
"So then how does a common cathode stage feed by a CCS work?"
The AC current path (loop) is through the load, into the anode, through the tube, out the cathode, and into ground (the return for the load). The CCS delivers DC current to the anode, but is not in the AC loop.
"BTW. A CCS (or a choke) at the cathode will block the AC current path through the tube in the same way as a CCS (or choke) at the anode will."
In the case of the cathode follower, the AC current path is through the B+ supply into the anode, through the tube, into the load, and through the load into ground (the return for the supply). The CCS below the cathode is not in the AC loop, it serves only to return DC current to ground.
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Buy Chinese. Bury freedom.
Just to illustrate what I mean, the two diagrams below show the AC current paths in a cathode follower and a common cathode amplifier. Installing a CCS (or high-impedance choke) anywhere in either loop will prevent AC current from flowing through the respective load.
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Buy Chinese. Bury freedom.
What is the current path for the upper output of this circuit?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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.
You have misunderstood. How is a MOSFET with its gate a fixed a certain voltage above the triode's cathode considered a CCS?
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
"How is a MOSFET with its gate a fixed a certain voltage above the triode's cathode considered a CCS?"
I never referred to a CCS in those terms. And before we go too far down this road, I want to reiterate something you already know. I never attach semiconductors to the signal path. The purpose of posting in this thread was to describe and demonstrate the benefits of a choke-loaded follower as a line stage. I'm not interested in discussing the addition of a transistor to this circuit.
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Buy Chinese. Bury freedom.
What I described is a cascode, and labeled it as such.
It is utterly irrelevant as your sacred text, or golden tablets, or what ever, prohibit the use of SS devices.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I'm planning to use a HV bipolar regulator and large filter cap. That should provide a very low PS impedance for the tube. Would there be an advantage to the MOSFET over that? I don't need to drop voltage for any of the tube types I tested. It's a problem getting enough current the way it is, due to the relatively high DCR of the choke.
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Buy Chinese. Bury freedom.
this is only a discussion if you are willing to answer a question. You can answer or not...
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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