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I am looking at several custom tube line preamps. Can someone tell me (and/or direct me to a source that will help me to understand) the comparative strengths and weaknesses of these two designs; what to look out for; and if it is possible to make any sonic generalities about the two.One design is a cathode follower that uses 2 12SN7GT or 6SN7GT tubes in the gain stage. The other is a mu follower that also uses 2 12SN7 tubes, plus 2 27GB5 tubes in triode mode as the current source(s) - it's balanced - plus a rectifier tube.
I know asking a question like this is like asking to know what sex feels like. Please try anyway. We all have to start somewhere.
Follow Ups:
The CF is said to be non-linear at certain parts of in its operation. This is explained and shown in the text on Tube Amplifiers (or similar title) by Valley and Wollman, but I cannot repeat the argument chapter and verse. Using a constant current source in the cathode of a CF goes a long way to linearize the operation and improve the sound in my personal experience. If you want more, Allen Wright has touted a souped up version of the CF, which he calls a "Super Linear Cathode Follower" (SLCF). He explains how to build it in his "Preamp Cookbook". He not only uses a CCS in the cathode but also a constant voltage source on the anode side. A highly skilled friend of mine built it into a driver stage for an amplifier and likes it very much. As for the mu follower, it was the cat's meow for a while. I did not particularly care for it in the context of a preamp using 6SN7s. I thought the SRPP that Poinz mentions was out of favor with the cognoscenti. Glad to see he likes it, because I have liked it too, as a voltage amplifier stage in an amplifier (not too different from a preamp linestage).
nt
I can't remember. Sigh.Try an SRPP; it's like a mu-follower without the second in-between resistor and cap. They look like this:
It's not slightly dry and slowish like a mu-follower, or strident like a cathode follower. Zout is almost as low as either, parts count is low, elegance is high. I like it.
You might like it.
Aloha,
....is there an advantage to using a pentode as the upper tube, with the screen regulated?
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
No idea. I have a basic aversion to elaborating circuits for kewlosity, though. You will almost certainly get a more perfect current-source load for the voltage amp; you get that with the extra resistor and cap (mu-follower). In that case, mainly what I hear is the extra cap sound. Pentodes are noisier.So, no idea. Sigh. Guess I'll just have to find out for myself. Ask Jacqui to dinner, maybe.
Aloha,
a negative rail appropriately below the ground of the output stage, then I cna direct couple the SRPP to the output stage tube grid?another alternative to the cap or direct coupling would be an interstage tx - do you like the sound of an SRPP driving an IS Tx?
I just have this inherent hatred for ridiculously expensive coupling caps, and would rather spend the money on either a better power supply or a really nice Lundahl interstage tx. what do you think?
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
The signal is taken off the cathode of the upper tube, or the plate of the lower tube, of a mu-follower/SRPP, so it's going to be a hundred volts or so above the grid , which is the input side of the circuit. Now, your input grid is at about -130v to zero reference, so you need a cap there. Since the signal is going to be a fraction as large there, probably hear the cap a multiple as much.That's the way it looked to me when I worked through it.
Aloha,
nt
No sonic comments, but a very good source of basic info on all the basic tube "building blocks" is available from GlassWare, namely Tube CAD, which is a Windows based computer program with accompanying manual which allows you to build and modify virtual circuits on you computer using a variety of tube types. IIRC, costs less than $50 and is more than worth it for the educational value alone.
The cathode follower employs a common cathode in the gain stage followed by cathode follower in the output stage.
Both can sound good in the right implementation. I am a fan of Mu stages in that most of the voltage comes off the plate of the bottom tube in the Mu circuit. In a cathode follower, the whole signal passes through the cathode. Does it matter in practice? I find cathode followers closer to the sound of solid state equipment and Mu stages a bit more tube like.
Long Live Dr.Gizmo
Thank you. I am getting this same information from other sources, too.
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