Need help understanding Audio Research's tube-hybrid preamps

Apologies as this may be somewhat broad, and I may be somewhat stupid.

As I understand, the first Audio Research hybrid preamp was the SP-11, which used several transistor/tube hybrid gain stages, in different numbers, for different parts of the circuit, with three in the line stage, for a total of six such stages used.

Then we get the SP-9, which has reduced the number of tubes to 2, one for the phono stage, and one for the line stage.

Later line stages such as the LS-25 and LS-26 use two tubes as well, this time both for the line stage.

I understand the basic four-tube line stage gain circuit. I assume hybrid designs replace one tube with a transistor (as in the transition from the LS-25 Mk1 to Mk2). So how do the single-tube designs work?

I guess basically I'm looking for an idiots guide to the topologies that ARC has used over the years. All I've found so far is Alex Megann excellent article about the SP-11 at, and of course the wonderful database
Joined 2011
This has most of the schematics.

The hybrid LS25II is a two fet balanced diff amp stage followed by two cathode follower buffers.

The LS25 had a two tube balanced diff amp followed by two cathode follower buffers.
The topologies of the two versions are the same, the MkII just swaps the diff amp tubes for fets.
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Joined 2011
No, the buffers are tubes in both versions. It's the diff amp that was changed from tubes to fets.

Also, the MkI had fet current source loads on the buffer tubes, while the MkII just uses resistors instead.

All the schematics are in the database.
Joined 2011
A single triode tube section can be used as either a cathode follower, or a common cathode voltage amplifier,
or a common grid amplifier. There are three possible configurations, since a triode has three terminals,
and one terminal must be common to the input and output. The three different basic circuits:
Common cathode is the usual voltage amplifier. Common plate is also known as a cathode follower.
Common grid is used, for example, as the upper tube in a cascode circuit.

If the circuit amplifies, it will usually include the common cathode amplifier. Sometimes both
the common cathode amplifier and the cathode follower are used in the same circuit, like in the MkI.

Many miniature tubes have two identical sections within the single glass envelope, like the 12AX7, 12AU7, etc,
which can be used for differing purposes, since they are independent except for the filaments.
Or the two identical sections can form a diff amp, as in the MkI.
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