[Patrick Turner]

Flipper mentioned here ....

Of course, one of the problems with bootstraps in general is they fall
apart faster than you can say "where did my voltage go?" at the
frequency extremes because the bootstrap condition fails

Indeed you make an excellent point.

However, in the case of McIntosh amps where they have 50% CFB in the
OP stage the McI OPTs have exceptionally wide bandwidth with low phase
shift. If one didn't have a wide BW OPT then the driver gain will sag
a bit too close to the AF band. But nevertheless gain shelving at both
ends of the AF band in many amps including most I build is a very
important technique to ensure unconditional stability will exist when
the the wanted response is -3dB at 10Hz to 65kHz.
If the amp OLGain is -3dB at say 20Hz and 8kHz, and this is not
unusual in many pentode or tetrode amps with pentode input stages then
the NFB will flatten everything out so you get 10Hz to 65kHz.
The amount of NFB at 10Hz or 65kHz is very much reduced below what it
is at say 400Hz where NFB may be at a maximum.

The mu-follower two triode gain stage is my favourite gain block with
good gain, wide BW, and high Z in and low Z out. It sounds and
measures excellently.
If one uses a 6CG7 with 10k0 between bottom anode and top cathode and
OL gain of top triode is say 16, then that bootstrapped 10k is made to
look like a load of 170k to the bottom triode. Because the top triode
is functioning nearly as a pure cathode follower its BW is very high
and its cathode is an ideal place from which to connect the
bootstrapped 10k anode load of the bottom triode.
Hence it is easily possible to get 0.1% THD at 10Vrms, as opposed to
having no bootstrapping and a much lower R load for the bottom triode
thus increasing THD to perhaps 0.4% at 10Vrms.
The top triode sees a cathode load or 160k, and its a win-win outcome
for all triodes concerned.

You may well state what you think are "general" tendencies but then I
find "exceptions to the rule".

The gain block which is technically better measuring is the normal
common cathode gain triode with CCS anode load and directly coupled to
a CF with CCS cathode load.
But how much better measuring is this? probably 3dB at most. I found a
12AU7 will sound magnificent with the dual CCS but then so does the Mu-
follower - hard to say which sounds best really.

The other benefit of the mu-follower which was called a bootstrapped
follower is that 5mA of anode current for 2 triodes in series is all
you need. With CCS loading for both triodes, you need 10mA.

Patrick Turner.