[Patrick Turner]

snip

A drawback - a despised electrolytic as a shaping component.

Btw, there's another means of achieving a modest 'LF shelf', using a
concertina, and that's to boostrap the gain stage load off the
concertina cathode. Gain drops at LF as the boostrap rolls off.

This has been done in some Dynaco schematics which have the concertina
cathode driving the top of a RLdc via an electro cap, therefore
supplying positive FB to the input pentode by means of increasing the
RL the pentode anode "sees" so that pentode gain goes much higher
therefore making a given amount of GNFB much more effective. Basically
while peter robs paul and you light a fire under both, they tend to
become very active indeed.

If the input tube is a lowly triode who's gain is determined by its
lower µ the PFB effect is minimal, and whether there is a bootstrap or
not won't change VLF gain very much, so the shelving networks seen
throughout my website are perhaps the better way with triodes.

It has the added advantage of boosting stage gain and theoretically
lowering distortion from the (idealized) 'infinite impedance' of the
bootstrap

The bootstrap from concertina cathode could be taken from an
additional triode cathode follower which is driven directly off the
cathode so concertina R values remain unchanged, ie, anode RL =
cathode RL. The CF can then be used to drive a two resistor R divider
to V1 anode. One has to be careful that cut off distortion with the AC
coupling does not happen.

The concertina resistors need adjusting to re-balance the load, and
that causes an offset toward B+, but that can be an advantage as well
since direct coupled concertinas on modest B+ rails can end up with
rather low anode voltage on the gain stage and the offset bumps that
up.

Yes, indeed. I would say I prefer the LTP driver with cathode CCS with
say 6SN7/6CG7/12BH7/12AU7 or EL84. Then the LTP acts as a balanced
pair with very low THD. Whatever the input tube is, pentode or
triode, it has only to produce a low signal in SE mode so THD remains
lower than if you have an input tube needing to make slightly more Va
than is applied to each output tube grid.

There's plenny of headroom for the function of the shelving network.
The LTP does not need to be directly
driven from input gain tube anode, so the LTP can have much more
headroom than found in most samples of amps like Leak and Manley Labs
et all.

Patrick Turner.