[Patrick Turner]

I'm not a big fan of parafeed anyway

Well who is a fan of parafeed?

The problem with parafeed for an SE OP tube is that you need a large
choke about the same size as the OPT. So there are 2 heavy iron wound
items, not just one. Then you must cap couple the OPT to anode. But
benefit of parafeed is that an ordinary PP OPT might be used, so that
a 50W rated 6k6 : 4,8,16 ohms can be obtained off the shelf from say
Hammond. a pair of parallel 845 might be used to provide the audio PO.
The anode is then cap coupled to the OPT via a suitable cap. For -3dB
response at 10Hz, only 2.4uF is needed, but one might have to use 3 x
10uF x 630V rated in series, with resistance dividers to keep the Vdc
across each cap equal at say 400Vdc if 845 Ea is at +1,200V, but it
could be +1350V if cathode biasing is used. 20uF polyprop caps rated
at 630V would be best for a pole at 5Hz. There is also the resonance
pole between C and the primary OPT L and the choke L. See how complex
things have become in terms of LCR set up?
So by the time you have all this LCR stuff at the OP stage, trying to
apply global NFB around say 3 C&R coupled stages while trying to get
LF stability may be quite a problem. HF will be OK, usually, depending
on the interleaving of OPT.

So the man who builds the parafeed and wants some 10dB global NFB to
get Rout 1 ohm for the 8 ohm outlet may have to be skilled at
reducing LF open loop gain with perhaps TWO LF shelving networks, V1
to V2, and V2 to V3 OP tube.

Its relatively easy for the DIYer to wind a Hugh Mungoose sized anode
choke. It should be about 50H at 150mA dc, or it could be TWO chokes
of 25H each in series. The two chokes offer much reduced self
capacitance of the winding, perhaps 5,000 turns, 0.4mm Cu dia wire on
E&I core with tongue 50mm x stack 50mm.

For those who LERVE irony-windy stuff, go for parafeed. Apply to a
local builder to strengthen floor supports. Try to remember to use a
separate PSU with a CHOKE input - of course! Mercury rectifiers. One
may as well go the whole darn hog, and make separate PSUs for each
channel because you want each chassis to be under 30Kg.

But the last time I used a pair of SE 845, SE air gapped OPT was GOSS
E&I, 50T x 75S, Np = 2,700t of 0.45mm Cu dia wire. No problems with LF
stability, and sound was Dreamy.

Patrick Turner