main reference for you is passive pre ...... even if passive pre is factually worst solution for driving power amp, regarding drive capabilities
you said that was working OK
if anything else is not working OK , culprit is in gadget itself , not F6
in your case , I repeat - preamp stage is fishy
you said that was working OK
if anything else is not working OK , culprit is in gadget itself , not F6
in your case , I repeat - preamp stage is fishy
Thanks, your reasoning is good. Only one catch: Pre has been built by real professionals, whereas f6 has been built by me (according to plans from a real professional, but still...) 
And passive pre works like this: dac->output buffer->transformer volume control->f6. So it really isolates source and amp, unlike other devices.
OK, I will concentrate on the pre (still being suspicous).
And passive pre works like this: dac->output buffer->transformer volume control->f6. So it really isolates source and amp, unlike other devices.
OK, I will concentrate on the pre (still being suspicous).
problem is that you need Linear Systems Matched LSJ74/LSK170 JFET pairs (Grade B) – diyAudio Store , which is sold out
if you buy 4 SK and 4SJ , hardly that you'll have 2 pairs of SK+SJ
look at sticky thread in Papaland : Replacement For Toshiba 2SK170/2SJ74
if you buy 4 SK and 4SJ , hardly that you'll have 2 pairs of SK+SJ
look at sticky thread in Papaland : Replacement For Toshiba 2SK170/2SJ74
technically , will the IXFN60N80P work in the F6 , no matter what it cost , size, etc..
and if not , what will be the very best option other than the IRFP 240 ?
.
From lm 329 data sheet
A new subsurface breakdown zener used in the
LM329 gives lower noise and better long-term
stability than conventional IC zeners. Further the
zener and temperature compensating transistor are
made by a planar process so they are immune to
problems that plague ordinary zeners. For example,
there is virtually no voltage shift in zener voltage due
to temperature cycling and the device is insensitive to
stress on the leads.
The LM329 can be used in place of conventional
zeners with improved performance. The low dynamic
impedance simplifies biasing and the wide operating
current allows the replacement of many zener types.
A new subsurface breakdown zener used in the
LM329 gives lower noise and better long-term
stability than conventional IC zeners. Further the
zener and temperature compensating transistor are
made by a planar process so they are immune to
problems that plague ordinary zeners. For example,
there is virtually no voltage shift in zener voltage due
to temperature cycling and the device is insensitive to
stress on the leads.
The LM329 can be used in place of conventional
zeners with improved performance. The low dynamic
impedance simplifies biasing and the wide operating
current allows the replacement of many zener types.
Nearly anything will work but there is a smaller number that will be optimal.
It depends what you are trying to achieve.
Start by deciding what voltage and current you want to operate at.
Generally speaking 60N80P isn't the most ideal part for F6, there are more suitable parts.
It will work though.
If you want to try some other parts IRFP150 or IRFP250 is a great starting point biased around 1.8A with stock voltages.
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My F6 is driving a pair of 15 ¨ Tannoys and it's seems to do the job properly , i though if some differents output will do "better" , better as quicker , lower distortion , etc..
what is the point(s) for an good Mosfet for that specific job : transconductance ?, Rdson ? , capacitance ? , inductance ?
.
what is the point(s) for an good Mosfet for that specific job : transconductance ?, Rdson ? , capacitance ? , inductance ?
.
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For a single device and gain stage you want high transconductance, good current handling and good power handling, and low levels of capacitance relative to transconductance.
If you don't plan to increase bias above 1.5A then IRFP240 is a very good device. If you have the necessary heatsinking to bias higher then other mosfets are suitable. IRFP150 and IRFP250 are very good at higher bias levels.
I'm using IXTH64N10L2 at around 2A bias, along with some tweaks to get the sound I like.
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