[Patrick Turner]

Prune wrote:

Patrick Turner wrote in
:
teflon, then they move to oilers

I don't get the oil thing at all, seems inconsistent to me -- oil has a
high dielectric constant, just the opposite of what the reasoning for using
Teflon seems to be (and of course the dielectric constant of polypropylene
is even lower than Teflon).

The only thing that makes sense is that foil would be better than film, for
the same reason Vishay makes metal foil resistors along their metal film
line (they have a white paper explaining why it matters, to what extent
it's just marketing I can't judge).

The dielectric constant is an issue with caps.
But not in coupling caps in tube amps where the
charge across a cap remains virtually unchanged
while the amp is running.

A 0.47uF feeding a 220k grid bias R forms an impedance divider network with
the cap having maximum Z at LF.
At 100Hz the ZC = 3.4k.
So where you have 100Vrms across the 220k the current = 0.45mA,
and so there is 1.5Vrms across the cap at 100Hz.
If the distortion in the cap is 0.01% which is all you'd ever expect to
measure, it could be 10 times less,
then the distortion voltage = 1.5 x 0.0001 = 0.00015vrms, and this is a tiny
fraction
of the 100Vrms across the 220k, and the cap caused distortion
would be 1.5uV, about - 116bD, well below the noise floor of the amp.
The worst case scenario for distortion numbers does indicate that any old cap
will do for coupling.

Perhaps there is something I have missed here about what makes caps have a
different sound.

But all the guys who say caps make a difference will not subject themselves to
AB tests where they are required to identify which amp has what caps.
They just like to believe pretty and expensive caps make a difference.
Its a bit neurotic. Christmas is a time for celebration of Christ's birth.
Having the best Christmas lights on a christmas tree does not celebrate
Christmas any better; the lights merely celebrate idiotic shopping habits.

Some people like to celebrate Christmas in July in a day or two and
some like to replace caps more often than underpants.

I leave them to happily enjoy their truths as they believe in. My best wishes.

Patrick Turner.