On 10/24/21 16:39, Luis Correia wrote:
A quinta-feira, 12 de fevereiro de 1998 à(s) 08:00:00 UTC, Robert Ang escreveu:
Try looking at my web page, as I have experimented with a 12AU7 direct-coupled with
a triode-wired KT66 through a Tango NC-20 interstage driving an 845. Enlightening
stuff.
http://opera.iinet.net.au/~rang
Rob.
Hi Rob.
I found your 845 circuit and I would like to know more about it. Where can I find more information?
Thanks in advance.
Regards.
Luis
wow tht link was from 1998 and is no longer working! tahat's nearly 2.5 decades ago!!
Can you paste the link you found, or maybe put it up on a sharing site
of some kind so the rest of us can look at it and see what you're
talking about???
You can direct couple 12AU7 to the grid of a power tube (like KT88) as
long as you put the correct bias on the 12AU7's grid and use a cathode
resistor to an appropriate negative voltage. You will need to maintain somthing like -50V on the power trube grid with no signal if I remember correctly (maybe it is more). It depends on the tube, of course.
Usually it's easier to RC couple. Just use a series resistor on the
power tube's grid to limit current if it goes into AB2 (which it will if
you want some REAL power out of it, depending). That causes some
nonlinearity but you can use NFB to compensate.
Anyway, I can think of a bunch of configurations using positive and
negative power supplies and clever voltage dividers (and isolating
capacitors at some point) that would let you direct drive a power tube's control grid from a 12AU7, even from the plate.
It's not like trying to do direct coupled with only NPN bipolar
transistors. You sometimes get clever with cascode and other configs.
https://en.wikipedia.org/wiki/Cascode https://worldradiohistory.com/Archive-DX/QST/40s/QST-1945-06.pdf (see
pg 17)
other references available, of course - these just popped up really fast
in an online search.
--
(aka 'Bombastic Bob' in case you wondered)
'Feeling with my fingers, and thinking with my brain' - me
'your story is so touching, but it sounds just like a lie'
"Straighten up and fly right"
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