Sander deWaal wrote:

"Fabio Berutti" said:

As per the output transformer, any good 3k5 unit will fit. BTW, being a PP
unit, it can be fed AC to the filaments, no problems. I'd keep separate
windings for each tube in order to have each one individually biased; this
allows to use any tube and it is safer in case of sudden failure of one
tube, but it is obviously easier and cheaper to have the 2 tubes of the PP
pair fed by a single transformer winding, having a center tap referred to
ground by the common cathode bias resistor (+capacitor). In this case, in
order to match quiescent current, tubes shall be matched on a good tester
(or bought from a dependable source as matched pairs),

Personally, I'm not a fan of common cathode resistors.
I used separates, each bypassed with a good 220uF/100V cap (philips
long life, good enough for me).
That way, I don't have to worry about unmatched pairs drawing unequal
currents, or worse, one tube taking it all while the other sits almost
idle.

1,000 uF are even better.

Patrick Turner.



--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005