Joined on 01-07-2006
bass solution, the idea continues to live.
Thanks to the invaluable help of Stephie B, I've comeback to the GU81M like
a boomerang with some progress. There are 3 new things:
1) FFT vs Sound: as an ad hoc assumption let's accept that the limit of torelability is when
the 3rd hamonic starts to dominate the 2nd; weather it's right or not I guess only
the experiment will show
2) Selectable Ultralinear connection as one more degree of freedome to play with
3) Incorporated low-pass filter around 12AX7 (I know nothing about x-overs, so correct me if there are mistakes).
Here are two versions: LC (Butterwoth???) filter and RC Sallen-Key type (Linkwitz-Riley):
Now the comments: there is a mistake, 6E5P has +214V on the plate of course.
Everything is DC coupled a la Chimera's Axiom,
which makes up for 73265876 power supplies
to supply 3 tubes (although the +/-200V 2mA is nothing). the -3dB point sims show at 5.5Hz.
The 6E5P driver is biased at -4V and not -3.5V to give it some
headroom. I'd love to have some provisions to adjust in some small limits
the bias of the driver and the ouput individually, as this all is very senitive.
Ad1) The sims show that the +/- optimum load is 2.9k giving 160W at the point when 3rd harm=2nd
(at I think around 10% THD).
(Stephie claims to have reached 187W with 6E5P pent. mode, but I could not reproduce it).
The tube can give much more, in excess of 200W but at considerable clipping. The search over
GU81M shows that amateur radio peple do use&abuse it quite hard, 450W free air is realistic,
and there are reports it can survive quite some more for years. Unfortunately I could not get
any feedback from audio users of GU81M.
Ad2) Thanks to Stephie I've resolved how to add an UL posibility -- since the grid needs it's own
supply the only chance is to add another winding on the OPT. This makes the OPT complicated, but
for a completely different reason than usually: the primary side now has 3 windings: the main plate load,
the selectable 0-20%-30%-40% screen load, and the compensating winding caryying filament current
to reach 130H@300mA with a reasonable core. I think the switchable UL shoud add some nice
degree of freedome, apart from changing the load. I could not simulate the UL as LTspice
gets stucked, so no idea how it "performs".
Ad3) I've assumed 12AX7 CF with 157k to be a good solution, following Romy's experience.
I know nothing about active filters, please educate me. The most "economical" seems to be
LC filters, but 99% of x-overs I've found googling (incl. the Marchand) are Sallen-Key RC
with positive feedback. Now, is S-K so popular because it sounds good or because
it uses cheap tubes and RC instead of costly chokes??? Any input highly appreciable.
I'm not even sure if my implementation of S-K is correct? Have to sim it.
The input sensitivity without the filters (still have to sim them) for 3=2 point is around 1.7Vrms.
Although the spiritus movens of the whole idea seems to have found the