Ridiculous Hallucinations

[kenpeter]

If a crazy man had a bifilar wound OPT to play with...
And I don't, so not to worry...

We know from Mac theory that pentode mode translates to UL.
But cross coupled triode strap turns to pure pentode mode???
Whats goofier here, screen currents cancel selves completely.

Also messing with flying heaters and a negative main supply.
No advantage found yet. Just food for thought experiments...

And a shunt regulated cathode bias, draws constant current
from the power supply regardless AB. Maybe this is worthy?

This many Henrys would have made about 5K Plate to Plate
had all primary windings been on the plate side. Just sayin
for comparison sake to something "normal"...

[TheGimp]

That looks a hole heck of a lot like a cross coupled multivibrator without the capacitor in it. And, stray capacitance would suffice.

[20to20]

We know from Mac theory that pentode mode translates to UL.
But cross coupled triode strap turns to pure pentode mode???
Whats goofier here, screen currents cancel selves completely.

Also messing with flying heaters and a negative main supply.
No advantage found yet. Just food for thought experiments...

And a shunt regulated cathode bias, draws constant current
from the power supply regardless AB. Maybe this is worthy?

If it was a class B2 that might work but you could run the filaments from the grid current and bypass the cathode with the Pr shunt to SG. That would unload the OPT and let more GNFB tame the hot Mu. Mu cancels the bias at the grid with the heaters. Duhhh! Mu, Mu, and more Mu! Meeeeuuuuuww....

[kenpeter]

Originally Posted by TheGimp

That looks a hole heck of a lot like a cross coupled multivibrator

Cathode to screen voltage is held almost a constant by the cross couple.
There is little or no screen feedback. I know what it "loooks" like. But you
have to account that cathodes and plate are moving equal and opposite.
The opposite plate swings the same as the local cathode. So go figure?

As drawn, sim is delivering 55WRMS into 16 virtual ohms in AB1. And draws
constant current from B- supply due to the weird cathode fixed bias shunt.
A third leg that runs up the middle and does nothing, except waste power.

When the amp is idle, the shunt is dumping a LOT of power... Not gonna
win efficiency contest. Compare shunt to ClassA for inefficiency, except
output tubes aren't taking brunt of the wasted heat.

Pic: Cathode and screen moving exactly together = Pentode mode...

[kenpeter]

Mac kept screens at fixed voltage while cathode and plates moved.
This bizarre relativity experiment was exactly equivalent to 50% UL.

I've tried that configuration too, but can't get more than 35WRMS
without violating AB1. UL + cathode NFB = far too much total NFB.
Would require driving G1 further than +0V to realize the full 55W.
At least with 6L6 at 450V, this seems too much feedback...

Feedback from cathodes alone, is almost but not quite too much.
Can keep it all in AB1, full power out, no big G1 currents to drive.
Abuse the same relativity to make Pentode mode instead... With
this much cathode feedback, did Mac actually ever need the UL?

Mac screen currents would have appeared in the cathode windings,
but not in the plate windings. I'm not sure the fancy Mac winding
scheme could extended balancing benefits to screen currents that
do not exist in the other half winding? But this crosswire triode to
pentode mode has complete balance. Cancel to the dot anyway.

[kenpeter]

So, no takers? I'm not too sure the shunt thing a grand idea as-is.
But could be crude beginnings for an autobias of some sort? Third
leg shunt maximum current corresponds inversely to AB crossing
minimum current. So is an easily detectible/comparible/correctible
minima. And could be low passed and fed back to the gate.

As for cross triode strappage, thats a sure winner with this OPT.
Assuming such an OPT exists separate from a classic McIntosh?
I wouldn't suggest desecrating history just to recover the iron.

If its a patenty thing that holds back new production, I can show
you an even better (takes same idea even further) winding for the
purpose. Most likely never been tried before. Ask me. Dare me to
whip it out... Goahead, see what it gets ya?

[dave slagle]

hey,

a few thoughts...

First as drawn the output is actually an autoformer. Not sure if it was your intention but it seems to make a difference since when wired as an autoformer your output voltage appears at the plate but not at the cathode. Still haven't gotten my head around what that means in reality but wiring the OT as a transformer changes things a bit.

Also,

I'm not sure how mac did it with the bifilar pairs but it appears that you could simply do a quadfilar and be done with it.

dave

[smoking-amp]

As far as I recall, Mac didn't use UL mode. The screens were either bootstrapped to opposite plates, or had a separate CT'd winding on the xfmr to supply isolated screen V+. (Some imitators like AR used the speaker winding for CFB and a simple fixed screen voltage giving UL mode) Some Macs had windings for feedback to the driver stage too.

You can build a Mac clone easily by just using two identical OTs wired up al'a Norman Crowhurst's Twin Coupled amp. He used cross-coupled caps between the opposite cathode and plate windings to sub for the bifilar effect. Looks to me to be completely equivalent. Edcor's CXPP60-MS-2.4K or CXPP100-MS-1.7K or 2.5Kcomes to mind.

Another alternative is the Elliptron configuration, this is half way between Mac/Twin and Circlotron. The tube plates go to the usual plate winding ends and the cathodes connect thru cross coupled caps to the opposite UL taps on the OT. A CT'd inductor supplies -DC to the cathodes then. This gives 1.4*1.4 ~> 2X the primary OT impedance equivalent for 40% UL taps, and gives about 29% CFB. Easier to drive than the Mac 50%. Needs an extra CT winding on the CT'd inductor for screen voltage distribution if you don't want the UL mode. Or use floating screen supplies (zeners, caps).
---------------------

Re: quadrifilar
The Mac used two bifilar sets of windings (each is bifilar between a plate winding and the opposite cathode winding) because those have identical (but opposite between bifilar sets) AC signals on them. Combining those into one quadrifilar set would cause enormous distributed capacitance problems. Also, of note, the Mac bifilar pairs have the full B+ difference between adjacent bifilar wires, this requires extra insulation enamel layers on the wire to avoid arc-over. The Twin avoids this issue. I would build a Twin, NOT a Mac as a first attempt. Although there are some super insulated magnet wire sources available nowadays.

[godfrey]

I like this!

For a practical build, couldn't you use two identical normal transformers - one for the plates and one for the cathodes, with their secondaries tied together in parallel?

[dave slagle]

oops... brain fart on my part. I was looking at the currents adding and forgot that the plate inductors and cathode inductors are is series so they cannot be bifilar to each other.

dave