ruffrecords said:OK, understood. So I drew a load line from plate volts =0 and Ia =30mA through the -5V Vg and Ia = 15mA point. From load line I estimate a 10V swing in Vg gives a 16mA swing in Ia which gives gm=1.6mA/V so Ra = 625 ohms. Have I gone wrong somewhere?
Winston O'Boogie said:I was wrong. Sorry.
No idea where my head was
John Broskie and Alex Cavalli have thrashed the WCF far better than I could.Winston O'Boogie said:...PRR ...Hopefully he can advise.
PRR said:John Broskie and Alex Cavalli have thrashed the WCF far better than I could.
1/gm is good for low-Z loads. High-Z loads should correct for load impedance. This happens when load is above half of plate resistance. So 6SN7/12AU7 driving 600r even 2.4k, correction is small; put a 6080 in there and you should work it out.
Broskie also notes "Plate resistance and transconductance vary."
(For more fun: if you push a WCF to MAXimum output, gm varies from near zero (say 0.5X) to about 1.4X the idle value...)
My tedious sims only show that a WCF's THD will null, only if *exactly* balanced. In practice this suggests trimming two +/-20% tubes to equal current and 2% equality of Vpk. Since real tubes are not even constant one minute to the next, you get a very low distortion residual which changes minute to minute. I wonder(!!) if that could be more distressing than a higher THD which stays about the same all the time.
Overload of a tube WCF (assuming near-correct Ra) is almost always about not running enough idle current.
PRR said:I wonder(!!) if that could be more distressing than a higher THD which stays about the same all the time.
ruffrecords said:Hey Winston. No problem mate. As I am fond of saying, the only people not making mistakes are those doing nothing.
PRR said:> the mechanism behind why the 2nd is raised
To a first approximation, both tubes must see the same current and voltage. This is usually not at top-grid exactly equal to half of supply. There's some lesser effects (especially if you do/don't have extra resistors). In sim it is a very fine trim. In real life it may not be practical to pursue a THD null which will drift.
So that's where the assymetry is, hence mo' 2nd. Ah-Ha! Thank you sir.PRR said:> the mechanism behind why the 2nd is raised
To a first approximation, both tubes must see the same current and voltage. This is usually not at top-grid exactly equal to half of supply.
kambo said:DC current on top and bottom is almost exact, 3.45 - 3.44 on lo - 10.45-10.42 on high current setting
Winston O'Boogie said:Of course it is, it always will be the same with this circuit.
We are talking about a push-pull balance which is aiming to supply an equal, but inverted "signal" to the bottom tube. None of this changes the standing current.
kambo said:nope, DC current is not the same, is there is a perfect tube out there that i dont know of it... at least with the tubes i have tried, u have to trim it!
at least my triode matched (as the sticker on tube says) tubes needed some trim!
Winston O'Boogie said:Earlier you said: "3.45 - 3.44 on lo - 10.45-10.42 on high".
That's the same.
Unless you're planning on cloning a few Tektronix tube 'scopes.
In tube audio however, it's a miniscule difference between 3.45mA and 3.44mA. I bet it changes more than that if you happen to look at it in the wrong manner. I bet Uri Geller could change it just as much with his mind. it isn't going to make any difference to your THD, or anything that matters.
If you don't like what you're getting, you need to look elsewhere in the topology. Or a different topology.
kambo said:those values are after the trimming 8)
PRR said:> can i dive 600:600 with White Cathode Follower
My car "can" climb Goose Hill in 4th gear.
But it sure isn't happy.
My little 4-banger is the 12AU7 of engines. If I replaced it with a 455 V-8, that would be like a 6080, and I could climb Goose Hill in 4th or drive 600r with authority.
The difference is cost/weight. Throwing cast-iron or empty vacuum at a problem is poor use of potential leverage. As long as you use a transformer, may as well use some leverage and slip the WCF into 2nd gear where it will be happier.
PRR said:My little 4-banger is the 12AU7 of engines. If I replaced it with a 455 V-8, that would be like a 6080, and I could climb Goose Hill in 4th or drive 600r with authority.