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In Reply to: RE: crow 10:1 posted by Ralph on August 06, 2007 at 10:57:16
Now that we can all agree on (I hope)! Might be nice if we could put the paraphase in spice too and see how things compare if tubes are "unmatched". The floating paraphase might end up doing better.
I know some guys don't like this forum as they feel we fight/argue too much. Personally I love it cuz I think we try hard to help each other reach a real understanding.
Might as well give me some of the crow too. After all I was the guy knocking the paraphase first and didn't see the point Dave Cigna was making at first.
Russ
Follow Ups:
Russ, if absolute balance is a priority, then it's hard to beat the LTP with a CCS in the tail. So long as the load resistors are matched so is the balance. Kinda like a concertina in that respect. Mismatched tubes should not affect the balance in any way, except perhaps to the extent that they present different capacitance to ground at their plates, but that's probably just academic in any real case. (W.T. Cocking wrote a five part series on phase splitters published in Wireless World back in 1948. He invested three or four pages analyzing the imbalance in signal that results from an imbalance of capacitance in the LTP. I had to ask myself why he went to the trouble. But, he considered the circuit unsuitable for audio because of the 1/2 gain thing. He was promoting it mainly as an vertical deflection amplifier for oscilloscopes where presumably precision at high frequencies is important.)
It's tempting to think that with mismatched tubes the impedances at the two outputs would be mismatched. I'm not sure that even that is true, considering the concertina and the fact that both of its outputs are balanced despite the fact that one comes off the cathode and the other off the plate.
I might get around to Spicing the paraphase. I admit that despite the seeming inelegance it can be made to sound good. Years ago, when I was experimenting with PP amps (EL84 and 6V6) I tried all of the common phase splitters. They all sounded good, especially compared to the SS Harmon-Kardon receiver that I had been using for years and felt pretty good about up until then. But, the paraphase seemed to give more of whatever was good about tube amps. The music came out of my little mini-monitor bookshelf speakers bigger with more dimension. I later found that SE amps, and then SE directly heated triodes, could give me even more of that. The rest is history as they say, and I haven't gone back to investigating PP amps very much. Lately I've been feeling the urge to retrace my steps, so I might take another look.
SwitcherCad/LTSpice is very easy to use. If you're really interested in modeling then you should download it and give it a whirl. Steve Bench has put together a few tips (not quite tutorials) on using it for practical modeling of tube circuits at the intact audio webs site.
-- Dave
Yes, I do need to drag myself out of the dark ages and learn it. Mr. Slage encouraged me too, but I chickened out. Too many irons in the fire thang. Maybe I will have a chance in the coming days. I'll need the right mix of ass kicking and encouragement. In particular, I don't really listen/learn until I understand how things work/interact on a root level. Looking at the spice programming I see I don't have a clue on that part.
We will talk about the balance if tubes were badly mis-matched when I make it over yonder, which will be at best a few days.
Russ
Russ,the beauty of LT spice is it is really easy to use. If you have a working design you learn by playing and asking just like modding a ST-70. you are welcome to anything i have and i'm sure others will share too. I have found it to be a very valuable teaching tool. I used to wonder what would happen if I tried something, and now i can sim it and see function prior to building it.
Just do it... Steve bench has a nice little jump start to get you going. The below link will get you started, and then read the rest of his posts in the section as you climb the curve. You will hit some quirks, but all you need to do is ask and someone will certainly help out.
dave
dave
Guys - So I'm still wondering if it's possible to accomplish this:
Run PP 6B4G in Class A (no feedback), cathode bias, using only four 2C22s (equivalent to one 6SN7 per channel) ? Keep in mind that I will be using a preamp that provides gain, not just a passive.
Paraphase or LTP with negative supply is fine with me. I don't want to use an interstage transformer since I'm trying to keep the cost down. I see how Poinz' LTP is built using a negative supply. And mikeyb explained how I could use the Eico HF-87 schematic and adjust the voltages downward to keep the same operating points by using a negative supply. And I did check out the PP1 schematic that Russ pointed me to which uses a CCS. My understanding is that a negative supply and a CCS are two methods of accomplishing the same goal. Correct?
Russ' comments help me understand what I need. . . I think. Are my interpretations correct?
"Pete's circuit has half of V4 operated as a voltage amp, which is directed coupled (actually, isn't it cap coupled? see C8) to the other half, which is operated as a split-load invertor. This alone is enough to drive tubes like a 6v6 or el84 but not a triode like a 6b4g. The split-load invertor is cap coupled to V5 which is operated as a push-pull amp."
So Pete's circuit uses V5 to provide additional gain. Will my use of a preamp be able to make up for the proposed elimination of this stage?
Also: "Notice one other big (and important) difference. V4's first section is running at 3.6ma (actually 3.0 I think - see R13), its second section at 4.5ma and V5 is running both sections at 6ma. The schematic you have posted is running about 1.3ma. This is important because it takes current to make a voltage change in the face of capacitance. The grids of the 6b4g have a good deal more capacitance the the grid of a pentode"
So I need more current than the 1.3ma provided by the paraphase. How much is needed for PP 6B4Gs in Class A? Is the current sent to the outputs the sum of the currents from those previous stages?
Can this be provided by the four 2C22/two 6SN7? In the Eico HF-87 schematic I posted the single 6SN7 per channel is running about 7ma. Is 7ma enough?
I've learned a lot from this discussion and I can build from a schematic but, frankly, I get confused by some of the math and charts. Ohms law I understand and I've read about drawing load lines in my tube manual and some online but it still confuses me. I'll keep trying but math is definitely not my forte.
So, is it possible? Can anyone suggest some decent operating points?
Thanks . . . Charlie
Charlie, here's a sample loadline analysis using a 6SN7's to drive 6B4's. The assumptions that I've made are that the 6B4's are biased with about -45V on their grids (relative to the cathodes) and that a supply of about 300V is available for the 6SN7's
The loadline in red is 47k ohms. One end is at the supply voltage of 300V and the other end is at 300/47k = 6.4mA. I chose an operating point of 3.5mA which puts the plate of the tube at about 135V. That's indicated in blue. The difference between supply and plate voltage (300-135=165V) is dropped across the 47k plate resistor. It all checks out with Ohm's Law: V=I*R=0.0035*47000=164.5. Close enough...
Green indicates the part of the loadline that will be traversed when driving the 6B4's to full power. Since the 6B4's are biased at -45V the plates of the 6SN7's will need to swing +/- 45V peak. The quiescent plate voltage is 135V, it will need to swing up 45 to 180V and down 45V to 90V. What is also shown in green, but is hard to see, are the corresponding 6SN7 grid voltages (really, grid to cathode voltages.) Just reading them off the graph, the quiescent grid voltage is about -4.5 and it needs to swing between about -1.9 and - 7.5 to drive the 6B4's fully.
Ok, that means you'll need about 3 volts peak input to the 6SN7, or twice that at about 6 volts peak is they are configured as a LTP. If you have a preamp that can deliver that, then you're golden. If you don't need full power, then you might get along just fine with less input signal.
-- Dave
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