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In Reply to: RE: You don't wanna use that posted by Dave Cigna on July 31, 2007 at 18:08:38
I agree with Russ57- the 6B4 deserves something better.
At the very least, the driver should be a differential amplifier (the so-called Long Tail Pair...). This insures that each power tube sees the same number of stages.
A single gain stage, direct-coupled to the 'LTP' can work very nicely. No need for negative feedback. You can generate the Bias supply from the winding of the power transformer that does the B+, if you set that up right, it can provide B- for the differential driver too. That works a lot better for the driver!
Follow Ups:
"...the driver should be a differential amplifier (the so-called Long Tail Pair...). This insures that each power tube sees the same number of stages."
But they wouldn't and that's my point. In the drawing above, the signal that arrives at the grid of V3 goes through V1 first. The signal at V4 goes through V1 *and* V2.
The AC voltages assume that V1 / V2 would have a gain of 20 in an ordinary common cathode circuit (same operating point, load, etc, blah, blah.)
-- Dave
When the signal arrives from the single-ended gain stage, the cathode coupling of the differential gain stage ('LTP' using the rather quaint vernacular) causes the other half of the differential amplifier to have an inverted waveform. There is an equal number of gain stages thus seen by both phases of the signal, inversion occurs in real time rather than have a propagation delay. You forgot the essential capacitor that goes to ground from the grid of V2.
The beauty of differential drivers has always been that the phase inversion is alway exactly 180 degrees out of phase- no additional gain stages for either phase. If you have trouble with this, read up on how differential amplifiers work. Their use in an amp if this type is elegant.
"When the signal arrives from the single-ended gain stage, the cathode coupling of the differential gain stage ('LTP' using the rather quaint vernacular) causes the other half of the differential amplifier to have an inverted waveform."
Right. That's why we use it as a phase 'splitter.'
"There is an equal number of gain stages thus seen by both phases of the signal, inversion occurs in real time rather than have a propagation delay."
Sorry, I don't see the equal number of stages. Could be that you're trying to tell me something and I just don't get it, but at the moment I just don't see it. I certainly don't see how it must follow from your first statement.
"You forgot the essential capacitor that goes to ground from the grid of V2."
I didn't forget any capacitors. I designed the circuit so that the grids of the input stage are at ground potential; no DC blocking caps are required.
-- Dave
If you have a single-ended input to the differential driver, the second grid has to be a virtual ground. Even if the voltage is at or near ground, the *AC* potential cannot exist- that's what the cap is for.
That way that half of the diff amp is trying to be constant current, insuring the proper cathode coupling. A differential amplifier by the way is always considered a single gain stage- its not two.
Look carefully at the drawing. The second grid is connected directly to ground.
I think I see where the confusion is coming from. Yes, the signal at the grid of V4 comes from both V1 and V2, but the signal at V3 also comes from both V1 and V2, not just V1. This is because the signal on the grid following the 'LTP' is derived from the difference between the two plates. Hence, differential amplifier. So, the signals at both grids go through one gain stage. Because each grid signal is taken from the different plates, they have to be 180 degrees out of phase (barring distortion of course).
Chris
I dunno why I am so anti-paraphase. I suppose it is because I saw it a lot early on and it was typically used in chessy amps with 12ax7's and low quality OPT's. So by all rights I am blaming a circuit when it was what surrounded the circuit that I should be blaming.
Sometimes I need a coulpe of good kicks before I let myself see the light. I still like the looks Jeremy's iron "babrbed wire" circuit though:)
Russ
that the paraphase has two consecutive grounded cathode stages in the non-inverting (actually twice-inverting) channel, whereas the LTP has a cathode follower followed by a grounded grid stage in its non-inverting channel.The paraphase can be difficult to balance and may never give really good balance across the audio spectrum, especially as the tubes age. It seems insufficiently stable to me.
The OP signals from an LTP, on the other hand, can be pretty well balanced for ever, by using a good CCS in the tail with well-matched plate loads. The balance does not depend upon the gain in each half; in fact, you could use dissimilar tubes and it would still work!
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