On Friday, 15 March 2013 09:03:16 UTC+11, John L Stewart wrote:
In a previous post John Byrnes asked why I used a resister rather than an MJE340 for the long tail in a phase splitter. The short answer is that it works & very well at that. The question has gotta be, ‘why does it work so well’? For success the circuit surrounding the splitter stage needs to satisfy the relation, Rk(mu + 1) rp + Rl. In some of the splitters I’ve built using the 6SL7 family the Rk has been 75K sourced from –150 volts.. That could just as easily been a 12AX7. The result is about one ma in each triode. Checking the 6SL7 curves shows rp to be ~58K & mu to be ~70 at that point. The plate resistors (Rl) used are 100K each. Plugging the numbers into the relationship we get- Rk(mu + 1) rp + Rl 75 ( 71 ) 58 + 100 (all resistances are K) and 5325 170 Here is the Mullard 520. The ECC83 (12AX7) is running about 0.5 ma in each section. So rp is ~80K & mu is ~100.. Neglecting the following grid resistors, Rl is 180K. Rk is 82K. Plugging the numbers into the relationship we get- Rk(mu + 1) Rp + Rl 82 ( 101 ) 80 + 180 So 8282 260 You might think that an N-Fet or MJE340 tail would be infinitely superior. But in practice that may not happen. Balance depends just as much on the two Rls being equal which can be done with good accuracy. The two sides of the twin triode may not track as well over their operating range. Could the difference be heard? Perhaps. Here is an example of what does not work well, for fidelity anyway. Some circuit designers in the past (30s, 40s) have tried to get self-inversion in the power stage using a long tail. A typical stage is PP 6V6s, loaded by 8K, p-p. So each Rl is 4K. The longtail in one of these circuits is a series connexion of two resistors of 0.27 K each. The grid bias is taken off at their midpoint. The Class A operating point of the 6V6 results in rp of 50K & gm of 4.1 ma/v, so mu is 205. Again, plugging the numbers into the relationship we get- Rk(mu + 1) rp + Rl 0.54 ( 206 ) 50 + 4 55.35 54 Ouch! The relationship fails If it made sense that could have been easily fixed by using a small iron cored choke, for example, the Hammond 157M. Notice I’ve picked this one since it is 259R, so can serve as the cathode bias resister as well. The 8H will still looks like 2.5K at 50 Hz. The 157M would have been too expensive but does demonstrate what is possible. Today another fix would be an N-Fet. No low frequency problems in that part of the circuit anyway. Some folks still like the self-inverting output stage. It is useful as a musical instrument accessory for the even order harmonics (2nd, 4th, .Etc) it generates. See one at this link- http://ax84.com/media/ax84_m276.gif +-------------------------------------------------------------------+ |Filename: Avater..gif | |Download: http://www.audiobanter.com/attachment.php?attachmentid=339| +-------------------------------------------------------------------+ -- John L Stewart
Lord Valve is right about the guitar players perceptions about 2H. When you see the squaring up of the clipped wave form in any slightly over driven guitar amp, you begin to learn to never take any notice of what non technical guitarists say about audio. But Jazz guitarists may indeed want to "breed up" 2H and 4H by having over driven SET input stages which then supply a distorted but not clipped signal to an OP stage for a speaker, and this is supposed to sound warm, not cold and hard.
LTP configured OP stages can't work in class AB because the "tail" is a high resistance or CCS or choke. They can only work in class A and most musicians want maximum Watts per $, so they nearly always go for normal amps with both OP tube cathode grounded so each tube can be hugely over driven so OP tubes cope by moving into class C during overdrive, with predominiantly odd H.
LTPs used for signal stages and with MJE340 CCS eliminate the need to have any concern for the same sort of devices each side of the LTP, and on one side you could have 12AU7, and other side 6DJ8, and as long as the RLa for each side is equal, Vo phases will have the same amplitude whether one side only is driven or both sides are driven with a balanced input.
BUT, even with exactly the same devices each side of the LTP, and with one side only driven with input, you have a different Va-k on each tube so the amount of 2H produced by each tube cannot be equal, so there is imperfect balancing and thus less than 100% cancelling of the 2H of each device. However, the amount of 2H imbalance only becomes significant where the tube gain of each device is low, and the higher the u, the better the 2H cancels. In OP stages, any form of production of opposite phase to drive a second OP tube grid will never result in lower distortion that is is routine with use of a balanced drive to both OP tube grids. This means hi-fi amps are never to be seen with phase inversion in OP stages, but maybe its done in some musicians amps, except that I've never seen it, and I doubt the Groove Tube book with so many guitar amp schematics has even one with OP stage phase inversion. Patrick Turner.