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Input and LTP driver schematatis at a.b.s.e
I have just posted 3 schematics at abse showing various input and LTP
driver arrangements with comments. Fig 1 is for a basic 6SN7 input tube with direct coupled LTP, with a common Rk. I have calculated the loads and working voltages as best i could, but expect some variation in practice. In actual use, this basic driver circuit must have R10 varied until balance of drive voltages is obtained. Once set, it won't change much, and although there is imperfect cancellation of 2H in such an LTP, it works and sounds ok. Output voltage is restricted by the Ea for the LTP, and Ea = 260v - 106v = 154v only, and a a large anode v swing is impossible. Fig 2 shows the similar input stage CR coupled to the LTP which has got a CCS instead of a common Rk, and this allows automatic voltage balance since RLs can be exactly equal and Ea = 200v - 30v = 170v, which is healthier and allows a higher v swing and lower thd at the same low v swing as in fig1. Fig 2 can also be stretched to make it give much more, and R13,R14 can be increased, 550v used instead of 400v for the supply. In one amp I have the supply for the LTP is at +600v, with paralleled 6SN7 for each half of the LTP, and max possible drive voltage is above 100vrms per anode The same 300v can still be used for the V1 supply, by increasing R6. The use of a paralleled 6SN7 for *each half* of the LTP is a good idea, since it slightly increases v swing, bandwidth, and lowers thd. I normally like to play with the value of R11, which sets the constant cathode current, and thus adjusts the value of Ea, and this is done with a big signal present and the Ik is adjusted for symetrical clipping ( with no output tubes present ). I like the LTP to be able to produce twice the required drive voltage needed to make the output stage clip. Fig 3 with CT choke and R loading for the EL84 triodes of the LTP has a response graph attatched showing the reponse at each anode output...curve A. Curve B shows the voltage at the winding ends of the choke, and this shows the variation of voltage across the choke compared to the anode output. In this test I used the primary winding of horrible old OPT suited for a pair of 6BM8, and which had a quite small amount of primary inductance. If the CT choke has 10 times the L shown in this case the curve for the voltages across the choke are much closer to the curve for the anodes, which would indicate that the choke's impedance would be a lot higher down to a lower F. Note the resonance dip and peak above 100 kHz, caused by stray leakage and capacitances, and note how the 4.7k R prevents the effects of such impedances from seriously affecting the response at the EL84 anodes. Of course, all of what I have shown is a lot more complex in terms of component count compared to what Lynn Olson has proposed in his PP amps, which he prefers to SET by the way. But I don't have to try so hard to get the bandwidth in my amps as I would have to with several cascaded transformer coupled stages. The iron distortions which are not mentioned by Mr Olson are not a problem in my designs. Mr Olson and several others and including Mr Sakuma of Japan with his restaurant full of tube amps, mainly SET types, all try to reduce thd as much as possible with wise loading of their tubes. Its not as if the measurements don't matter to these dudes, they surely do, and they all mention linearity and distortions often, as I do. These other dudes my well wax lyrical about the sound, or complain about the sound of a stray cap here or there, and they surely hate capacitors. I don't, and find capacitors are the least blameable reactive coupling element, since they are purer than almost any inductor, which users damned iron, and which has odd resistance to windings and stray C and damned resonances, not to mention the appalling tendency to create crossover like harmonic distortions. But at least the iron has its better side, that of vastly increasing the loads seen by the anode circuits of voltage stages, and this reduces the triode thd, and we only need to have Ra nice and low to naturally reduce the iron thd to negligible levels. I believe that if a triode amp meaasures well, and has good spectral content, ie, freedom from much high numbered odd harmonic products, it should sound OK, and in fact its hard to make a bad sounding triode amp if it has plenty of ceiling and measures OK. All PP designs produce some 3H. With the LTP, or in the case of most class A PP triode output stages the 3H isn't just due to the 3H which is in the SE triode transfer curve being uncancelled by the balancing in the LTP by the PP action. The gm of the two halves of any LTP vary each side of the zero point. As one tube turns on with more Ia, the gm tends to increase, and the other triode which is turning off tends to have its gm reduce, and the change in gm isn't equal, so as the output voltage is increased the net differential voltage gain of the LTP ( or output stage ) tends to decline a bit, resulting in the 3H we see gradually increasing from low voltages, and rapidly increasing near clipping. But set up right, the LTP can be engineered to produce astonishingly low thd figures at say 75 vrms with a couple of EL84 in triode with a choke as in Fig3. Unlike an output stage which is forced to endure a large current swing and voltage swing to make some power, the LTP should be engineered to have to make as little power as possible, which means minimizing current change in the triodes concerned, even with a high Vswing. Thus the thd of the LTP can be kept to much less than the thd of the output stage connected, assuming we have made the output stage in triode, UL or CFB, and mainly class A, so its thd at clipping is only 1%, ( no global FB ). Often it may be so easy to have 2% thd just because we have not got a low thd LTP driver. When I built my first 8585 with a Quad of 6550 to each channel with 12.5%CFB windings on the OPT, I was able to get 0.7% thd at 40 watts class A without any global NFB, and this was the straight measurement of the whole amp without deducting the LTP and /or input stage thd. A lot of the measurements quoted for triode or other PP output stages may not be taking the thd of the LTP into account. Since it is impossible to easily and naturally engineer any LTP with a rise in 3H which is oppositely phased and of a similar rate of increase for most output RLs to the peak flattening 3H in nearly all output stages, it is left to the designer to try to engineer the most linear LTP he can, since there isn't any natural other better way to minimise thd. Voltage distortion cancelation is impractical in PP amps, a somewhat different situation to SE amps where the much larger quantities of 2H in drivers and outputs can actually cancel to good effect, providing we don't deliberately set up a driver stage to produce lots of 2H just to cancel the large 2H in the output, by means of adversly loading the driver. Its better to linearize the output stage to get its 2H minimized so that the slight 2H produced in an SE driver will usefully cancel but not produce many second order harmonic and imd products. The simplest way, but by no means the cheapest way to linearize amplifiers is to have a high power ceiling, so that a small amount of class A is all we ever will ask of our amps, even if its a class AB amp. Patrick Turner. |