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KT120 tests, compared to 6550.
Over the last fortnight I rebuilt my benchtop power supply to be able
to test KT120 with a regulated 600Vdc anode supply, at up to 320mA. I placed shunt regulation for the screens on the breadboard for the pair of KT120. Fixed bias was used, with a balance pot to equalise Ia in each tube. Operation in all tests were limited to class AB1 with no grid current. The OPT is a 8Kg monster with negligible winding resistance, and lots of interleaving, so its character does not affect the function of the tubes. The OPT has 20% CFB, so that where there is 200Vrms+ at an anode, there is 50Vrms- at the cathode. Depending on load, applied grid signal is about 60Vrms at middle load values. OPT ZR = 1,233:1, TR = 35.1:1. The drive signal maximum for test1 below were the highest, and up to 115Vrms was needed to be applied to each KT120 grid. This was obtained by using another tube amp and taking out the anode to anode signal via 10uF caps, and applied to quite high value biasing R. The maximum available drive was 276Vrms from each 6CA7 anode from an ultralinear power amp with 17dB global FB, so THD in the drive signal 0.1% at 276Vrms. At the levels of 115Vrms used, THD was considerably lower than for 276Vrms. Using an OPT with 20% CFB meant that the distortion in all the tests remained low, and the Russian KT120 exhibited no nasty surprises when compared to average condition Russian 6550 used to explore the differences between the two tube types. KT120 test1 :- Ea = +600Vdc, Eg2 = +400Vdc and Idle Ia = 50mA, idle Ig2 was 4mA per tube. Fixed Eg1 at -47Vdc. At absolute maximum PO possible, Ig2 rose up to 17mA per tube. Po versus RLa-a as follows, at clipping onset :- 2k0 = 60W, 4k0 = 102W, 4k8 = 115W, 6k0 = 102W, 8k0 = 80W, 12k0 = 56W, 16k0 = 45W. KT120 test2 :- Ea = +500Vdc, Eg2 = +500Vdc, Idle = 50mA, Ig2 slightly higher than in test1. Max Ig2 rose to 22mA per tube at absolute max possible Po. Po versus RLa-a as follows, at clipping onset :- 1k0 = 90W, 2k0 = 127W, 2k5 = 130W, 4k0 = 100W, 4k8 = 85W, 6k0 = 72W, 8k0 = 58W, 12k0 = 40W, 16k0 = 27W. Conclusions. The effect of having the higher screen voltage in test2 raises the knee of the diode curve. The diode curve in fact is the line for Ra when Eg1 = 0V. With Eg2 = +400V, the curve is a straight line from zero to 50V x 350mA, then the line rolls over to the left for 100V x 500mA, and 150V x 550mA. The knee shape could be lower for where class A is intended, so Eg2 could be lower. But for highest class AB Po with the lowest RLa-a load, the Eg2 should be ket as high as possible, and well regulated. With Ea at +600V, I recommend Eg2 not be higher than +450V, which would increase max Po possible to about 125W. With Eg2 = +500V, then the diode line is nearly straight to 100V x 650mA, before rolling over at the knee to 200V x 760mA. The extra +100V of Eg2 gives the tube a much higher max Ia capability with low load values. The curves I plotted for test1 and test2 show that test1 has the higher spread of power, 80W or more is available between 3k2 and 7k9. Test2 shows 80W or more of power is available from 1k0 to 5k3. 6550, Test3 :- The 6550 was not tested wth Ea = +600V and Eg2 = +400V, as it is felt by this technician that reliability can suffer with such high Ea for 6550. But it was tested with Ea = Eg2 = 500Vdc. 1k25 = 90W, 2k5 = 112W, 3k7 = 93W, 4k9 = 77W, 7k3 = 57W, 9k8 = 45W, 12k0 = 32W, 16k0 = 24W Conclusion. The results show that rather a small amount of extra Po is available if the KT120 is used to replace 6550, or KT88 as currently produced. However, the tendency of the KT120 to over heat with abuse is much less than for 6550, so that use with Ea at +600V and Eg2 at +450V would be OK. I fond that a KT120 tended to draw less Ia at idle if plugged into a tube socket which previously had 6550, so it would appear it is safe to plug in the KT120 to replace all other beam tetrodes, ( although please don't assume anything, and don't assume KT12 will be OK where there previously were EL34 or 6CA7. ) The distortion character witnessed in all tests was unremarkable, and what I expected when a substantial amount of CFB is used, 20%, in this case. The tube electrode signal relationships with 20% CFB is the same as for UL with 20% taps. I tested output resistance of the output stage as follows :- Set up amp with 10.0Vrms, no load connected. The connect 4 ohms, without changing anything else. The Rout = Vo drop / current in 4 ohms. In this case, with KT120, Rout = 1.0 ohms, which includes maybe 0.1 ohms of Rw at the secondary. With ZR = 1,233, then Ra-a is effectively reduced to 1,233 ohms, or Ra of one tube is around 600 ohms. With the same test performed on 6550 in the same circuit and op conditions, the Rout = 1.5 ohms, giving effective Ra-a = 1,850 ohms, or around 900 ohms per tube. So the KT120 must have a higher figure of merit, ie gm and µ is higher, so Ra must be lower. In conclusion, the KT120 performs rather better than is indicated by the attrocious data sheets currently available from the makers, drawn up by lazy *******s who seem to have forgotten what it takes to really test a tube properly, and to provide curves without lies. The KT120 is a slight betterment on the KT90EH which I like a lot. For those NOT interested in trying to force huge amounts of class AB1 Po from any tubes they lay their hands on, the KT120 allows those interested in pure class A to have Pda at a safe 40W per tube continuously, thus allowing up to 17W of class A1 Po, or 34W from a pair. I have seen ppl try to run KT88 and 6550 at Pda = 40W, and seen how the tubes didn't last long. But with the higher 60W Pda rating, 40W should be OK. I have not tested KT120 for triode connection, but I suspect they should be good. 12Watts in SET may be easily possible from 1 tube, ie, about twice what one gets from a single 6CA7 or EL34. The simplest way to use KT120 for best fidelity and for home hi-fi use is to use the UL connection because CFB OPTs are very rare off the shelf. The KT120 will very much suit use in amps such as my 5050 at http://www.turneraudio.com.au/Integrated5050.htm Everyone knows huge Po is possible from PP KT120, but what audio enthusiasts was is finesse, which means the first 30W is the most important. I think they may find KT120 a fine replacement for all previous octal beam tetrodes. Patrick Turner. |
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