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KT120, more test results.
Hi all.
Tonight I tried a few more tests of KT120 and also NOS Sovtek KT88 from 2001, and the 6550 I had tested. This time, I re-configured the OPT without CFB to allow a better look at how the tubes functioned in pure beam tetrode, UL and triode, and to get an idea about general comparable distortion, especially in pure tetrode mode. Idle conditions were Ea = Eg2 = +500V, Ia in each tube = 50mA. KT120 :- In tetrode, +500Vdc regulated, I could get 137Watts max with RLa-a = 1k5. THD looked awful, maybe 6%. Because the power band above 90W is between loads of 1k0 and 3k5, it is somewhat a useless amount of high PO because the loads ideal for the tubes would be ALL between 3k5 and 12k0. Use of RLa-a loads less than 3k5 could overheat anodes and or screens. 6550 :- Up to 122 Watts at 1k8, and all power below 110W and for loads above 2k5, PO was almost identical to KT120 for the same load values. But the 6550 had the least THD. KT88 :- Sovtek, coke bottle small type. PO max = 136 Watts at same RLa-a of 1k5. then PO was the the same as 6550 and KT120 for loads above 2k5. In these tests the class A PO max is quite low for loads likely to be used between 3k5 and 12k0. So the operation of any of these tubes to have high Pda at idle to get maximum class A PO does not make much difference to the fidelity. The idle condition is 500V x 50mA = 25W, so total idle Pda = 50W, so max PO class A = 23Watts, with RLa-a = 18k0 approx. As RLa-a rises, I found the THD also rises badly, so even class A tetrode with the high RLa-a is quite awful compared to CFB, Triode and UL, with the CFB best, followed by triode, and then UL. To estimate the amount of class A with any RLa-a *below* the RLa-a for max PO, the class A PO = 0.5 x idle Iadc squared x RLa-a. So with RLa-a = 5k0 where all these tubes produce 70Watts of AB PO, the class A available = 6.25Watts. If Ia was increased to say 65mA, safe for KT120, class A PO increases to 10.6Watts, a small increase. So from my tests, the KT120 gave almost identical performance to the early Sovtek KT88. Minor differences included Ra for KT120 being much lower than for KT88 or 6550, but this does not matter because the Ra is so high anyway. Triode operation. Ea = Eg2 = +500V and Ia+Ig2 = 55mA. The KT88 was tested first and PO max = 43Watts with RLa-a = 2k3. For all RLa-a between 4k5 and 12k0, triode PO max was about 1/2 the tetrode PO max, to be expected. The KT120 was then tested and it gave PO max = 52.5 Watts at 2k0. Above 2k0, the difference between KT88 triode PO and KT120 triode PO diminished to less than a Watt by RLa-a = 18k0, all to be expected. The triode PO is limited by the Ra curve for Eg=0V, and KT120 has a lower triode Ra than KT88 triode. At the idle condition around which an amp will be used, triode Ra = 1k4 for KT88 and 1k1 for KT120, so that the Ro of an amp with KT120 will be marginally lower than with KT88 ( or 6550 ). I tried the KT120 with Ea = Eg2 = 400V, and Ia = 100mA, for idle Pda = 40W per tube. PO max = 32 Watts at RLa-a = 2k3. But Ra dropped to 834 ohms at the idle point. Triode µ = 6.76, so gm at the 100mA point = 8.1mA/V. With all such tubes, the gm quoted is usually the highest that maybe easily obtained, often by operating the tube at say Ea = 250V and Ia = 140mA, Pda = 35W for a KT88. Then gm might reach 11mA/V, and probably gm for KT120 is a bit higher than for KT88. For where Ia = 50mA, and Ea higher and Pda lower, gm is MUCH less than the quoted amount, and Ra usually MUCH higher than the data figures say. The gm is easily measured by altering the fixed grid bias one volt each side of the idle value, and gm = Ia change / 2V. At 50mA, gm is between 4 and 5 mA/ V for all these tubes. Ultralinear. With 37.5% screen taps, and Ea = Eg2 = 550Vdc, Ia+Ig2 = 55mA. I only tested KT120. Its been well known since 1957 than under the nominated op conditions, KT88 make 100Watts safely. This implies anode efficiency = 60%, so power input from psu is 166W, so Pda total = 66W, or 33W per KT88, and below the 42W rating. But not much has to go wrong and overheating can occur. The KT120 gave me 115Watts at RLa-a = 3k5. This is 20W less than tetrode max PO with Ea at 500V. But it matters not, because between 3k0 and 6k0, PO is an average of 20W more than the tetrode case with lower Ea. The OPT I have used may have much lower winding losses than the one tried in 1957. Sec Rw is negligible, and Rw P = only 60 ohms. For hi-fi, the UL gives extraordinary PO, and very much like with 20% CFB, except that the CFB gives far lower effective Ra of about 400 ohms. UL gives Ra = 3,330 ohms at the idle condition, so that as usual the Rout of the amp becomes similar to the RL without additional loop NFB. Distortion with UL was much lower than for tetrode, and I suspect proabably lower than triode, but never lower than using CFB. From all this, my KT120 recommendations for hi-fi would be as follows :- Pure AB1 Tetrode. Don't use it. Its for headbangers who like noise. CFB and 37% to 50% UL, or triode, good with Ea = 450V, Ia = 77mA, RLa- a = 5k0, Ea = 500V, RLa-a = 6k0, Ia = 60mA, Ea = 550V, RLa-a = 7k0, Ia = 50mA. For pure class A maximum, one should use much lower Ea of +400V or lower, and then it becomes safe to use Pda = 67% of the Pda rating, so 40W with KT120. Patrick Turner. |
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