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發表於 2013-7-22 09:53
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Wilmer威馬 發表於 2013-7-22 00:26 
可惜 MP-501 講唔到比人和 KT120 同 KT88 究竟有咩分別，因為 MP-501 莫講話 KT88，連 KT90、KT100 都唔 ...
"It is safe to use Tung Sol KT 120 in ALL Prima Luna amplifiers". Prima Luna 的Dialogue 5 係 auto bias, 本身以經可用EL34, KT88, KT90 及6650整 係列互用, 正如前文講, 最主要看POWER SUPPLE 可否乘受比KT88 多少許發熱電量, 而用120的好處是 UltraLinear 的class A 輸出會多咗, 更線性. 有人用Electro Harmonix 胆作 loadline 分析, 得出以卞結註論.
Real benefits of KT120 would be most possible for where LOW power is desired for hi-fi
and where clipping will never occur, and the power is nearly all pure class A. The maximum pure
class A possible from any two beam tetrodes or pentodes is about 45% of the total Pda at the
idle condition, so that if two KT120 are used with Pda at a safe 40Watts each, Pda total = 80W
and max class A PO = 36W, and this is substantially above a pair of KT88 with safe Pda total = 60W,
and class A PO max = 27W.
For class A, RLa-a is always higher than for class AB, and maximum peak Ea swing will always be a
high fraction of Ea. Peak Ia swing in the class A amp is never more than twice the idle Ia.
With the class A idle Ia set higher than for class AB amps, the gm at the operating point is both
higher and more constant, resulting in much more linear operation. Screen supply voltages seldom
need to be higher than +300V, and this allows lower g1 bias voltages easily generated using cathode
biasing with R&C networks between cathode and 0V, and without wasting much power in hot running
Rk. The class A tube has high voltage gain because gain = gm x RL (approx) and where local CFB is
used as in the Acoustical mode ( like Quad-II ), remarkably linear operation is gained.
For example, consider KT120, Ea = Eg2 = +400V, Ia = 100mA, and RLa-a = 5,600 ohms.
Peak Va swing = 360V, and PO = 46W, AB1, with the first 28 Watts in pure class A1.
The loadline analysis shows the 60W Pda limit line for KT120 well above the class B load line
of 1,400 ohms, so there is no risk of tubes overheating. Even if the load is reduced to 2,800 ohms,
max AB1 PO = 68W, and there is still no risk of tubes overheating. From this it is possible to use
KT120 at an idle Pda = 40W reliably so that Ia = 100mA, Under such conditions, distortion is
minimized, and music may sound much better than if the amp were set up with Ea at +550V, and
Ia at idle of 50mA, and RLa-a = 2k0, and class A max = 2.45Watts
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發表於 2013-6-24 17:20
