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작성일 : 05-10-30 22:03
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[막가표 발판 시리즈(2) 2A3/45] Glow Ultrapath
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글쓴이 :
막
 조회 : 1,377
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울트리패스시 콘 안쓰고 정전압관으로 해결한 똘이 이야기 -- no C 의 개념과 유사. 구슬은 콘의 성질을 가지고 있다.
Posted by Mark Kelly ( A ) on May 26, 2002 at 21:20:37
After reading Lynn Olsen’s stuff about “glow tube” voltage regulators I was thinking about changing the active feedback shunt regulator on my 71A preamp. A while back I exchanged posts with Kurt and a couple of others regarding what I refer to as the split ultrapath cathode bypass – C1 cathode to B+ and mu x C1 cathode to ground. I posted my ideas about why split ultrapath doesn’t work as well as it could above.
It occurred to me that the AC impedance of a VR is so low – of the order of 80 ohms for the 0D3 - that a useful analog of the split ultrapath connection could be made by connecting a VR from cathode to B+ and putting a resistance of 1/ mu times the AC impedance of the VR from cathode to ground.
Taking my 71A as an example, a 0D3 from the B+ connection to the cathode gives about 145 Volts plate to cathode. A 27R resistance from the cathode to ground gives 1/mu times the effective impedance so the voltage division ratio is 1 / (mu +1). This can be used as is with fixed bias on the grid or some arrangement can be made to give cathode bias with the total effective impedance to ground equal to 27 ohms.
I’m in the process of building this, I will post again when it’s finished and tested – I’m away from the bench for two weeks so don’t hold your breath.
As it says above this can be used with fixed bias, unlike the split capacitor ultrapath. The range of cathode to plate voltages possible is limited to available VRs and series sums of these, but that’s just about anything from 75 Volts up in 15 volt steps. As is it can only be used with “high Z” V/Ts like the 71A, 45 and 10Y as VRs don’t like being connected in parallel. For higher current applications however you could use a VR version of an amplified zener circuit – say a series of 5651s from B+ to the grid of a power pentode such as an EL34 or KT88 acting as shunt element – yes I know this is a shunt feedback amplifier but the level of feedback is relatively low.
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