MAC Autoformers?
ramtubes What I’ve been saying all along, they are a band-aid fix for amplifiers that are "not capable" of doing the job properly without them. They are also "not able" to make an amp that "is very capable" without them sound better. If anything they make them sound worse. Cheers George |
@ramtubes --- Roger, thanks for responding. Yes, ... there is only one bias pot to control the bias of a pair of KT-150s. The bias pot is used to adjust the bias in mVs of the set tube. ARC recommends 65mVs. The other tube in the pair is a slave. If the tubes are well matched, the slave will measure 65 mVs, plus or minus 3 or 4 mVs. No, … I do not have a schematic and I do not know if the drivers are direct coupled to the output tubes. As you probably gleaned from the ARCDB website, each pair of KT-150s are driven by one 6H30 driver tubes; 4 in total. Does the following ARCDB excerpt give you a clue about your direct coupling question? The ARCDB website states that the " power-supply energy storage has been doubled to some 1040 joules. All-new interstage coupling capacitors using technology and materials first incorporated in our Anniversary Edition preamplifier effortlessly link input stage to output stage, which is powered by two matched quads of Russian KT120 [KT-150s in the SE version] output tubes driven by 6H30 twin triodes. There is more than ample multistage solid-state regulation. Output stage coupling is a combination of “ultralinear” and Audio Research’s patented 'partially cathode-coupled' topology, which is superior to conventional pentode or triode operation." The New Sensor website reports that the KT-150 has a maximum plate dissipation of 70 watts. See https://www.newsensor.com/pdf/tungsol/kt150-tungsol.pdf I seem to recall reading that ARC runs the KT-150 at half that max amount. The New Sensor website says that the KT-150 "CARRIES A 70 WATT PLATE DISSIPATION RATING WHICH PROVIDES FOR PUSH-PULL AMPLIFIER DESIGNS UP TO 200 WATTS OUTPUT." I believe that a quad of KT-150s in the Ref 150SE is configured to produce only 150 watts. So I surmise that ARC chose the tube because it sounds good and is pretty robust. And ARC is not pushing the KT-150s to the max Q limit. As far as sound is concerned, I ran my amp with KT-120s and KT-150s. IMO, there is a world of difference in terms of sonic saturation and a sense of bass extension and control. In plain terms, the KT-150 is a great sounding tube, but expensive. Your comment about tap selection is interesting. In a post directed to Ralph Karsten I mentioned that my speaker are nominally rated at 8 ohms, but that is a misnomer. The impedance and phase angle curves are rock and roll. See https://hometheaterhifi.com/reviews/speaker/surround-sound-speaker-systems-reviews/a-secrets-speaker... In particular, impedance drops below 8 ohms between 65 and 700 Hz and below 4 ohms between 70 and 150 Hz. There is a big impedance peak between 600 and 6000 Hz. The good news is my speakers are rated at 92 db sensitivity. Based on a Stereophile article done on the Ref 150 some years back, Atkinson suggested that one try the 4 ohm taps for the reasons you suggested above. Weird thing is that I have gone back and forth between the 4 and 8 ohm taps any times and I keep settling on the 8 ohm taps. Distortion and damping factors considerations aside, the 8 ohms taps sound better, even though I sense the amp is adding a little not too unpleasant flavor to the presentation. And yes, …. please keep me in mind that if you decide to expand your tube repertoire to include KT-150s. I sense it is the power tube de jure for many manufacturers and as the KT-150s age, the need for replacements will make a market for replacement tubes. Care to guess what the useful life of the KT-150 is in my amp given what I posted above? Is there any way to measure tube life and quality with a tester? Thanks and I love your YouTube presentation. BIF |
Roger, I'll ask the following for myself, but your answer may well be of interest to others. You rate your RM-200 Mk.2 amp at about 100w/ch at 8 ohms, and, unlike a "normal" tube amp, about the same at 4 ohms. The advantage of using the 4 ohm tap is less distortion, lower output impedance (higher damping factor), and longer tube life. With a tube amp which produces less power at 4 ohms than at 8 (typically half as much), the user forfeits power to gain the advantages afforded by using the 4 ohm tap ("light loading"). Since the RM-200 produces about the same power on both taps, what possible reason is there for NOT hooking up a nominal 8 ohm speaker to the 4 ohm tap? Thanks---Eric. |
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