#16
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With +60/-60 rail voltages I would feel a hell of a lot better about such a topology. The one 6AS7G amp. I'm familiar with uses +140/-140 volt rails. That makes me nervous and for good reason. It also calls for a potentially lethal hot chassis design. No power transformer, no output transformer, hell, no inductors at all! A bit scary to have something like that plugged right into the wall... |
#17
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The output's purpose has usually been more focused on efficiently coupling the needed power to the speaker instead of raw signal gain....If it weren't then there would be no preamp/driver stage tubes in all the amps made, and all one would have is outputs.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#18
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OTLs are IMO the best of both worlds in amps; combining and balancing the redeeming traits of the characteristic sounds of tube and SS gear.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#19
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Maybe this has something to do with it - source: 1955 Radio Shack Catalog
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#20
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Fair enough. It wasn't a cheap tube so in the case of damper service it was the bean counters that won out. But, for a push pull amp, you only need 1 6AS7G vs 2 6L6s. The cost in tubes is still comparable.
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Audiokarma |
#21
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Fair enough, but a pair of 6L6 was usually used in 20-30W amps, for the 10W class there was the 6V6.
And triodes are hard to drive, needing more stages. Triodes in general lost favor over time, even the 2A3 is found in hardly any hifi era products. |
#22
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I'm not going to argue the merits of the triode vs pentode/beam power tube. That is an ongoing battle that has been beaten to death from the dawn of Hi-Fi. I think it is sufficient to say that if the demand of triode output stages didn't exist, hundreds of thousands of Williamsons, with their triode strapped KT66s, 807s, and 6L6s wouldn't have been built. You wouldn't have had articles in Audio Engineering magazine extolling the virtues of the triode strapped 6550s circa 1954. I don't fall into either camp. I think you can build an excellent sounding amp with a pair of 6V6s and a pile of garbage with a pair of 300Bs. The circuit matters far more than the output device of choice. Sure the 6V6 could do 10 watts in a push pull pair... at 5% THD, with a shitty damping factor, and virtually requiring some sort of negative feedback. A push pull pair of 6L6s could get you ten cleaner watts without much effort. A 6AS7G or pair of 6B4Gs or 6A5Gs could have done even better yet, and with less effort... There's a reason Paul Klipsch demonstrated his Klipschorn in the early years with a single Brook 12A and not a push pull 6F6 or 6V6 amplifier. It is very easy to build an excellent sounding push pull triode amp. It is much harder to achieve the same level of quality with pentodes/beam power tubes. |
#23
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Ok true, but the culture of the time was to use as much negative feedback as would make the amplifier unstable, and then back it off a bit. Given this sort of treatment, beam power tubes formed the basis of high fidelity amps from the early 50s until the end of the tube era. Triodes were still around but no longer mainstream territory.
The triode revival of the 90s was another story. I do remember in the early 90s hifi world made a stereo amp with a 6080 (industrial 6AS7). There have been others too, and also the OTLs. I run single ended 2A3 in one system, and push pull 807 in another. I like em both |
#24
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I'm a fan of negative feedback myself, and built it into my OTL...It is not needed at lower volumes, but if I want to crank it to 11 the feedback adds some headroom before distortion.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#25
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I would say that post-1954, neither the straight beam power tube arrangement nor triode strapping was terribly popular in comparison to the distributed loading arrangement popularized by Hafler and Keros. No matter what camp someone is in, commit to one and don't go the distributed loading route. It is quite simply a mixture of the worst characteristics of triodes with the worst characteristics of the beam power tube/pentode. |
Audiokarma |
#26
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The main advantage of ultralinear, so far as I can tell, aside from damping factor, is that you can use a screen voltage the same as the plate voltage, and avoid complexity in the power supply. Beam power tubes really work better when Vg2<Vp, but this of course asks for a more complex and expensive power supply, regulated screens being best.
I don't really like it either, but it does offer a lot of value for money, in terms of performance versus cost and complexity. |
#27
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You're 100% right, the UL connection was a cheap way to handle the problem of the screen grids. It also ignores the fact that a great number of output valves simply can't have the screen at the same voltage at the plate. About the craziest example I can think of is the 6146. 200 volts max on the screen, and 600 or 750 volts IIRC on the plate. Yikes! Good for 100 watts, so the entire neighbourhood can hear what you're listening to |
#28
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There's a certain segment of the audiophoolery crowd that eschews feedback of any kind. I prefer to handle it in smaller local loops when possible only for reasons of absolute stability into any load, but still, anything that lowers distortion and widens bandwidth at the expense of gain, which is not exactly hard to come by, is a win-win in my book.
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#29
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If you spend a lot of time playing with the curves, even the usual audio tubes benefit greatly from a lower screen voltage. And since pentode gain is largely determined by screen voltage, regulation pays big dividends.
My empirical experience supports your conclusion that pure pentode or triode is better than ultralinear. As for negative feedback, it's a tool. It obviously is a very useful tool, but like any other tool it can be mis-applied. I think lots of approaches to tube amp design have merit. |
#30
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Here's the article I was looking for from Williamson:
http://www.keith-snook.info/wireless...perlatives.pdf Cathode feedback windings are great, IF you can drive the output stage. In the case of the typical triode or triode strapped pentode/beam power tube, cathode feedback windings just won't work. In the case of the pentode/beam power tube, this is certainly an approach that deserves more exploration. But again, it comes at a cost, higher driving voltage at the grids and a much more expensive and unusual OPT. |
Audiokarma |
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