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In Reply to: RE: "Until you compare them to equally good no feedback circuits I can imagine you don't hear the consequences.." posted by Lew on February 23, 2015 at 08:07:14
LewIMHO original older J.Futterman OTL designs which uses triodes but not pentodes in asymetrical SEPP(Single Ended Push Pull) OPS can probably have almost that SE amp sound signature , without of any of my doubt that Futterman amps are really PP amps and also regardless of use of significant amount of GNFB .
But must say that only some THD analyzer probe can reveal real output THD signature , so if THD spectrum consist from mainly second harmonic followed with much lower level of third harmonic and even almost non existand traces of higher order harmonics only than I can say that Futterman amps perfectly mimics SE amplifiers sound .Best Regards !
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"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 02/23/15Follow Ups:
The only such amplifiers I can think of were those old models made by Fourier. One used 6AS7s and the other ("Panthere", I think) used 6C33C, I think. I owned the smaller version for about two weeks, or until I could re-sell it. It was nowhere near in the league with a Futterman H3aa or needless to say an Atma-sphere amplifier. Its sonic failings may have had as much to do with its poor quality of construction, as anything else about it. But so far as I know, Julius never built a triode output stage for any of his various models, of which I owned at least 3, at one time or another. As you probably know, he eventually settled on the 6LF6 as an output tube. He did build a headphone amplifier; maybe that one used triodes.
LewYou are basically right ,
but at that old time (1956) Harvard Electronics Co. NY under license was made J.Futterman OTL( model H1 , 15W ) amps based on 12B4A power triode tubes for AB class SEPP OPS , actually every early Futterman OTL amp was based on that 12B4A power triode , I have few original J.Futterman articles from 1954 & 1956 which clearly show that .
But of course that later he switch to use only TV horizontal sweep power pentodes probably with main intention to even more rise up the AB class OTL amplifier efficiency .
Any way my original point was that Futterman OTL PP amp under few condition can easy reveal typical SE amps sound signature ,or even to sound much better than any typical low power SE amp .
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 02/23/15
Thanks for that information. Even as I was typing my post, I was wondering also about the very early models, built in the early to mid-50s. I have seen only one or two in the flesh, but I did not realize he used the 12B4A in those days. Too bad Fourier made a hash of it in the later years.
I have an old HiFi Guide somewhere from the early 1960s that lists a Futterman OTL that used 6AS7Gs for power tubes. It had distortion specs that were not matched with transistors until a few years later.
LewIt is not surprise for me that Fourier do not have big success by replicating original Futterman amplifier concept .
Since even if that early triode based Futterman OTL amp looks very simple on the paper ( schematic ) basically they are not so simple , since surprisingly they are designed almost as some modern SS PP amps , with very high OLG ~60db ,with~ 35 dB of GNFB , with one extra bootstrap loop around of phase splitter and with final target to represent close as possible pure voltage source for at that time standard 16ohm load(speaker).
Even simple wrong layout of passive and active elements on amp chassis can cause unstable operation for that type of amp, sometime inserting of some extra compensation elements is needed to suppress amp oscilation ...
Any way to make long story short ,original Futterman OTL amp concept can not successfully replicate anyone , pretty big knowledge and experience is needed for that , and even bigger for original Futterman OTL amps based on TV power pentodes.
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 02/24/15
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