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In Reply to: RE: Triode outputs on a Futterman... posted by Lew on February 23, 2015 at 14:45:18
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 .
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"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 02/23/15Follow Ups:
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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