On the diagram I am looking at he has a FET input stage/6SN7 pre and driver stage Driving a RCA-50.
It's a circuit that I have been looking (Just keeps drawing my attention) at for a few years and wondering what all the fuss is about
He also has the modification to drive an 845 to about 4.5Watt per side.
It is the 4 stage topology (Prof. Morikawa)...Apparently the FET has a big effect on the amp (so he says)..The driver stage (cathode follower) is DC coupled to the OP tube..He uses a radio shack automotive choke in the FET supply. I have one from 1970..LOL so it might be a way to use it...
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Regards the OTL I use it most of the time with the Mozener mod works great!
I just want to know what is it thats so special with the 50..Or is it a damp squib..LOL
Regards
M. Gregg
That sounds interesting..
Do you have the web address?
Perhaps he would post the Schematic in this thread just for interest..
Regards
M. Gregg
hey there, the original "dinosaur" was a fun evolution of the loftin white DC concept, where the bias for the various stages could be somewhat compensated for by the bias of other stages... i don't remember exactly what i did. but i got DC for the FET from the cathode resistor of the output tube. it is related to the "professor morikawa" circuit, but that isn't unique or important. the main thing i learned from that circuit was that a cathode follower solves most of the problems a large output triode creates... especially when the swing necessary to drive them gets above 80 volts peak to peak. cathode followers had gotten a bad name in those days, for various reasons, some real and some fashion. mostly fashion. power triodes are not so easy to drive. they have a variable input impedance that changes with level, and a large grid to plate capacitance. the 50 tube is more so, than many other tubes (more than a WE 300B, but often less than a modern remake). the idea of using an RC coupled driving stage, with a large grid resistor and a high impedance driving stage (all the rage at the time because everyone wanted to make a one or two stage single ended power amp), had bottomed out in my view, because the amps sounded so different at small signal levels than they did, balls out. the highs and bass compressed like crazy and mids got dirty. all amps fall apart when they clip, but this was a "long" oscillatory recovery and slewing... i had begun to really get into testing things. the next obvious step was to drive the output tube with a low impedance driver stage, and to get rid of the cap... the one between the driver and output stage. that led to getting rid of all the caps: an obsessive design feature i have pursued for years.
i should say that others had discovered the same problems, and went the other way: by using interstage transformers to keep the impedance more optimum. i did it too, but the quality of the interstage transformer became an issue. in the mid 80's, all you had was the tango NC16 (i believe that was the number). it was hard to get in the usa. very little of the recycled american interstage iron was good enough. that was when UTC became "you take a chance". there were some peerless parts that were amazing, but it was slim pickings for iron, in those days. it had to have vastly superior bandwidth than the output transformer did, or the two could combine in a destructive way. there were many who didn't care... or, even liked the sound. i accept that. but it wasn't my taste or interest. i was looking to exceed the "high end" hifi sound, not go nostalgic.
the dinosaur was a fun stepping stone to things that came later on. if you look at my blog, there is some discussion of the ideas. "some interesting DC circuits" and other posts. hope this helps?
i should say that others had discovered the same problems, and went the other way: by using interstage transformers to keep the impedance more optimum. i did it too, but the quality of the interstage transformer became an issue. in the mid 80's, all you had was the tango NC16 (i believe that was the number). it was hard to get in the usa. very little of the recycled american interstage iron was good enough. that was when UTC became "you take a chance". there were some peerless parts that were amazing, but it was slim pickings for iron, in those days. it had to have vastly superior bandwidth than the output transformer did, or the two could combine in a destructive way. there were many who didn't care... or, even liked the sound. i accept that. but it wasn't my taste or interest. i was looking to exceed the "high end" hifi sound, not go nostalgic.
the dinosaur was a fun stepping stone to things that came later on. if you look at my blog, there is some discussion of the ideas. "some interesting DC circuits" and other posts. hope this helps?
well, i have used the 211 types more than any other large transmitting types. 845's had already become scarce in the 90's. for me, the tube type is not the interesting thing... it's the end result. they're all good, and on any given day, any one of them might become a favorite. 211's need less swing, but it is not true to say that they are easier to drive. grid current changes are large from quiescent to full swing and make a high performance from a high Z driver (12AX7 or cascode/pentode) not possible. that circuit will work fine with 211s. are there better ways? sure. that is a good simple way to do it, though.
Not sure if you noticed my last post..
Is it Ok to post the Dinosaur circuit from sound practices mag or is it copywrite protected?
Its only so everyone knows what is being discussed..
Regards
M. Gregg
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