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In Reply to: Bogen HO50 1949 AB2 vintage amp posted by ivan_terrible on October 19, 2020 at 00:00:46:
I have to assume you mean the Stancor Williamson amplifier CH-2133 rated at 10W??
These were sold as kits with the A-8054 OPT, and the 807 screen grids tied to the anodes in an attempt to imitate the PX25 triode.There were major problems with Williamson amps, because without a decent transformer they turn into great RF oscillators.
As the British knew, they turned to Partridge for a proper interleaved winding arrangement.In reality the KT66 with higher screen grid ratings is better than the 807, which is why DTN W used it.
This triode curve tracer graph is for the KT8C, which is, an exact (war time) forerunner of the KT66,- a failed transmitter valve.
Stancor made a Stancor A-8072 "ultralinear version" which is an anachronism. 6L6/807s and the like with their aligned grids don't basically work with "ultralinear" because the screen grids have inadequate gain.
Probably would work a lot better with the EL37, forerunner of the EL34.Apparently Stancor, UTC with their LS series and Peerless were about the only people capable of making decent interleaved OPTs.
The Bogen amp seems to be pretty impervious to HF oscillation,I suspect because the OPT is well made and because it's AB2 is HALF the usual A-A load. (ie. low impedance).
I never checked the KT8C to see if you could run the screen grids at KT66 voltages, but the only data I have is stating 300V MAX, although with G1 +10V they make 350m/a Ia at 100V, +20V quotes 430m/a.
RCA is quoting much higher at about 0.5A from the 807 at those voltages, but I can't see how it can do it with such a small cathode.
(KT8C has much higher perveance.)I dismantled one and measured it.
Here it is:-The cathodes are huge in the KT8C, but the wire failure rate particularly in the wires that went down into the (ceramic) base to my mind were totally unaacceptable.
Then one guy sold me one "so called" NOS which in fact was a complete pile of rubbish burnt out junk...
That put me off them for good.Cathode designs took huge strides forward as TV horizontal scans and military needs demanded them.
My little mil spec pentodes designed in 1955 for nuclear bomber radios & missiles (& still made in 1970-73) have cathodes 30% larger than the 2nd World war 807...but a package 1/20th the size.
Edits: 10/20/20 10/20/20 10/20/20
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- RE: Bogen HO50 1949 AB2 vintage amp - ivan_terrible 00:09:45 10/20/20 (0)
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