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In Reply to: RE: DHT "El Cheapo" posted by Eli Duttman on March 12, 2017 at 20:27:20
Did you look at 46 prices lately?
Re. transformer, why do you need a custom one? Do you need special NFB winding(s)? Custom won't be cheapo.
Any insight why Class B?
I already have the quartet of #46s on hand. :> D I've got nice sockets too. ;> )
I have a custom filament trafo on order from Heyboer, which is not particularly expensive. The main power trafo is a stock AnTek toroid, which (again) is not a "big ticket" item and already on hand.
The PP O/P "iron" needed is simple. 3.5 Kohms end to end, without UL taps. I'm going to go for at least 35 W. power handling. An "El Cheapo" uses "textbook" GNFB that includes the O/P trafo and that requires power handling capability above the power O/P. Core saturation due to a LF error correction signal SUCKS. Stability is pretty much a non-issue, as the only caps. in the signal path are between the LTP and driver FETs.
As for Class "B2", the #46 contains 2 grids, but is always used as a triode. Tie g2 to the plate and the #46 is much like the #45. OTOH, tie g1 and g2 together and the tube is nearly cut off, with 0 VDC of bias. Look at the data sheet. A near Class "B2" operating point ( slightly positive bias) to tame notching/crossover trouble is pretty (IMO) obvious, with 15+ WPC power O/P not being a stretch in the slightest.
Power rating of 20 W is very conservative. 3K and 5K primary. Has a variety of secondaries, which is good for tweaking NFB. May seem pricey, but it sold for ~200 in today's dollars back then. It is highly interleaved, highly symmetrical, and has exceptional bandwidth. Cheap OPT may cause stability problems when inside NFB loop.
BTW, I am hunting down 46s for use as drivers for 1624 triode-connected.
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