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In Reply to: Re: BDT preamp details posted by nubz69 on July 10, 2004 at 11:02:38:
I agree completely!
This looks like a fun simple and elegant design.Will
Follow Ups:
JohnI haven't communicated since the Austin Bottlehead meet, but a friend said you had an interesting new preamp design up.
I looked.
It really is "something new under the sun". It looks like everything swings in this design. The pentode current sources the pseudo triode pair, which really has the grids in parallel with the current, instead of in series. I'm not enough of a mathemajician to suss out all the implications, but intuitively it is the equivalent of "massless". It is not resisting any particle in motion, merely directing the stream. It looks like something that would have a limited ideal range of voltage swings, which would change with all of the plate load and current conditions. How much minimum and maximum current does this come out to be?
I went through all 3 of my RCA manuals and only found this noted as a discontinued type in the last one. A flash in the pan!
One of my Sylvania manuals has a page, but no graphs. This lists the Miller capacitance of the deflectors as 4.8 pF, pretty durn close to Zero...
It looks like a push-pull choke on the plates of this tube would make for a good differential device, and it would allow the use of a variable cathode resistor as a volume control, while keeping the plate voltages pretty constant. That cathode resistor would be bypassed with a large value cap, of course. This should get rid of noise coming in through the cathode circuit. Cut-off is at -14V for grid #1 as listed. You would know what your actual grid#1 voltage and current conditions are for practical use.It seems that this tube might be pretty linear over a wide range as I look at the specs. The "peak deflector #1 and #2 voltage" is +/- 150V. It looks like they weren't trying for much gain out of this, but a clean, cheap phase inverter with a large voltage swing. It could be a very nice input tube for a push-pull power amp, or even a driver for the outputs, direct coupled downstream from a voltage gain tube. I'm not sure how to calculate the equivalent plate resistance, but the Grid#1 transconductance @ 10 mA and 250V on the plates is listed as 4000 micromho. This is similar to a 12AT7, 5965, or 6DJ8 under similar conditions (250V and 5mA per plate).
Nice to see that beautiful (and cheap) mind of yours at work again!
John Day
Austin, Tx.
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