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Old 13th March 2009, 10:13 PM   #21
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Quote:
Originally posted by Miles Prower
^^

Who uses AM for Part 15 anymore? CW, at the very least, or digital modulation these days.

Even LowFER's use QRSS60 and computers running all night to record the signals.
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Old 14th March 2009, 12:47 AM   #22
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Quote:
Originally posted by wrenchone
Speaking of the 6J6, I've thought of using one in a passive equalization phono preamp using LED bias at the common cathode, figuring that the low dynamic impedance would more or less decouple the stages from each other. Some more bias current for the LED via a separate JFET or ring-of-two current source would lower the LED impedance some more and help out the isolation between stages. I've got a fair number of 6J6s lying about, so I figure I might as well do something with them. This would probably work with the 6X8, too, though I only have a couple of those.

Dude,

JMO, the 6J6 was "born" to be a LTP phase splitter. I don't know if it's quiet enough for mV. level audio work. The upper 30s mu could be problematic too.

You have mail! Reply and a schematic will head your way.
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Old 14th March 2009, 03:57 AM   #23
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Splitter is not a bad supporting role for what would otherwise be a "plinker".
I'm not convinced that the 6J6 would be a rotter in the phono role. A mu of 38 doesn't disqualify it for me, as I commonly work with a front stage gain of 40X and a back stage gain of 30x for my passively equalized sand state projects. This gets me an overall gain of 40dB, which is very compatible with my system. As always, noise performance will probably vary between manufacturers and individual units. I have a bunch of RCA NOS units, as well as a passel of used tubes of various sorts. The design isn't meant to be the be-all and end-all, just a a stab at using something that would otherwise be auctioned off at my estate sale when I kick it. When I get really serous about doing a phono preamp, I'll probably use some of the 6AM4s and 6HF5s (as well as those 6JK8s) I've been collecting. The 6AM4 looks eccentric enough that I'd want to use it for something. The elements are arranged horizontally, so it looks like a little barrel inside its peanut-sized envelope. Specs aren't too bad, either.

Attached are a couple of back of the envelope schematics for an RIAA preamp and a unity gain buffer/line amp using the 6J6.

For the RIAA stage, the current source loading on the input stage is a kneejerk thing. Cheapskates could probably get away with using a 22k load resistor on that stage, There is enough current going through both tubes so that the bias LED is pushed up into the low impedance region of its characteristic curve, so extra bias current may not be necessary.
I like the MOSFET current sink for the buffer circuit, as I like current source loaded followers. The output cap is sized to operate into a 10k load without a lot of bass sag.
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File Type: gif 6j6_circuits.gif (58.8 KB, 375 views)
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Old 14th March 2009, 04:25 PM   #24
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^^

Why not just use the thing as a differential? Add another triode, wrap a NFB loop around the thing: Hollow state op-amp.
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Old 14th March 2009, 06:30 PM   #25
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The only thing I don't like about the differential approach ( it occurred to me the first time I saw the 6J6) is that there's no assurance the two halves of the tube are well-matched, so there could be a lot of picking and choosing. With the approach I've shown I don't have to care about that.
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Old 16th March 2009, 02:33 AM   #26
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For the sake of accuracy, I meant 6HM5/6HA5 rather than 6HF5 in my previous post. I admit, though, that It'd be neat to see someone persuading a big sweep tube to work as a low level amp. That said, I won't hold my breath waiting for it to happen.
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Old 18th March 2009, 03:18 AM   #27
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Am I "nucking futz"? Assuming grid leak bias with 20 MOhm parts at both 6X8 section control grids, I make a 159 pF. cap. coupling the 47 KOhm cart. load resistor to the pentode as taking care of the 50 Hz. (3180 S.) pole of the RIAA curve.
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Old 18th March 2009, 04:10 AM   #28
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Doubles nicely. But what about the self noise of such a huge resistor? Won't be amplified by first stage, strongly worsening SNR?
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Old 18th March 2009, 05:21 AM   #29
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Originally posted by salas
Doubles nicely. But what about the self noise of such a huge resistor? Won't be amplified by first stage, strongly worsening SNR?

You are correct about caution with those huge grid leak parts. Fortunately, Caddock makes some very quiet parts that meet the need. Mouser stock # 684-MK632V-20M is what I have in mind.
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Old 18th March 2009, 05:39 AM   #30
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Good for ppm, size and sonic fingerprint, but nV/Hz^0.5 isn't still related to nominal resistance and Johnson?
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