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Old 12th March 2009, 07:46 PM   #11
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Triode-strapping the pentode section may be an idea.
According to Tim,
"After some testing, I discovered that it's (6X8) really quite reasonable, its apparent similarity to the 6J6 being no accident. In fact, with the pentode section wired as triode, it may well be identical to a 6J6."

From:
http://webpages.charter.net/dawill/t...Compound2.html

Just a pre-coffee thought.
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Old 12th March 2009, 08:56 PM   #12
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Quote:
Originally posted by Miles Prower


Not necessarily. You never know until you try. The spec sheet for 6BQ7s make no mention of any sort of audio useage at all. Yet, when used to make a cascoded LTP with active tail load, these really rawk. Click the image to open in full size. More gain than the usual solutions (12AX7, 6SL7) none of that pentode nastiness problem mentioned with pentode LTPs. These, too, are cheap since they have no audiophool pedigree. I also discovered that the Sylvania and Motorola versions with the parallel connected heaters aren't microphonic at all. (Different story with the ones with series connected heaters.)

Besides, the fat cathodes score extra glowey bottle coolness points.

6BQ6GTBs also are "non-specialized" with no mention than anything other than TV horizontal deflection duty. Sounds as good as PP 6V6s, but with better than twice the output power. Given the need for pulling bigamps through the deflection coils, these require less impedance transformation, so that the OPTs are easier to design, and work better. Also, cheap with no audiophool pedigree. (But they've been going up lately. I guess the good folks at the VT store figured something was up when I bought a whole bunch at $1.30 a pop.)
Yes Miles, I just told it is typical and not systematic.
Then the tolerable noise or microphonic level can be good for you but not for someone else...etc etc...
In my system, jfet's, bjt's and some op-amps simply work better for low level signal.

45
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Old 12th March 2009, 10:18 PM   #13
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Quote:
Originally posted by Miles Prower


Yep. I can use 'em.

Email me a wishlist. I'll see what kind of box I can throw together for you

Cheers!
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Old 13th March 2009, 01:38 AM   #14
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Quote:
IMO, the 5755 as input tube of a MM phono stage can really stand out.
Yeah, I saved the ziplock bag full of those. They all have pretty yellow printing on them too.
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Old 13th March 2009, 01:48 AM   #15
Salas is offline Salas  Greece
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This one also behaves really well as input tube for phono.
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Old 13th March 2009, 02:33 AM   #16
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Quote:
Originally posted by salas
This one also behaves really well as input tube for phono.

The Russian 6Н2П (6n2p) pins out like a 6992, but the triodes are the "same" as those in a 12AX7. Therefore, some MM carts. and LOMC SUTs will experience an adverse interaction with the substantial Miller capacitance.
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Old 13th March 2009, 02:48 AM   #17
Salas is offline Salas  Greece
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It works OK with really low noise floor sans shield cap and zero microphonics even when knocked on with finger nail. I get 60dB from cart in to line out using a Lundhal 9206 set at 10X, this Ruskie, passive RIAA, and 6N8. 2 Mu followers like in the Dude. No weird interactions. Riaa curve is the same with or without the step up.
Will show that phono here soon.
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Old 13th March 2009, 06:06 AM   #18
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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.
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Old 13th March 2009, 04:40 PM   #19
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The 6X8 would probably make a good single tube exciter/modulator for a Part 15 AM transmitter.
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Old 13th March 2009, 04:52 PM   #20
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^^

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