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Old 24th June 2005, 12:12 PM   #1
docali is offline docali  Germany
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Default 6H30 phono with passive RIAA

Hi,

i would like to ask the community about suggestions to build an tube phono preamp using exclusively the highly praised russian 6H30 tube. Some commercial designers seems to having changed their designs from ECC88/6DJ8 to 6H30. But the gain of 6H30 is only 15 compared to 33 of 6DJ8.

We need at least 40dB of gain (60dB minus 20dB RIAA attenuation). I think this precludes a two stage design. A possible circuit may be a CCDGC (constand current draw grounded cathode) three stage design with split RIAA. What do you think?
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Old 24th June 2005, 12:23 PM   #2
SY is offline SY  United States
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What's the advantage of the 6H30? From the data I've seen, it's a current hog with unexceptional transconductance; ECC88-oids do much better. And mu is pretty low for a phono stage first hole.
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Old 24th June 2005, 12:36 PM   #3
docali is offline docali  Germany
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Ok, this is a citation from BAT:
http://www.balanced.com/products/lin...0SE/6H30P.html

"The trial results surpassed all our expectations. The 6H30P proved to be better than the 6922 in virtually every respect - whether in its measured performance, or more importantly, in the quality of the resulting sound. From that moment on, our goal became clear - to obtain this new SuperTube and bring it to the West for use in our flagship products. "
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Old 24th June 2005, 12:43 PM   #4
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I agree with SY here (as usual). The first tube in an RIAA stage really should have both high transconductance and relatively high mu (some call this desirable combination a high “figure of merit” tube). The high gm gives good noise performance in the first stage and the high mu also improves overall S/N by raising the signal higher above the noise sources in subsequent stages. Also the high mu gives you a good head start on the needed gain for RIAA, particularly when passive equalization is used. The good old 6DJ8 is fine, but real thoroughbreds for this application include the 417/5842, 6C45, and the triode-connected 7788 (my favorite right now). If you find a 437A or an EC8020, those would be great, although I’d sell them on eBay, use a 7788, and buy more LPs with the profits
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Old 24th June 2005, 12:43 PM   #5
SY is offline SY  United States
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Quote:
Ok, this is a citation from BAT:
Well, that certainly trumps a technical analysis.
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Old 24th June 2005, 12:47 PM   #6
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Brian: From what I see, the tube's major virtue is a low plate resistance. That's useless in an RIAA stage since the plate resistance will presumably be padded upward for the RIAA equalization...

I certainly have no specific knowledge, but I'm willing to bet that BAT got a pile of them on the cheap.
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Old 24th June 2005, 12:53 PM   #7
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Marketing hyperbole from BAT aside, the 6H30 is probably an excellent tube for many other applications, such as for line stages, power amp inputs/phase splitters, etc. I have not used it yet, but I don't doubt its merits.

But one tube does not fit all applications, and it's just not the best choice for the critical first stage of an RIAA amp with a mu of only 15 (which is too low here ) and a gm of 18mS, IIRC (which is just OK here). The equivalent noise resistance of a triode is Req=2.5/gm. Hard to get around that little rule.
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Old 24th June 2005, 12:56 PM   #8
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gm of 18mS, IIRC (which is just OK here)
Not even that good. Steve Bench reports the gm at 20mA is something like 12mS. At 10mA, it's under 8mS.
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Old 24th June 2005, 01:02 PM   #9
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Ughh. As you say, Victor Khomenko, whose products I do respect, probably got a pallet-load of these tubes cheap. Audio Research seems to like them too (but not in RIAA stages) .
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Old 24th June 2005, 01:16 PM   #10
docali is offline docali  Germany
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To summarize your opnions, the ECC88/6DJ8/6922 is a better choice for the first gain stage.
Audio Note uses 6072 tube in their RIAA stage, it has a much higher plate resistance and a lower transconductance than 6DJ8. What do you think of 6072?
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