Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jojokeo on September 11, 2014, 10:20:53 pm
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JJ's come out with a new "medium gain" 12AX7/ECC83 tube! Wondering if anyone's tried it yet? It says, "This medium plate tube is tight with a lot of presence and articulation in the low mids." Here's a couple of screen shots: (sorry if it's already been discussed prior?)
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Hmmmm..... What would be the advantages of a medium gain 12AX7?
Brad :think1:
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Ummm...
hang on a sec...
gm = 1.6mA/V
ra = 62.5k
mu = 100
not to mention the other tube characteristics that spectacularly fail to distinguish it from any other 12AX7 tube datasheet.
Don't understand why it says 'medium gain'
Does that mean that its a normal 12AX7 that's prone to micro phonics if used in a normal way? but more likely to work longer if only set up for 'medium gain'?
(Is this another one of their advertising gimmicks like their 10W 'EL84'?)
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Total marketing BS... the characterisitc curves are nearly identical to the regular 12AX7s to be within the manufacturing tolerance.
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Assuming JJ isn't just putting different labels on the same tube, what I would take away from this is that the data sheet numbers & curves are meaningless. It all looks like the same data reprinted from every other 12AX7 data sheet that ever existed.
If the tube had higher internal plate resistance compared to a "normal 12AX7" the gain in-circuit would be slightly reduced. This may be a way to sell tubes without rejecting any at the factory, and without getting a reputation for inconsistent product.
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This may be a way to sell tubes without rejecting any ay the factory, and without getting a reputation for inconsistent product.
THAT seems like the best explanation.....low performers get thrown into the "mid gain" bin. :think1:
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why don't they just spin a 5751 or 6072?
agree, marketing crap. maybe they mean the physical size of the plate, as in small, medium, and long plate like GE & sylvanias, and not medium gain.
NOS or no stock for me anyhow.
--pete
EDIT: never mind... read the data sheet - it's crap.
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HBP, nice insightful comment. I should've included a bit more info on where I saw this and about the descriptive comment made. It was from the AES/CED sites in how they put various tubes in an AB763 circuit and make determinations to help explain various tubes & their performances as to how they each relate to each other. So I made a cut & paste of mostly their various JJ tube's ratings, performance, & descriptions and also included a new Mullard (top of the gain list) and down lower a rating for good ol' NOS RCA's etc. for another comparison of their "comparison system".
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After reading the methodology for the testing, I came to two conclusions:
1) The chart is mildly misleading
2) I want to build the "tube switcher" box.....it would be cool to run this kind of test for myself :headbang:
As far as the marketing goes, I really don't have a problem with selling weaker (but still functional) tubes under different branding. It keeps the consistency of the same type tube high and overall prices low (its not like they didn't do that in the golden days, they just called them "selected" tubes and charged more*) they probably should have called it something else though......
*I know that the "selected" tubes were better "quality" hence the price increase
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According to the chart, the "Mid-Gain 12AX7" has more gain than the regular JJ 12AX7...
Only thing you can trust is putting the tube in an amp, applying a fixed & repeatable test signal, and measuring the output.
That said, I wouldn't be against buying some & trying them out. I once bought ~10 or so different-brand or -model 12AX7's just to see if I heard significant differences between new production stuff. I still have most of those hanging around, but don't remember what conclusion I came to and haven't had a need to replace the old production 12AX7's I have.
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It is a "mid gain" tube in the sense that it is a "medium plate" in the vein of the "long plate" tube. So, now they have the "short plate" which is just the normal 12AX7, the "mid plate" (this tube we are discussing) and the "long plate" they've had for a long time.
The chart attached further illustrates this showing that this "mid gain" tube has more gain than the standard "short plate" tube but less gain than the "long plate" tube.
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If the tube had higher internal plate resistance compared to a "normal 12AX7" the gain in-circuit would be slightly reduced. This may be a way to sell tubes without rejecting any at the factory, and without getting a reputation for inconsistent product.
This was my thought, as well. It reminds me of the time Groove Tubes re-branded some 6L6 tubes that didn't make the 6L6 grade as 6V6 tubes (6V6 HD or some such). Their logic was that it gave players another option since there was little being manufactured in the 6V6 realm at that time and NOS had really dried up. Never mind pesky issues like heater draw. JJ should cut the gimmicks and concentrate on manufacturing a tube that rivals those of the device's heyday.
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> the characterisitc curves are nearly identical to the regular 12AX7s to be within the manufacturing tolerance.
Agree. Manufacturing and *measuring* tolerance. See attached. GE 1953 versus this MG.
We know the old data is a limited number of points (all hand-measured, and a junior engineer's time is worth something), then smoothed over with a French Curve (ask your daddy). The new data is probably machine-taken, and could be very detailed, but the MG curves show kinks which are probably insufficient points dotted together by a graph program which does not know what the curve should be.
One "difference" is the zero-grid curve around 25V and 0.5mA. The GE line rises quite abruptly. This has always been suspicious.... electrons aren't that easy to push around. And in 300V tube amps we never get into this area.
Another is the spacing near say 250V and 0.2mA. This would be where a Fender-tradition amp stage "compresses". The MG appears to compress more, but the kinkiness suggests they just didn't plot enough points in the 0.1mA area.
The thin blue line is a typical 300V 100K load-line. Along most of this line the match is exact.
And IMHO both could come from the same production line, even in the same bottle. You wind super-fine wire around two rods, you never get two grids exactly alike. Or two cathodes with the same surface activity. Plate spacing depends on stamped mica and mica hates to be stamped.
And if it truly IS "medium gain", that could be a way to market tubes which fall on the low end of the tolerance range.