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In Reply to: RE: Amazing JJ 2A3-40's posted by Palustris on March 12, 2015 at 18:18:22
I will grant that there is a slightly different spacing of the grid. Everything else is pretty similar. I hope I did not offend you.
Follow Ups:
Sophistry is not something I have associated with Chip - having strong opinions and being forthright maybe, but not sophistry. But hey, maybe that is his sophisticated plan ;^)
I vaguely recall JJ stating the plates were that same between the and 2A3... but my memory is not what it used to be.
Also, those graphs use different scale, so the differences look more pronounced than they actually are... but yes, there are some differences.
Cheers.
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki
Well, it was not my intention to offend anyone either, but sophistry is such a fun and cheerful word, I couldn't help but employ it under the circumstance. I must admit to a certain frustration when it comes to the frequent apocryphal assertions regarding the JJ 2A3-40 and the JJ 300B and, if I were to be completely candid, the inexorable assault on our shoreline, only one hundred feet from our home, by acre sized mats of Sargassum seaweed have infused every pore of my being with a less than fruity fragrance and my mind with less than salubrious intentions.
Casting macro algae aside for the moment, I can't fail to notice that I have been blessed by the winds of fate with both the JJ 2A3-40 and the JJ 300B in my arsenal of thermionic devices and find that each of them conforms to the expected operating conditions implicit to the archetype. Consulting the charts so thoughtfully provided by Mr. Trey, it should be clear to the naked eye, without resorting to the benefit of load lines, that the JJ 2A3-40 biased with an Eg = -45V and Ep = 250V will pass 60mA of plate current. If this does not conform to the expected operating conditions of a 2A3 I will chow down on an acre of Sargassum. It should be noted that under the same operating conditions e.g., with a bias of Eg = -45V and Ep = 250V the JJ 300B will pass 80mA of plate current. The implications of "new math" not withstanding, I submit that there is a 20mA difference in the operating conditions of the respective tubes under equal bias and plate voltages. That would appear to this casual observer to be significant
Looked at another way, at 250v/60mA the necessary bias resistor is 44v/60mA=733 ohms, which the 300B needs 47v.60mA=783 ohms. The difference is only 7%. At the time these were introduced, a 10% resistor was about as tight a spec as you could readily get.
Yes the nominal "bogey" 300B has a mu of 3.9, vs. 4.2 for the 2A3 - a 7.7% difference. The difference is smaller than individual tube variations. Transconductance off the spec sheets is 5300 vs. 5250, only 1% different, though it will typically drift 30% lower over the tube's operating lifetime.
The 300B and 2A3 are often interchanged in amps. TJ even did the 2.5V filament 300B, for use in 2A3 amps... and some folks even like it ;^) Others however preferred "native" 2A3s, or at least something more 2A3-like.The differences between the graphs could be (well) within natural production variation / tolerances for any given tube. With no detail provided about the graphs - like do they represent some kind of measured "average" or centered target value - you can't deduce much from them.
I have not tried it myself, but others have reported the JJ 300B and 2A3 versions as sounding very similar (or same) when used in the same amps (with filament voltages adjusted accordingly). Of course, others will have different and equally opinions.
Irrespective, I have found the JJ 2A3 to be a decent, uber-dynamic sounding tube that represents solid value, providing they are reliable. I am looking forward to trying them in a more optimised amp.
Edit: in a branch below, details are provided of the 2A3 and 300B JJ versions biasing up very similarly in an amp - certainly well within production variation.
Cheers.
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki
Edits: 03/14/15
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