|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
76.90.39.24
In Reply to: RE: Amazing JJ 2A3-40's posted by Chip647 on March 12, 2015 at 06:39:17
According to the plate curves on the data sheets they're not the same.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
"According to the plate curves on the data sheets they're (the JJ 300B & the JJ 2A3-40) not the same."
Now that's just not fair: you can't bring the harsh reality of technical details into the discussion when sophistry is the prevailing theme.
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.
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
Would the different filament voltage cause the variance between the two? A 300B running at 2.5V could be cooler than at 5V. A real 2A3 and a 300B both dissipate 6.25 watts at the correct voltage (2.5V @ 2.5A and 5V @ 1.25A). Is that the case with the 2A3-40?
The data sheet for the JJ 2a3-40 says 2.5v 2.5a
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre'
Of course it does, but my point was, I'm running it at 5v, sounds just fine - maybe a lot better than fine.
how much current are you drawing at 5V?
dave
With a JJ 300B in the socket, I get H = 5.0vdc (this is unreg by the way), I = 80ma, B+ = 450, Vpk = 370, Pd = 29.6w.
With the JJ 2A3-40 in I get H = 3.36v, I = 76ma, B+ = 450v, Vpk = 374, Pd = 28w.
I didn't measure heater current in either case, but clearly, higher with the 2A3.
The 5V supply dropping to 3.36V makes perfect sense. say it is drawing 3.25A and the extra 2A of current draw on the supply accounts for your lower supply voltage. I was trying to picture 5V across a 2A3 and how bright it would be drawing over 4A of current.
Thanks
dave
Almost forgot, the hum at output is 10mv with 2A3 and 5mv with 300b
So if the filaments are running at 3.36V they would be drawing 1.86A if the filament wattage is still 6.25W?
2.5V / 2.5A = 1 ohms with the standard 2A3 or with a 300B, 5V / 1.5A = 4 ohms
3.36V / 1.86A = 1.81 ohms with your 2A3-40
Is this correct? It doesn't seem right to me. Where did I mess up?
The filament supply can't handle the load so the voltage dropped. That throws everything off.
If the supply was up to the task then the 2a3 would have 5 volts across it's filament and be drawing 5 amps of current from the supply.
But that wouldn't happen because the "Hot" resistance of the 2a3 filament with 5 volts would be higher than the "Hot" resistance with the normal 2.5 volts.
So it would naturally end up some where in between.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It's clear now.
Right! As I said, the filament supply is unregulated.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: