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- Head-Fi Buying Guide (Desktop Amps & DACs) 2
For 6AS7G tube rollers here ..... - Page 27
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Its all in the respective datasheets. The 421A does indeed have higher amplification and transconductance than the 5998(A).
Here is a post I made earlier with 421A info added to that for 6AS7G and 5998(A):
The real confusion comes from the fact that the 5998A, while having 6AS7 plates, also has:
* The lower heater current of the 5998; 2.4A for 5998(A) vs 2.5A for 6AS7 (2.4A for 421A, same as 5998(A))
* The higher amplification factor of the 5998; 5.4-5.5 for 5998(A) vs 2 for 6AS7 (6.1 for 421A, higher than 5998(A))
* The higher transconductance of the 5998; 14000-15500 for 5998(A) vs 7000 for 6AS7 (20000 for 421A, higher than 5998(A))
References for Tung Sol versions of all tubes:
The similarities are enough that the 421A is almost certainly just a cherry-picked 5998.
But I should also point out again that the vast differences between the 5998A and the 6AS7G indicate that they are *not* the same tube as you commonly suggest.
It's not that the 5998A and 6AS7G(A) are the same electrically - it's that they feature the same construction, which is not the same as the 5998. Look at the GE 5998A - it's clearly identical construction to their 6AS7GA. Not sure what they do exactly to make it spec like a 5998.
I have a question...
Since one of my RCA Reds arrived broken whilst my 5998's and 7236's arrived from another source I was forced to use my older driver tubes,
Black Metal Svetlanas "6SJ7"-wannabe's and Sylvania 6SJ7GT
So my first round of listening was 6SJ7GT + 5998's .... Mmm... Fantastic sound. Albeit not that much gain that I was expecting. I was running the amp at 9 to 10 o'clock. Wide, liqud soundstage and magical texture in the bass... When I tried to switch to the Svetlanas, which seem to have more gain than the Sylvania's, the sound distorted. Like a soft fuzz/overdrive-effect for guitars (I'm an electric guitarist). I was a bit shocked and thought that I had somehow broken the 5998's. Switched back to 6SJ7GT. No problem.
Tried the 7236's, sound great with both the 6SJ7GT and Svetlana "6SJ7". Actually better with the Svetlanas. Insane gain. Really great bass quality and quantity. Almost too loud at 7'o'clock. Not the same magic mids and wide soundstage as 5998's, but not that shabby either. Great for modern music.
Why did the 5998's behave like that with the "6SJ7's"?
Edited by ValentinHogea - 8/9/12 at 7:01am
This was reported in the Woo owners thread a couple of times too, and we never really came to any conclusion about what was causing it. The prevailing theory is that the drive tubes might be getting old or weakening in emission and aren't quite cutting the mustard for properly driving the 5998 where as they might still be up to the task of driving a 7236. If the internal resistance of the 5998 is lower than the 7236 then it would be more demanding of the input tube driving it. That's just one theory, and based on my pretty shallow knowledge of tubes so take it with a grain of salt.
From looking at the operating parameters, it's hard to tell what might be the potential interaction issue. Hard to know, though, unless you know what the diffence between a true 6SJ7 and the Svetlana is.
No. They are electrically very similar, but the 5998A has plates that look more like the 6AS7, not the dimpled plates of the 5998.
Its actually pretty funny, because I have seen MANY people say how much they dislike the 5998A, but love the 7236, and they are the same damn tube.
My 2399 most definitely sounded like a 5998, at least it did when they weren't humming. Just a touch warmer because of the copper grid posts. The difference in gain and presentation would have been pretty clear if it was a 6AS7 electrically. I think the 6520 is the more interesting specimen personally since they used both plate styles.
- For 6AS7G tube rollers here .....
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