[hodgjy]

I read some spec sheets, and they say the two tubes work differently in class A amplifiers.
 
5998:
Class A Amplifier
Plate Voltage ................................. 110 V
Grid No. 1 Voltage Derived from
Cathode Bias Resistor ....................... 105 Ω
Amplification Factor .......................... 5.4
Plate Resistance (approx) ..................... 350 Ω
Transconductance .............................. 15.5K μ
Plate Current ................................. 100 mA
 
6AS7G:
Class A Amplifier
Plate Voltage ................................. 135 V
Grid No. 1 Voltage Derived from
Cathode Bias Resistor ....................... 250 Ω
Amplification Factor .......................... 2
Plate Resistance (approx) ..................... 280 Ω
Transconductance .............................. 7000 μ
Plate Current ................................. 125 mA
 
Sources:
http://www.nj7p.org/Tube4.php?tube=5998
http://www.nj7p.org/Tube4.php?tube=6as7g

 

[hodgjy]

I've only heard the bottom getter, and it's lovely.
 
Quote:

Has anyone heard a GE 6AS7G 6AS7 blackplate with dual getters?  I got one off ebay to try against my RCA.

 

[Skylab]

The 5998 has twice the transconductance of the 6AS7G, and as such (given the slightly higher plate resistance and that most of the rest of the parameters are the same), the amplification factor is more than double.  Amplification factor (mu) is the product of transconductance and plate resistance.
 
However, none of that should effect heat.  The tube is either on or off.  What effects heat is the current pull from the transformer, and the tubes are very close in that regard.
 

 

[hodgjy]

Muy interesante.  

  I don't know then.  All I know is when I use the 5998, the amp as a whole, including the transformer housing, is noticeably cooler than when using a 6AS7G or 6H13C tube.  Don't know why.  It's just interesting.  I agree that it can't be from a 0.1 amp difference.  Something else must be going, perhaps the resistive properties of the metals themselves inside the tubes.
 

 

[les_garten]

My 6080WB ran about 350F and the 5998s run about 250F

 

[hodgjy]

I've done some heavy tube rolling and listening, and I'm ready to rank my favorite to least favorite flavors of the variety I've tried.
 
1=most favorite
 
1) RCA black plate 6AS7G.  Yep, this el cheapo is magic to my ears.  It doesn't have the precision, bass thump, or tempo of the 5998 or the 7236, but it is just wonderful.  It has beautiful tube bloom and the bottom line is it's just a very musical tube.  Warm and sweet, but under control.
 
2) Chatham 6AS7G.  A wonderful tube.  Warm and sweet.  However, it's just a tad colder than the RCA with a little less tube bloom.  Still an excellent tube, and I will cherish it in my collection, but it doesn't have enough to best the RCA.
 
3) Tung-Sol 5998.  Yep, while this is many people's favorite, it is my third favorite.  It is also an excellent tube.  It is precise, with good bass extension and rhythm.  Best all-around tube.  Jack of all trades, master of not all.  It has less tube bloom and harmoic distortion than the RCA.  It is a great all-around tube, but to my humble ears, the RCA just sounds more romantic and musical.  Don't get me wrong, I still use and love the 5998, but when I want to melt into my chair to some music, I think of the RCA first.
 
4) Sylvania 7236.  This is a decent tube.  It is the most solid state sounding tube I've tried.  Good precision and rhythm.  Bass champion.  However, it is a little dry and sterile.  Good for metal, techno, and electronica.  Not so good for acoustic bliss.
 
5) RCA 6080.  This is the stock tube that came with my amp.  It doesn't do much well at all.  Slow, sloppy, and lazy.  Not engaging at all.  It is warm, but not in a musical way.
 
6) Svetlana 6H13C.  This really isn't a bad tube, but due to other circumstances, it automatically takes last place for me.  This tube isn't terrible, but it isn't really great, either.  It has good tonal balance, decent bass, and acceptable rhythm.  It is warm with a little tube bloom.  However, it is very veiled, wooly, and congested.  This, unfortunately, is torture to me.  It is extremely fatiguing to my ears and gives me intense pressure headaches after an hour or so of listening.  I will never use this tube again because of that interesting side effect.  Thus, last place.
 
All in all, I think anyone using tubes 1-4 will have a good experience.  I could see myself using any one of them at any time depending on my mood and musical choice.  Tubes 5 and 6 will never see the socket of my amp again.
 
But, to muddy the waters, which tube is in my amp right now?  Well, it's the 5998 of course!  

  It's the best all-around tube.  I tend to change genres during my listening sessions, so it makes sense to have a great all-arounder in the amp.  However, if I know I'm going to be listening to light music, or perhaps acoustic-only, for long periods of time, I'll roll in the RCA and melt away.
 
Hope this helps someone considering rolling options.

 

[tdogzthmn]

What are some good 12AU7 replacements I should be on the lookout for?  Also the 5998-A seems to be most common, is this the version that most are referring to?

 

[Skylab]

Quote:

What are some good 12AU7 replacements I should be on the lookout for?

 

[hodgjy]

No, different tube.  That is essentially a renumbered/rebadged 6AS7GA.  Not a bad tube, but there are much better ones out there.
 
Quote:

  Also the 5998-A seems to be most common, is this the version that most are referring to?

 

[Skylab]

Indeed - the 5998-A isn't worth more than $10 NOS.

 

[tdogzthmn]

Quote:

very nice, where did you find this?  The only place I could find the 5998 was on tube depot.  Ebay has not had any for a few days.

 

[Skylab]

I almost hate to post this, because once I do, they will be gone, but you can get NOS 5998's from Radio Electric Supply: www.vacuumtubes.net
 
That chart has been circulating for ages, but I've always found it pretty accurate.

 

[hodgjy]

Tubeworld.com has them, so does and vacuumtubesinc.com.
 
However, don't over pay for them.  You can get them for no more than $40.  I'd say that at a dollar more, they aren't worth it considering the RCA is $11.

 
Quote:

  The only place I could find the 5998 was on tube depot.  Ebay has not had any for a few days.

 

[tdogzthmn]

I do have a matched pair of the RCA blackplates which I do enjoy very much but I have found they lose their magic with faster genres which is why I am looking for a 5998.

 

[hodgjy]

That is the perfect reason to try the 5998!!!  I hope you get your hands on some.
 
I also found the 7236 is good at keeping up to speed with fast genres.  It's not as musical as the 5998, but yet it costs about the same.  However, they are everywhere and easy to find.
 
Quote:

I do have a matched pair of the RCA blackplates which I do enjoy very much but I have found they lose their magic with faster genres which is why I am looking for a 5998.