SED EL34's.Musical,liquidy.Don't get the ones that say Svetlana.The name changed to SED.$100-$120 quad.The 7199's don't make that much difference as long as they are good.You could get buy with a Sovtek 5ar4 if you don't have any Mullard's laying around.The SED EL34's are the best choice for what you want.
My vote goes to a matched Quad of EH 6CA7's in the ST70. But you should replace them all at the same time. But I also reccomend that if the mods to the input board, beefing up the B+ and power supply as well modding the bias circuit as well then you will miss all that the Stereo can deliver. I see you said rebuilt but some call putting all new parts in the amplifier rebuilding it. I think 7199's should be replaced in each rebuild, mod, new build. I don't consider that a rebuild. The problems that existed still exist. The Stereo 70 is a solidly built amplifier. If you want better detail than the tubes mentioned try the EH's. Not the Fat ones. Many don't like them because they are so fat in it's sound. If you can afford to find some good deals and try a couple different sets. You may be in for a surprise.
If the amp didn't come with a Mullard(Blackburn Plant) GZ34/5AR4 rectifier; get one(NOS). It will probably be the last one you'll ever have to buy for the unit. That and NOS 7199's(Philips ECG=best, or RCA black plate, Sylvania) will both make major improvements in your presentation. The original Genelex Gold Lion was a warm/fat sounding tube(the sound it appears you want, per your post). If the Russian copy is anything like it; they may please your ears better than the others mentioned. Get a skinny aftermarket power cord(ie: the Zu BOK or Mother fits nicely, but one of the Audioquests would work too) and you can install it through the existing hole in the chassis with a small rubber grommet. Some sources for tubes: (http://www.upscaleaudio.com/bEL-346CA7-Typesb_c_86.html) (http://www.tubeworld.com/5ar4.htm) (http://cgi.ebay.com/7199-JAN-Vacuum-Tube-Tubes-PHILIPS-NOS-USA_W0QQitemZ370295123083QQcmdZViewItemQQptZVintage_Electronics_R2?hash=item56374f2c8b)
if it sounds great..use the tubes that are in it.
I have a Will Vincent rebuilt st 70. It is able to use 6L6, kt66, EL34, or KT77 tubes.
The amp came from Will with jj kt77 tubes and it sounds incredible with those. I have since tried gold lion kt66( they sound very good but with slightly less 3d midrange), sed "winged c" new matched quad which has good bass but do not have the "magical" midrange and are so rolled off on top that it sounds like I threw a quilt over my speakers!
I have also tried a new matched quad of Electro harmonix el34's and they had a "thin" sound with little bass compared to the other tubes and had a "grainy" top end extension that was not pleasing in my amp.
All tubes were run in for at least 25-30 hrs.
The output tubes make the only major difference in sound quality or character in my amp. I am currently running a vintage 1961 blackburn mullard gz34, I have used an nos telefunken gz34 and chinese 5ar4. None of these had any effect on sound quality.
I have also changed from ge 7199's to nos rca blackplate 7199's with no audible change in sound from the amp.
I think that the speaker, amp, preamp synergy plays a major role in how different tubes will sound in your amp.
I would rather listen to my old marantz 2220b ss reciever than my dynaco with sed winged c's or electro harmonix el 34's in it. But if I switch to jj kt77's or gold lion kt66's, there is no contest. The dynaco is warm, magical,and lifelike sounding so the marantz is back collecting dust again.
My system consists of classe audio dr6 preamp, will vincent st70, quad 11L monitors. I have dialed in the sound I like and am very happy with my system now.
Good Luck and enjoy your tube rolling.
Just to follow up on the system synergy part of my previous post... I have a pair of klipsch heresy's with new crossovers and also a pair of klipsch kg4's.
I have set up both speakers in my system and did not like the sound.
I have a bryston 3bst amp also, and both of the klipsch speakers sounded better with the ss amp than the dynaco?
Most people swear that the klipsch speakers beg for a tube amp but they did not sound good in my setup.
My quad 11L mini monitors have better bass (tight and quick) and much better detail than any of the klipsch speakers..... but where the 2 way mini monitors shine are the incredible imaging and soundstage width and depth they produce compared to the klipsch speakers.They "dissapear" and sound so lifelike with the st70 powering them. If I put the bryston in the system, it sounds good but no where near the level of tube power to the quads???? 86 db speakers sound better than 96db speakers with tubes???
System synergy..... only experimentation will find what sounds best to you.
It's important to note that there are many variables that will determine, or limit, a listener's ability to discern improvements/changes in presentation. ie: A system(source/pre/speakers/cables) that does not present or pass the info on the recording, poor recordings, room acoustics, aural acuity(actually; the lack there-of), the untrained ear, and the ubiquitous mental block(a refusal to hear what is obvious to others, because it doesn't agree with your mindset). Don't let the opinion of another stop you from experimenting with your own system. There are numerous simple/inexpensive mods and tube upgrades for the ST-70, that will yield much musical pleasure, if you ignore the naysayers(again- providing the rest of your system, and your ears, support them). Besides- It's just fun to experiment! Happy listening, all.
Another voice heard from. I run Klipsch Fortes with mine and would say that the biggest difference was going to a Mullard 5AR4. Any Stereo 70 can use 6L6, KT66, EL34, 6CA7, KT77, etc. For power tubes, I would suggest getting away from the EL34/KT77 family and moving to some nice RCA 6L6GC or similar.
I will agree that *most* st 70's will bias 6l6 or kt66 tubes properly, but some will not without a resistor change in the circuit.
My amp has new much higher quality bias pots with a wider adjustment range than stock.... some stock bias pots may not turn the bias low enough without a resistor change in the circuit for the 6l6, kt66 tubes... while this may not seem like a big deal to most people here, the reason some may buy a rebuilt amp is they have no desire, expertise, tools, etc. to change anything in the circuit.
I have been in this hobby for almost 35 years and have owned countless speakers, seperates, recievers,etc.You are on the right track going with a rebuilt st70. It is the best sounding amp I have ever owned and cost much less than many of them. I will never go back to solid state amps again!
As mentioned by a previous poster don't let anyone's personal experience be the end all to your decision.
What works in my listening room may sound horrible in yours.
Power tubes for a dynaco st70 are cheap enough to play around and have fun with....
A Dynaco brought up to spec's is ok done right.A repainted one hides the transformers.There are transformers with cloth leads that sound totally different that the early ones.I went through about 20 or more of them.When everybody went to SS,there wasn't much choice for someone who wanted tube amps.You had to buy vintage amps.Some say they started out sourcing the transformers without the employees knowing about it.You could make them bias up for a 6L6 family tube,but any tech knows it is not made for them! At 4300 ohms for the A470 vs. the over 6000 ohms compatible for the 6L6 family.It's easy to buy a rusted Dynaco cheap and repaint it.There are people out there putting different transformers in the old Mac amps too.If there is a buck to be made,somebody ruthless out there sure will jump in.The early Dynaco 70 that has transformers similar in shine to the old Mac gear are the best ones.
Gentlemen: Esp Dave65mus, Rodman99999,& Viridian
I am so gratefull for the time you have taken to respond.
As a novice in this field of audio I appreciate your knowledge and passion.
I"ve got the new NOS ordered and look forward to Listening but also following your opinions on the threads.
I also own a Will Vincent rebuilt ST-70 wired in triode. I ended up changing all the tubes. RCA 7199's made a positive difference. I tried SED "winged C" el34's and a quad of Siemens el34's. I much prefered the Siemens, winged c's being too rolled of at the top for my taste.
But getting a Mullard fat base gz34 made it sound like a whole new amp. A much better amp. I actually couldn't belive what a positive impact it had on the sound. So if I had a limited budget for tubes, I'd spend my money on a Mullard gz34 first.
If you want the top(price doesn't matter) EL34,go for the Bugle Boy "square D" double getter.It gets along with every EL34 amp I've owned,and heard it in.At $800 and up for a NOS quad,watch out for counterfeits.They will out do all of the above mentioned.
Tommy 583, that is a great description of the sed winged c in my amp also.
IF I had tone controls on my preamp(good gear does not use them so I have none) the winged c's would be like turning the treble adjustment all the way down..... dull and lifeless.
I just put in a new quad of jj kt77's tonight... I had one go out after about 1 year of use and had been using kt66 gold lions since.... now with the vintage mullard gz34, rca blackplate 7199's, and jj kt77's, No more tube rolling for me! I am going to stock up on jj's and rca blackplates and enjoy the music!
Have you ever tried the gold Lion KT-77's? I was thinking about trying quad of them when funds allow.
Dav65mus, you've more or less ended up in the same place I have with my ST70.
The one difference being I prefer the JJ E34L to their KT77. You give up a bit of low frequency performance, which isn't the strong suit of the ST70 anyway, but the midrange and treble open up a noticeable (in a glorious, but not harsh way)amount more.
For the record, I have a bunch of highly regarded amplifiers, and while the ST70 doesn't outclass them in terms of power or low end, not one of them can match its overall tone. To me, that's why it's the amp I enjoy the most. For all of the shortcomings people love to chirp about, I shake my head at how far things have not come since the mid - 1950s.
1959, actually. One of the most important components in the building of a good sounding tube amp is the output transformer(if you're going to use one). The originals in the ST-70 were pretty darn good. With upgraded RCAs/speaker terminals/PC/chassis wire, more capacitance in the power supply, better coupling caps and resistors; the old jewels can really sing, even with the original driver board. That's not to mention the fun the modding provides.
You might want to look at a qusd of NOS Tesla EL 34 if you can find them.