dynaco st 70 what tubes to replace first

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.

Good Luck

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.