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)

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.

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.