What tubes for ST-70

What tube or tubes would make the biggest change in sound on this amp. The GZ-34, 7199's or the EL-34's. This amp is a Dynaco ST-70 rebuilt by Will Vincent. Also what would i gain or lose by using KT-77's instead of the EL-34's?
No reason to replace the GZ-34 unless it goes bad and that may take decades.Not a fan of 7199's - they don't age all that gracefully - might want to consider some of the alternative driver boards - nothing actually just substitutes for the 7199 - weirdly enough - I'd consider replacing these first -simply because in a couple of hundred hours they don't sound all that good anymore.When I owned and serviced 70's - had a strong preference for EL-34 Telefunkens -am a little shocked what purported NOS's of these go for nowadays.
You WILL hear a difference by replacing the GZ34/5AR4 w/a Mullard 5AR4(unless there's already one in the unit). Any improvements in the power supply will be very audible. Sylvania and Phillips ECG 7199's are the two cleanest(least colored) brands that I've tried in the ST-70's that I've worked on. Ditto- the Svetlana EL-34, from the St Petersburg plant(all that's now available at a sane price). If you're considering mods: Contact Joe Curcio (http://www.curcioaudio.com/) Here's been a major player in the Dynaco mods business for decades. Upgrade that pitiful AC cord too.
Thanks for your input guys. The amp came with a JJ Tesla GZ34, i just picked up an RCA GZ34 from a guy that lives near me. He also has a few Mullards i could get, but i didn't have the money on me when i stopped by. That being said, the RCA does sound better to me. The amp also has an IEC inlet, so i'm using a Signal Cable magic power cord. The 7199's it came with say Tung-Sol on them. The power tubes don't have any markings on them at all. I'm not even sure if they are EL34's or KT77's. My local tube guy doesn't have any 34's or 77's to try before i buy. So what ever i choose i'll have to get them off the net. I just want to make sure i don't make a mistake that i'll be stuck with for a while.

Oops i stated above that the GZ-34 i bought was an RCA. It is a GE branded tube.
If you purchase your output tubes from Kevin Deal: you'll never have to worry about getting "stuck", in ANY sense of the word. He's first class, all the way: (http://www.upscaleaudio.com/view_category.asp?cat=86)
Thanks for the link Rodman, he has alot to choose from. I just ordered a quad of the winged c Svetlana el34's. I'll post once they arrive and i have a chance to listen to them. Thanks guys.
I don't think you will be disappointed! =8^)
I got a pair of RCA 7199's from my local dealer yesterday. All i can say is WOW. It's like a whole new amp. Alot more detail, but still sounds smooth. Now i really can't wait for the new EL-34's to arrive.
I would heed the advice of Stonedeaf....The 7199 is a medium mu triode / sharp cutoff pentode that is very prone to capacitance problems when it ages causing hum and premature clipping on one side of the waveform. That is, asymmetrical clipping and increased distortion. If you are into soldering I also would recommend replacing the mother board with and update that allows for 2 front end tubes instead of one per channel as the 7199/6AN8 was really not up to the task of properly driving the EL34 or the 6550 to clipping @ low distortion. G
My experience was the same as Tommy583's.

Had a pair of Sovtek 7199 in my bone stock ST70. The sound was so-so, maybe a "C" or even "C-" on the traditional letter grade system. Put a pair of RCA 7199 and it was literally the biggest improvement I've been around when it comes to rolling tubes. No, it's still not the last word in power (though more than enough for my efficient speakers) or low end response, but the amp is now a solid "B+".

I get so much enjoyment out of this amp in my second system that I spend about 80% of my time listening there now. This amp has now caused me to question what exactly has improved over the past 50 years when it comes to tube amps.
Circuit topography hasn't changed much at all and the ST-70's came with excellent output transformers too(very important). The major differences in sound that can be appreciated with later designs comes from the improvements in passive components. Replace the old Black Cat coupling caps with some good polystyrene/polypropylene and foils(ie: auriCap, MIT, DynamiCap or WonderCaps) and the carbon composition resistors in your signal path with precision metal film resistors(Vishay or Caddock), and you'll understand SOME of the improvements. I won't even mention the cheesy RCAs, speaker terminals and power cord on the ST-70(OOPS- I guess I did after all).
Ok i put in a quad of the Svetlana EL-34 winged C, RCA 7199's and a GE GZ-34. The sound is alot smoother overall. I do think i lost a little bass when i installed the EL-34's. This leads me to belive the old tubes were KT-77's. The soundstage is a little wider and higher. The top end is more layed back and not harsh at all. I think i'm going to get the Mullard GZ-34 and see if that makes a differance. Over all i'm pretty happy with the sound, just looking for that last little bit.
Your going to find that the KT-77 has a faster, more aggressive bottom end vs. the EL-34. This is also true with the MK-3 amps when the 6550 is replaced with the KT-88. You will also get a bit more power output out of the KT-77 vs. the EL-34. When I had Mk-3's I would also keep them in Genelex KT-88's. That was a while ago when they were still available and reasonably priced. G
I've seen some good things about the remake of the Gold Lion KT-77's. I may try a quad of those next. I've seen them priced around $180-$200 a matched quad. I wonder if they are worth it though?
I have not tried them or tested them so I could not say...G
Well guys my tube rolling is done for now. In the end i settled on a pair of RCA 7199's, a quad of winged "C"' el34's and a Mullard fat base gz34. While the 7199 and el34 tubes were an improvement over the tubes i had to start with, the gz34 brought the amp up to a new level.
I could not belive the improvement i heard after i installed this tube. The bass is much more full, the treble very smooth and the mids are to die for. I'm just glad my local dealer doesn't charge the crazy prices i see on the net for these tubes. But in truth if i spent $150 for this level of improvement i wouldn't be disapointed.
In this case- it's a pleasure to say, "I told you so!"(re: the Mullard in the power supply) If you're handy with a soldering iron, replace the Black Cat coupling caps with something newer(auriCap/Multicap/DynamiCap to list a few good, reasonably priced ones), and you'll find even more music hidden in your discs. Enjoy your tunes. =8^)
Well Rodman, you were right thanks for your advice. About the caps, this is not a stock st70. It has been rebuilt from the ground up. The only thing that is original on it is the transformers. From what i can see from the top there are solen caps in the 7199 board. I will have to open it up to see the rest of the components that are in there. But for now it sounds so good i don't want to waste any time on taking it apart. I'll just listen for a while. :)
If it's using the 7199: It's probably still the stock design board, and the signal path components are on top(no need to open it up). Solens are way better than the oldies too. Happy listening!