Leave it original. Just maintain it to spec with equivalent component values if necessary.
Buy a kit to play with. Then tell us what YOU think.
leave it original if you want a price point design that isnt anywhere near what it could be. Otherwise, update for huge benefits.
The AVA stuff is great. I also like the boards originally sold by Triode electronics, and SDS I believe it was. Very similar circuit than the original, almost identical, but uses an nice pentode and half of a 12au7 instead of the 7199's triode and pentode sections. Much better and can then drive all sorts of tubes the original circuit couldnt of dreamed of.
My favorite sounding ST 70 rebuild I've had, although one most people have had the most difficulty with is the Welborne kit. While not a simple rebuild, correctly done is a real winner.
The original is a classic, but it was built to a price point and a lot of good thinking kept it simple, worked well, but was quite a compromise to get to that price point as far as sonics are concerned.
At the very least, get rid of the selenium rectifier ( and the bias caps ) and replace with modern components, replace all the coupling caps and definately the power supply caps. this will at least keep it safe to operate. By the time you have finished, you will have spent close to what many rebuild kits go for. So why not make it better than it ever could be in its original package? The boards are getting so old I've had them fall apart like an old cracker, traces lifting and so forth, just not a good platform to begin a rebuild with.
If you want to keep it original and can live with its limitations, at least find a replacement board, several are offered that are duplicates of the original. Again, by the time you are done restuffing it, you'll be at or past the cost of one of the good rebuild kits.
Perhaps things have improved at Welbourne labs, their kit is dynamite, but is more complex than many and a lot of people have had trouble with it. It isnt a very intuitive kit, but if you get it right, it is the better one in my opinion.
Next would be the Triode labs/SDS boards or the AVA boards. As far as simplicity and clarity of instructions, the AVA stuff is out in front by a country mile. Very clear concise instructions like Dynaco used to provide, even more clear! Lots of support as well.
The curcio rebuild kits are well thought of, the original regulated the B+ and I personally didnt care for the effect, but that was a long time ago, and I think now Curcio offers more than one rebuild kit.
To simplify, yes try the AVA kit, it will not dissapoint. If you rebuild it original,
you will likely spend as much money, and the amp just will not be as good as these modern kits.
Have fun, its a great little amp,
Russellc
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