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Hi everybody,

I am new to this forum and I want to share my experience with the rebuild of yet another ST70. I bought this one on ebay not so long ago. It was in a descent state but I stripped the entire amp and started all over again. It gave my the fun of building a ST70 myself.

What I don’t like about the ST70 is the PCB on top. So I decided to eliminate the PCB by point-to-point wiring underneath. I had a cover made from chrome steel plate for the ‘PCB gap’ and gave it ventilation slots similar to the ones near the EL34’s, just for aesthetics.

The chassis is still original. That is the advantage of an empty chassis; it gives you room for a thorough cleaning. After that I polished it with brass-polish and finished it of with a layer of Turtle wax.

I did not alter the schematics except for the mono/stereo switch that I left out. I also placed a new transformer (230V AC in my country) and a new multicap 80-40-30-20. Of course I replaced al the resistors and capacitors. I reused the tube sockets, which I found in a perfect state
The amplifier lost al its hum and sounds crystal clear. I am very pleased with the results.

Pictures of my amplifier:

http://www.mijnalbum.nl/Album-J4WJS6CU-Foto's-in-Emmeloord-Foto's-van-Voorwerpen.html

Your comment, please.

Your chassis really cleaned up nicely. You have an interesting looking st70, with the chrome plate in place of the driver board.

Thanks Kheper.
I wonder would it be possible to change the EL34’s for an other tube without changing the hardware and improving the sound quality? Which one do you recommend?

There are plenty of other types that will work, KT66, KT77, 6L6GC, 6CA7, etc. I've been wanting to try the new GEC KT66.

Rovano wrote:Thanks Kheper.
I wonder would it be possible to change the EL34’s for an other tube without changing the hardware and improving the sound quality? Which one do you recommend?

I like the new production Tungsol Kt66 tubes. However, you may need to adjust the bias supply to make it put out a more negative voltage.

Some people swear by KT77 tubes. The only manufacturer producing them (which I know of) is JJ. From past experience, I shy away from JJ tubes.

Terry:

You mention a lot of tubes that could be placed, but what kind of sound quality do they offer? What is your opinion? Do you also swear by the KT77? What about KT88?

Kheper:

Does the KT66 really need a more negative voltage or will the original bias supply do? Did you just have bad luck with your JJ’s or do you think that JJ is an inferior tube? I have JJ el34’s on my ST70 and they sound alright to me, but then again I don’t own the ST70 that long.

Trying different types is totally subjective as some like EL34's, some like the larger 6CA7. I am suggesting types that will function and others have tried in a '70.

The EL34, 6CA7, and KT77 are direct subs for each other but are totally different in construction.

6L6GC and KT66 are in the same family, but different.

FWIW, my experience with JJ KT77 (bought an year ago from
AES) has been pretty good. I estimate the tubes have
about 70ish hours on them, and they are doing fine
and sound excellent.

The only nit is that the glass envelope is not
uniformly fitted, so a couple of them are off vertical
when the tube base is seated properly in the socket. Looks
a little wierd, like the leaning tower, or as if the heat is
melting them out of their base.