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Output tubes in ST-70--how important to sound?

cpditter

cpditter

Member
Hi:
My system is built around a Tubes-4-HiFi ST-70 amp kit which I built. Preamp is their SP-10 tube preamp, and my phono preamp is the Bottlehead Reduction kit. I've replaced all preamp tubes in all three with NOS vintage tubes. My output tubes in the ST-70 are Electro-Harmonix EL34s...for budget reasons it's what I went with when I built it.

So my question is, how important are the output tubes to the overall sound of the amp? I read a comment somewhere that said in a tube power amp, the output tubes really define the sound of the amp. True?

NOS vintage EL34's are pretty pricey for 4 of them, so looking to understand if this is something I should do or not. Thanks for any input.
 
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Thay can. I tried various different output tubes in my VTA ST70. If the EH EL34's are anything like the EH 6CA7's I'm using they should sound pretty good. If the EL34's your using sound good to you, no need to change them.
 
I find in my admittedly limited experience with an "idealized" OEM Dyna Stereo 70 and a custom one-off VTA with A-470 OPTs that the Dyna works out better with the EH 6CA7s and the VTA more forgiving with either. The SED EL34s sound good in it.
 
I have a ST-70 and output tubes will definitely make a difference in sound. You have a nice set in there now, I would get a matched quad of JJ EL34's or the reissue Mullards. Also Lavane's advice is good, the EH 6CA7 tubes sound great in my amp and are inexpensive.
 
SteevG said:
I have a ST-70 and output tubes will definitely make a difference in sound. You have a nice set in there now, I would get a matched quad of JJ EL34's or the reissue Mullards. Also Lavane's advice is good, the EH 6CA7 tubes sound great in my amp and are inexpensive.
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The EH 6CA7's are my favorite so far. The KT-77's a close second.
 
Thanks for all the replies. When I bought my current EH tubes it was a matched quad.
I think I will sit tight with what I have...I am overall pleased with the sound, and I am guessing I would not really hear much improvement by switching to an expensive quad of NOS tubes.
 
cpditter said:
Thanks for all the replies. When I bought my current EH tubes it was a matched quad.
I think I will sit tight with what I have...I am overall pleased with the sound, and I am guessing I would not really hear much improvement by switching to an expensive quad of NOS tubes.
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I have a hard time letting go of big wads of cash for NOS premium tubes myself. Especially when it comes to power tubes. If you loose one, now you have to find a matching reacement. I buy new manufacture and the companies I buy from list the matching test results either on the base of the tube, or on the box. So if one happens to fail, I can order a new one to match what I have. There are some decent sounding power tubes being sold that won't break the bank. The EH 6CA7's are a good example. New tubes like the Sovtek EL84M and JJ EL84 are good examples as well.
 
Most noticeable sq change here (other than drivers) was adding a bucker to the power supply - current VAC levels tend to be much higher than what these amps were designed for. Mine is a current gen clone (Latino ST120) and even that benefited a bunch.

bucking-transformer.jpg
 
New production tubes are fine. I originally was using low end Shuguang tubes and they were just fine in my VTA.
 
The EH 6CA7s may be a touch better. Have tried both. Not a huge difference, but if I had to pick would select the 6CA7s. the other area you may consider is looking at what you are using for coupling caps....tons of options there.

The other area that helps on the 70s is using the optional larger power trans from Tubes4hifi. Bolts right in place; no mods needed.
 
Is it making rated power?
I'd say if the amp is making close to 35 watts per channel and it seems to sound good, you're better off saving your hard earned dough.
You know fix something that's broken...
On the other hand, it doesn't hurt to have extra tubs stashed away in pairs and quads, in case on day you find out that you cant buy them... The power tubes you're using right now will eventually wear out. You'll know they're worn out because they'll no longer make any power....
 
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