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Old 27th March 2010, 10:24 PM   #1
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Default EL34 for ST70

I have stripped down an old St70 and am doing a complete mod on it, using the Curcio 4-tube driver circuit and regulated PSU. I've also got 4 new EL34 marked "National Electronics" and "made in China". Does anyone here have experience of these tubes or this manufacturer? I got them cheap so I am not expecting too much....

Another question: How important is it to match output tubes on the ST70

Thanks / Michael
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Old 27th March 2010, 10:56 PM   #2
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Quote:
Originally Posted by MichaelK View Post
I've also got 4 new EL34 marked "National Electronics" and "made in China". Does anyone here have experience of these tubes or this manufacturer?
National manufactures no tubes. They only re-brand whatever is cheapest that week. In your case, it was Chinese tubes, probably Shuguangs. Is there any type of Chinese tube other than Shuguang?
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Old 27th March 2010, 11:24 PM   #3
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How important is it to match output tubes on the ST70

Michael,

It's quite important that each pair be well matched for both gm and cathode current. A Dyna ST70 has a bias adjustment for each channel, not for each tube.
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Old 28th March 2010, 07:43 AM   #4
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Perhaps I should incorporate a bias adjustment for each tube. I guess it is only a matter of having a separate cathode resistor and a separate bias source/pot for each tube? I was thinking of reducing the cathode resistor to about 1 - 2 ohms instead of the current 15.6 ohms
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Old 29th March 2010, 05:40 AM   #5
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Quote:
Originally Posted by MichaelK View Post
Perhaps I should incorporate a bias adjustment for each tube. I guess it is only a matter of having a separate cathode resistor and a separate bias source/pot for each tube? I was thinking of reducing the cathode resistor to about 1 - 2 ohms instead of the current 15.6 ohms
Yes, that's a very good idea. Reasonably well-matched tubes is a good start but having the means to adjust them individually is also good.
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