EL34 for ST70

[MichaelK]

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

[Ty_Bower]

Originally Posted by MichaelK

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?

[Eli Duttman]

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.

[MichaelK]

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

[ray_moth]

Originally Posted by MichaelK

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.