2 Quad EL34 sets for Dynaco ST-70

The way I do it on my ST70 is I first measure, and turn down the bias adjustment. Then I swap the tubes, measure, and turn the bias back up to the correct operating point with the new tubes.

This makes sure you don't se the bias too high. Also, recheck the bias, and readjust it once the amp has warmed up, and again a few days later, as the bias will change after the new tubes have been heat cycled a few times.

John C.

With the amp off, and the tubes cool, pull the tubes straight up, holding them at the base of the tube, not the glass. This may take considerable force, try not to rock them back and forth too much as you do this.

Your Dyna uses two sets of matched tubes. Keep the two tubes from the left hand side of the amp together and the ones from the right side together. Do not mix them up.

Before inserting the new tubes, turn the bias controls all of the way down. That is counter-clockwise. At the bottom of each tube, in the center is a plastic key that prevents the tube from going into the socket the wrong way. Make sure that the key in the base of the tube lines up with the hole in the center of the socket.

Insert the new tubes and turn the amp on. If any of the tubes glows red or seems to be having issues, turn off the amp immediately, you have an issue. Go back to your old tubes. If everything looks fine, put your voltmeter into the sockets and bring the bias voltage up. Re-check the bias voltage again in a few minutes, and again in a half hour. For the next few listening sessions, test the bias voltage at least once.

Enjoy, it's not too hard.

Thanks for your information. I will try them tonight.