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.

What the bias measures BEFORE you remove the tubes doesn't matter. The correct bias adjustment for the ST-70 is 1.56 Volts. Be certain that's what you end up with, after all the other steps mentioned by Viridian are followed. Also: The bias needs to stabilize for 15 minutes to 1/2 hour(let the amp warm up) before testing/adjusting the bias(every time). One more thing: Each channel will fluctuate according to adjustments made to the other. Don't just set one, then the other and forget it. Check/adjust back and forth, until they both read 1.56 Volts. Happy listening!

Thanks for all your help. I did not try to replace all tubes yet as I want to make sure what I am doing. Old tubes still sound good but I want to test Mullard Russian tubes. I want to have procedure to replace tubes for my future reference replacement.
1. Do I have to removed 4 old tubes or one side first and set biaset for one replacing channel.
2. Power off and remove tubes. Make sure amp hooked up with speakers. Turn biaset down (counter clockwise). Put new tubes set. Turn on power. Set DC on DMV, one test point to line 1.56V and other to ground as screw on side panel. Turn left and right knob to get 1.56V on DMV for 2 channels. The bias needs to stabilize for 15 minutes to 1/2 hour. Do I have to test with sound or pre-amp must off for stable. When listening to music, can I check the biaset for both channels.

At the risk of opening a horrible can of worms, when the Dyna was designed, the tube of choice was the Mullard EL-34, though many owners used the GE 6CA7. These were quite different in terms of build quality than the stuff that we currently get from Eastern Europe and China. Even though the new tube says "Mullard" it is no more a Mullard than that Rolex that you bought on your last trip to Hong Kong is a Rolex. The new tubes are much more frail than their forebears, even though many sound quite good. The Siemens that you had mentioned earlier are quite frail as well. I think that a lower bias voltage is appropriate for these tubes in the Dyna. Perhaps 1.4 or 1.3V. How can you determine what will work best for you? Well, the lower the voltage is, the longer the tube will last but, at some point, the sound suffers. So if you can't hear any difference biasing at 1.3 than you can at 1.56, go for it. If you feel that 1.56 sounds best and don't mind a little shorter tube life, then that is best for you. Interstingly, the 1.56V was arrived at because the inexpensive volt meters of the day were not particularly accurate and a battery put out precisely 1.56V. So all the user had to do was measure a battery with the voltmeter and bring the needle up to exactly the same place with the bias control. And now back to your regularly scheduled program, already in progress..................

Thanks for all your information. I did change 4 Russian Mullard El84 tubes. Sound is much better than Svetlana tubes. Tubes need more time to burn in. I set biaset around 1.40V for both channels. Less money but sound is much better than solid state costing more than $1500. Klipschorn sings better than before. I did try Siemens EL34 tubes but sound is not much better than Russian EL34. For me, system matches speaker, amp, and pre-amp. Expensive systems do not mean sound better just look and brand name. For amp connection, I found better right adapter for high end RCA. I want to keep original connection. radioshack.com/produc/index.jsp?productId=2102594

Sag, which Klipsch are you using; I use the original Fortes and like them quite a lot. I have been thinking of moving up to Cornwalls? The LaScala's are not for me, but a pair of Altec Valencias may be in my future.

I currently use Klipschorn with Dynaco St70 and PAS 3. My friend has Altec Valencia which sound is good with mid-range. I used to have Cornwalls with sound is good but La Scala is much better with vocal for Jazz.

Nice rig there. The La Scala's just don't do it in the bass for me. I enjoy full orchestral music, rock and dance, as well as jazz and bluegrass so I need a kind of all around speaker.