2 Quad EL34 sets for Dynaco ST-70

I am new to tube amp and I really need your help. I have one current quad Russian EL84 running in ST70. My friend gave me another brand new quad Mullard EL34 Russian tubes for me to test sound. What is the best way to remove old quad tubes out and replace new quad tubes? If I do not like new one, I also can put my old tubes back.
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.
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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.
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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
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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.
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