2. amps-preamps
  3. 1053108423

upgrade tube for SF line 2

I am new in tube amp, I just pickup a used SF line 2 a month ago. They use soviet 6922, manufacture @ 03/2000. I heard changing the tube will make a big different. Can anyone recommend some brand and model? I do some research and find siemens CCa is a of the best. however, it is hard to find and the price is high. Some body told me I can just change the two tubes in the back and leave the front four as is. that can save some money. Should I change all six to some good NOS tube (but not the best) or should I put all the money in the two that is in the back? If I change just two in the back. In the further will I get benefic by changing another four? One last question, how about bias? Do I need to re-bias them?
waip7

Showing 1 response by kana813

kana813
956 posts
 
As Lafish points out, if you are using single eneded RCA outputs the bias trimpot position doesn't matter, but for XLR balanced outputs you should adjust the bias after replacing any of the tubes. The easiest way is as follows:

1. Disconnect cable from preamp to power amp.

2. With the preamp warmed up for 20 minutes, play a 1 kHz sine wave from a test CD or signal generator.

3. Use a voltmeter set to AC, and measure voltage from pin 1 to pin 2 on left channel XLR output.

4. Adjust volume on preamp so voltage output is approximately 1V.

5. Measure voltage from pin 1 to pin 3.

6. Adjust left phase balance pot until pin 1-2 voltage equals pin 1-3 voltage - check 1-2 after adjusting 1-3 and adjust again as necessary.

7. Repeat for right channel.

The bias tool that comes with SF Power series amps
is perfect for adjusting the pots, otherwise you'll need a long shank small blade screwdriver.
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