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In Reply to: RE: 6SN-7's posted by throwback on January 04, 2015 at 20:24:09
Good advice, guys. Thanks. But I am still curious as to whether I would need to replace the whole string or whether there would be any benefit to replacing just some of them.
Anyway, I'm off to CES today in Vegas. Unfortunately, the THE show has abandoned that particular ship for Newport, so fewer crops to harvest this year.
Cheers
Follow Ups:
I have MA-1 Silver Editions, and have not found an audible difference when replacing a single bad 6AS7 vs a pair. It's much more important to focus on the 6SN7s in the cascode circuit.
I can't stress strongly enough how important a great tube tester is. The Amplitrex allows us to test in both fixed bias, and continuous bias mode. In continuous you get a true reading of how a tube behaves, especially as it warms up.
You start by testing in pairs. Look to see how the left and right banks age. Upon completion you total the readings from each bank. Don't just total GM. Voltage V is just as important. The first thing you'll notice, over time, the banks equalize themselves. What a great amplifier the MA-1 is.
Since your strongest 6AS7's do more work, they age a little quicker. After time things start to balance. Pay attention to have the same number of newer stronger tubes per bank.
I have never replaced all the tubes at once. Be careful to add up the totals of each banks tubes. It is preferable not to have one side stronger than the other. Placement in the bank appears not to make a difference.
Years ago Jack Williams commented that while GM is important, make sure the V (for voltage) is also well matched. Discard any run away tubes. These are the one's that just keep rising while under testing. I'll guess that I encounter one out of every twenty I test.
Chuck, You wrote, "I am still curious as to whether I would need to replace the whole string or whether there would be any benefit to replacing just some of them." The answer depends upon what "benefit" you seek. If the existing output tubes are nearing the end of their useful lives, you'd best replace them all in one fell swoop. If you perceive that one tube or more than one is noisy, then you'd need to do some experimentation to identify the culprits and replace them specifically. But to the general notion that replacing one pair of tubes at random in a 14-tube output stage would have some aural benefit, like bypassing an electrolytic capacitor with a good film capacitor, nix on that, IMO.
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