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QUESTION about Installing "Matched" Tubes

Hey folks,

I have Conrad Johnson Premier 12 monoblock tube amps, 140W/side. And I’m replacing the old 6550 output tubes.It’s 4 6550 tubes per monoblock.


I usually just replace the tubes with whatever CJ recommends a the time. But this time I splurged for Svetlana Winged C NOS tubes. I bought two pairs of matched quads.


My question is: How do I go about placing them in the amps? Is it required, or best, to put each matched quad in each amp? Or does it matter which monoblock all the tubes go in?


I’m asking because at the moment I’m not sure which tubes are actually matched. They were measured before shipping, with the Plate Current and Transconductance number’s written on each tube. But frankly, not being an electronics guy, I don’t really know what they mean and how it helps me match the tubes, if they do.


So for instance, there are numbers on each tube:



46.8/4740
42.2/4410

41.4/4870
38.8/3970

42.8/4830
39.5/4470
41.3/4810

41.9/4770

Do I, for instance, try to find the 4 tubes in which the Plate Current numbers are closest in value? Or the Transconductance number?


NOTE: My CJ amp allows for user biasing of each 6550 output tube!


Thanks for any info!
prof

Showing 4 responses by atmasphere

atmasphere's avatar
atmasphere
11,566 posts
 
 
Does that not indicate working tubes to begin with?
Of course! But that is very different from saying that they won't arc right after shipping. Its like I said- if you make it two weeks after installation and you don't have a failure you'll probably be alright.
atmasphere's avatar
atmasphere
11,566 posts
 
The burning smell is likely either a cathode or screen resistor.

I suspect this may not be the amp's fault- the tube itself might have simply arced. They are most likely to do that right after shipping. It does not matter that the tube is brand-new- it was likely shipped untested from the factory.


Generally speaking when buying brand new power tubes, if you make it past about two weeks of normal operation they'll be OK. I would send the tube back and get a replacement since they do have a 90 day warranty.
atmasphere's avatar
atmasphere
11,566 posts
 
Yes
atmasphere's avatar
atmasphere
11,566 posts
 
I go with the transconductance value if I can. Plate current works too but IME is less accurate.

You'll have to bias the tubes up anyway, but if they all show the same transconductance values the bias setting will be nearly the same on all of them. But the thing is, what the tube is doing at half and full power is important too. That is why I go with transconductance.
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