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What is involved in proper tube matching?I pulled the EL34's in my ST70, checked the bias voltage on each, one at a time in the same socket. One was much higher than the others - I set the bias pot to give me .78 volts on that tube. The three remaining read something like .44, .46 and .49.
Is measuring the bias difference in this way meaningful? If I find a tube tester, what do I want to look for in matching these tubes?
Can I get information from a tube tester that would allow me to search for another original Mullard EL34 that would match, without having it in hand?
Thanks much for any information. BTW I'm around Berkeley CA if anyone knows where I might go to test tubes...
Mark M.
"Politics is the entertainment branch of industry" - FZ
Follow Ups:
Power tubes have several concerns.
First, you must make sure that the cathode can emmit enough current, per tube's spec for max current. That's a very basic test that cheap emmision testers perform.
For tubes that run in parallel or push-pull, you must also match the gain curves THROUGHOUT the operating points of the stage. Easier said than done.
Mutual conductance testers only tell you one operating point!
It's like saying a car engine is great after hearing it at idle only... what about the rest of its operating range?!
There are some computerized testers that could help you with curve matching, and those are the right instruments to get, if you are VERY serious about your equipment.
Of course, most manufacturers would tell you that the negative feedback fixes all these issues. To an extent, that is correct, but logically you can imagine different scenarios where one tube carries significantly more current than the other (like in your amp!). Does that seem like a "healthy" situation?!
You be the judge...
Do any of the retailers offer this level of matching?
"Politics is the entertainment branch of industry" - FZ
Well, if you send me your e-mail address I might be able to help you out in testing your tubes. I'm in Mountain View.
CW
mmmalmberg@comcast (dot) net
"Politics is the entertainment branch of industry" - FZ
Ooops - forgot my address!
SAT,
cw
Check out Bill Thomas' post on tube matching. Sounds like you did it correctly. The difference in tubes is quite significant. Maybe you should try another socket just to confirm or look for a tech with a good tester.You really only have to match 2 pairs not all 4. The readings you received on the other three are close. When I bias my ST70 the bias readings are not constant, they go up and down slightly.
It would be hard to find a matching Mullard, unless you were able to test that tube with your others. Different tube testers will give different readings.
David
Sounds like a good band name...I was thinking I could probably call the two lowest ones a match and then find a match for one of the other two. I see what you mean about different testers - thanks David.
"Politics is the entertainment branch of industry" - FZ
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