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Hi all.
I have 4 6SL7 tubes that I keep on hand for the preamp of a Valco style SE guitar amp. I know they have a reputation for being microphonic, and they all are, except for 1. I have a CBS Hytron which is the only one of the bunch that isn't microphonic. Two of the tubes are JAN tubes, both Sylvanias I think. And the other is an RCA. All 3 of the microphonic tubes have the longer plates. The CBS Hytron is a short plate tube.
To get to my question ... in general, is there a better chance that the short plate 6SL7's wont be microphonic? I picked up a short plate, brown base Sylvania WGT on ebay just to test the theory. But it was cheap and eabay is a crap shoot. If it works, great. But I'm wondering if there are any generalizations that can be made regarding the plate type/length and microphonics?
Also, ran across quite a few GE chrome plate tubes that had large but shortish plates that looked like they may be less prone to microphonics. But that's just a wild guess.
As far as tone goes, it really doesn't matter much in this amp. They all sound a little different but the amp isn't played clean and they all seem to do the overdrive just fine so I'm not after the tubes with the best reputation for tone nor the price they demand. Just hoping to find a couple inexpensive spares that aren't super microphonic.
Thanks for any advice you can share.
Follow Ups:
I know a dealer that tests for microphy. Thre is a video demonstrating test method. Look up Brent Jessee at audiotubes.net {no affiliation at all etc.)
Personally I don't think microphonics is bad if the gear is stationary and you have to hit the tube to hear it. If microphonic at rest obviously that's out.
I would agree with that. The issue is the speaker output being enough to excite the tube and then the microphonics feeding back through the speaker. Even with the amp being located off of the cabinet.
At any rate, I got a small plate Sylvania WGT and it's OK. It's not a new tube so hopefully it will stay quiet enough. It sounds good too. I'll probably pick up a couple more of the same type.
Thanks.
I think it's a simple matter of physics. The more mass and/or length the structure has, the more likely it will vibrate at audible frequencies. The stiffness of the support structure obviously affects this too.
--------------------------
Buy Chinese. Bury freedom.
Consider making adapters from Loctal sockets and Octal plugs, to hold 7F7s. The 7F7 is electrically equivalent to the 6SL7. Forget about branding, Loctal tubes were made by Sylvania at Emporium, PA. In any given time frame, Sylvania used the same "cages" in both 6SL7s and 7N7s. However, Loctal tubes have glass button bases, like the 7 and 9 pin mini types, and they were intended for mobile service, with the associated vibration.
Eli D.
"in general, is there a better chance that the short plate 6SL7's wont be microphonic?"
Yes, in general the smaller the plate area the less likely the tube will be microphonic.
Of the "normal" type 6sl7 "round" plate tubes, I find the short bottle GE 6sl7 to be the least microphonic.
You might look for the 6188, shown above, with ladder type box plates kind of like a 12ax7.
Those should be even less microphonic.
There is also a 6sl7 with real short black plates. See link.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Almost as much as 5691 which also should be non microphonic. The 5691 tube was built for rockets so it could withstand something like 200Gs.
I thought the 5691 and 5692 were built mainly by RCA as a long lived tube but did not have the incredible internals of a Bendix with their ceramic spacers eg and Nonex glass etc.
Actually 5691 and 92 were made mostly by GE for RCA. I guess they had the excess jigs to make them. And not made to the standard of the Bendix tubes, but way above the needs for an ordinary 6sl7 with 10,000 hour lifespan if recommended specs were followed.
Yes, a Sylvania with the plate type in your link (the one on the right) is the type I purchased as a test. Don't have it yet to try.Also interesting that the structure of the CBS Hytron that I have that is not microphonic is very much like the 6188 that you pictured. Of course, that could just be luck.
Thanks
Edits: 04/12/17
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