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6SL7

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Posted on April 12, 2017 at 05:14:23
Laudanum
Audiophile

Posts: 1031
Joined: March 1, 2003
 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.

 

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RE: 6SL7, posted on April 12, 2017 at 08:08:17
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002

"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"

 

RE: 6SL7, posted on April 12, 2017 at 09:11:19
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

RE: 6SL7, posted on April 12, 2017 at 10:20:38
Triode_Kingdom
Audiophile

Posts: 10047
Location: Central Texas
Joined: September 24, 2006
 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.





 

RE: 6SL7, posted on April 12, 2017 at 10:57:06
Laudanum
Audiophile

Posts: 1031
Joined: March 1, 2003
 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

 

RE: 6SL7, posted on April 12, 2017 at 14:07:15
Laudanum
Audiophile

Posts: 1031
Joined: March 1, 2003
 Interesting. A Loctal socket may drop into the existing Octal hole. Maybe I'll pick up a couple 7F7's and buy or build an adapter. If it works out, I'll swap sockets.

Thanks !

 

RE: 6SL7 there are sellers who test for microphony, posted on April 13, 2017 at 07:23:25
Mechans
Audiophile

Posts: 1804
Location: East Coast
Joined: May 23, 2004
 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.

 

6188 are getting expensive., posted on April 13, 2017 at 11:52:58
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 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.

 

RE: 6188 are getting expensive., posted on April 13, 2017 at 17:46:14
Mechans
Audiophile

Posts: 1804
Location: East Coast
Joined: May 23, 2004
 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.

 

RE: 6188 are getting expensive., posted on April 13, 2017 at 19:17:36
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 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.

 

RE: 6SL7 there are sellers who test for microphony, posted on April 15, 2017 at 15:02:53
Laudanum
Audiophile

Posts: 1031
Joined: March 1, 2003
 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.

 

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