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I have a problem and need some advice about 6SN7s. (Long.)

Posted by jeffreybehr (A) on November 14, 2006 at 23:50:11

I have dozens of 6SN7s that may or may not be microfonic.* These ‘N7s have reputations for being very-good to excellent--Ken-Rad staggerplates and parallelplates, Tung-Sol grayplate MM-ears, new ElectroHarmonixs selected for now noise and low microfonics by the retailer, and Tung-Sol roundplates. All these were purchased, generally thru eBay and both used and NOS or close, for use in Antique Sound Lab Hurricanes.

My plight started when I replaced my rather-insensitive (83dB-rated) Eminent Technology LFT-8s with homemade line-array systems with a sensitivity of around 97dB. Suddenly my ‘Canes were making noises I never heard before. I started swapping front-end tubes and discovered that some hummed and some produced ringing or rattling sounds thru the speakers as I tapped them. I quickly discovered that one amp is MUCH more-susceptible to microfonic tubes than the other, and since I was busy building speakers and pursuing new ASL SETs to replace the ‘Canes, I took the ‘Canes out of the system.

In came the new ASL 1006/845s, each of which uses 2 ‘N7s in the frontend and one in the regulator for the frontend. I ran the 5-dozen ‘N7s thru these amps, culling tubes into 3 piles--those silent or virtually so in the frontend-gain position V1, those moderately microfonic in that V1 position, and those quite microfonic. Subsequently I discovered that many of the 2nd group were silent in position V2 and that many in the 3rd group (and all in the 2nd) were silent in and could be used in the regulator position.

So of 60 ‘N7s including 11 roundplates, 7 MM-ears, 6 EHs, and many-dozen KRs, I have 13 (!) tubes in group 1, about a dozen in group 2, and about 4 dozen in group 3. ALL in group 3 are usable in the regulator position, but in my lifetime, I’ll use maybe a dozen.

So what do I do with these ‘microfonic’ tubes? Each was purchased without any guarantee so they’re all mine, and my conscience won’t let me sell them even with a not-DOA guarantee and a statement that they sounded just fine in my ‘Canes with ET8s.

Anyone interested in trying these in their amps? Since we know they’re suspect, I’d price them quite low--say $4 each for the 3 TS-MM-ears, $8 each for the KRs (20 staggerplates and 4 parallelplates), and $25 each for the SEVEN roundplates (which cost me probably over $700). I’ll sell these tubes only to those who will promise NOT to peddle these as good tubes if they ever try to sell them.

What do you think?

BTW I do use Herbies dampers on them and doing so reduces microfonics substantially, but their use won’t turn a group-3 tube into one usable in V1 position.
.

* I define ‘microfonic’ as passing noise into the audible signal when tapped litely by a pencil. AFAIK, I’ve NEVER noticed the effects of microfonic tubes in the music signal.

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Tin-eared audiofool and obsessed landscape fotografer.
http://community.webshots.com/user/jeffreybehr

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