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Old 29th November 2006, 02:56 PM   #1
JoshK is offline JoshK  Canada
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Default 6F8G's any good?

I recently was browsing some notes on tubes and I came across the 6F8G. I read in the spec sheet that its sections are very similar to the 6j5 apart from interelectrode capacitances. I have also been reading Morgan Jone's chapter on tube (valve) selection and the analysis he did on the *sn7/*n7 family.

I am wondering how the 6F8G ranks among its peers. It certainly has cool factor written all over it. I thought it was strange that according to what I can tell from its spec sheet that one grid is hooked to the grid cap and another is to pin 5. That seems strange to me. Does this make one triode any different than the other, via interelec cap?

I was wondering if anyone has any experience with it, tested it or compared with the *sn7 family?

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Old 29th November 2006, 04:40 PM   #2
kevinkr is offline kevinkr  United States
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I looked it over and according to the data sheet it is basically a pair of 6J5 in a single envelope - other than the obvious grid cap issue I suspect it would perform very much like a 6SN7. Good find I'd say.. I'll bet it is not too common though, I haven't run across many in my travels, but not being the current tube-du-jour it is probably pretty inexpensive too.

On the differences in inter-electrode capacitances I would expect that the differences would be swamped by external capacitances anyway, and the one that counts most Cgp (miller capacitance) is the same in both sections.

I'd certainly use it..
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 30th November 2006, 12:15 AM   #3
GRD is offline GRD  United States
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I built a VTV linestage using 6F8G's instead of 6SN7's for 2 reasons. First, a lot of people praise the 6SN7's that are quite pricey. The 6F8G's are basically old stock 6SN7's that are lower priced. So, you can get round plates, etc. and see if they are indeed the holy grail. Second, and equally important to me was they looked cool. I haven't compared to any other 6SN7 so I can't comment on how they stack up. The other SN7 oddity is the 2C22 - a rabbit ears 6J5. And then there are the 7N7's - a 6SN7 in the local socket. The tube limits are a little different than some of the 6SN7's, so see the datasheet.

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Old 30th November 2006, 02:04 PM   #4
JoshK is offline JoshK  Canada
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What prompted this obviousy was Morgan Jones's discussion on the *sn7 family and how good they were. I had always liked the sound of octal line stages I had heard over other tube line stages (not controlling for circuit obviously).

I came across some old 6f8g's and wondered if they were good, and of course, they look damn cool.
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Old 30th November 2006, 03:28 PM   #5
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I built a PP 807 amp a while back for a friend and I used 6F8G tubes in place of 6SN7s and it sounds great. 6C8G is another variant and sound nice in linestages. 2C22 tubes as already mentioned also work great.

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Old 30th November 2006, 03:49 PM   #6
GRD is offline GRD  United States
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One observation on the 6F8G - They can be somewhat microphonic. I ended up using Herbie's tube dampers which fixed any problems. I think it might be related to the size/shape of the bottle.
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