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Old 30th December 2008, 12:36 AM   #1
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Default 6sf5 = 1/2 12ax7?

From what I can gather from the data sheets, the 6SF5 seems to be 1/2 of a 12ax7. I am rebuilding a bogen made challenger CH-18, and it has two 6SF5's in the preamp. I would replace them with a single 12ax7, but I think it would be cool to build an all octal fender Deluxe 5E3, and 6SF5GT's are $4 NOS.

Are these close enough to call equivalents?
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Old 30th December 2008, 12:44 AM   #2
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Meh, why not? I dunno about noise, hum or microphonics, but I'd give it a shot

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Old 30th December 2008, 02:15 AM   #3
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yeah I was worried about that too, but shouldn't the glass variants be slightly better?
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Old 30th December 2008, 10:37 AM   #4
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It has nothing to do with the envelope, but internal structure.

If you're going to be operating it in a listening room, on its own shelf with a foam (el-cheapo foam is OK) pad, no worries for microphonics.

I always recommend that people just use a tube they have, breadboard it and if they like the sound and noise level, run with it.

IMO, the 12AX7 has its uses, but is WAYYYYY over rated (I like 5751's a zillion times better ).

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Old 30th December 2008, 07:42 PM   #5
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If I wanted word to get around I'ld say the 6sf5gt is a spectacularly linear tube which into a follower swings obscene voltages with low distortion, shows almost no visible harmonics above the third, and if I recall saw early duty in microphone preamps. Since I don't I'll say it sounds like a choke fed reversed biased diode.
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Old 17th April 2009, 02:20 AM   #6
mp9 is offline mp9  United States
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rdf, do you have any preference to brands of 6SF5GT such as arcturus, philco, ge, raytheon, ...?
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Old 17th April 2009, 04:19 AM   #7
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No, sorry. The only brand I've tested to date is Sylvania.
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Old 17th April 2009, 05:06 AM   #8
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The 6SF5 and 12AX7 look to be very similar. The 12AX7 runs a bit higher g(m), and a bit lower in r(p). It doesn't look to be a big difference, and VT circuits are quite "forgiving". You can check 'em out at Frank's.

If you decide in favour of 6SF5s, remember that you don't want the cathode to be more negative than the metal shell, otherwise, the can will act like another anode and throw off your cathode bias resistor calculations and/or lead to other forms of nastiness.
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