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Old 16th November 2008, 02:40 AM   #1
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Default 7-pin Low Rp tube

Does anyone have a suggestion for a 7 pin tube, either a triode or a triode strapped pentode, with an rp below 2K?
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Old 16th November 2008, 02:49 AM   #2
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6T4.  Great toob, it's like a power tube but with the smaller power rating (3.5W, which is plenty in small signal usage).

I have some experience with this type, if you're interested.  It's like a teensy 6V6 / 6AQ5, without the microphonicity.

Aloha,

Poinz


edit; you'd think I could spell by now.
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Old 16th November 2008, 05:52 AM   #3
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Great, right. I was thinking it had a 5K rp.

Thanks for the offer, but I have a pretty good stash.
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Old 16th November 2008, 03:58 PM   #4
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No, no.  I'm operating at ~70V 20mA, and getting about 1.6K or 1.8K.  If your operate at low volts and max (30mA) current, you can get down to ~1.3-1.4K.

Good tube for drive.

Aloha,

Poinz

What brand of 6T4 do you have?
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Old 16th November 2008, 06:41 PM   #5
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I think mine are all Westinghouse.
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Old 17th November 2008, 02:13 AM   #6
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Interesting.  All mine are RCA.  I asked Jack what he had, and the RCA seemed like the most reliable suspect.  I got about 170 of 'em.  Makes for very nice pairs and quads.

Aloha,

Poinz
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Old 17th November 2008, 06:16 PM   #7
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hey-Hey!!!,
6AF4 meets your spec...though it is more a signal amp than a power tube.
cheers,
Douglas
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Old 18th November 2008, 01:57 AM   #8
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It looks like a pretty much exact equivalent to the 6T4, Douglas.  Same specs and ratings all around (including the very restrictive Vh-k rating), both are applied 'UHF oscillator' .  I much suspect that they were simply developed by different companies.

When I initially went looking for a new lineamp device, I went through the manuals, and there were at least three or four of this almost exact type (sometimes differing in pinout).  I gave the list to my vendors, and the 6T4 was the one that came up available.  Love it when Jack sez, 'We're holding that type; how many hundred you want?'

Aloha,

Poinz
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Old 18th November 2008, 09:24 AM   #9
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If you wanted something more linear than the 6T4, you could try the GEC Marconi A2134, a 7-pin pentode. Connected as a triode it has an anode impedance of 835 ohms and a mu of 10. You can see the triode curves on Frank's tube pages. The only problem is how to get hold of them in the USA. You can find them at radio rallies here in the UK but I suspect dealers would want a lot for them.
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