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Old 15th November 2005, 08:54 AM   #1
nafunga is offline nafunga  United Kingdom
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Default 6bl7

Hi,
I recently built a little SE amp for my office using a single 6SN7 and 6BL7 for both channels - works very well. Decided to go to B9A size and am using a 6CG7 replacing the 6SN7 but need a substitute for the 6BL7 - any one know of a direct replacement please? Save me diving in to the data books!

Thanks,

David.
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Old 15th November 2005, 09:49 AM   #2
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I'm tempted to say 'if it ain't broke don't fix it' but there are alternatives. The most obvious that comes to mind is to use two 6S4A instead of the 6BL7. 6BL7 has a mu of 15, 6S4A a mu of 16.5. So that should be a good solution. You may lose a bit of power - the 6S4A is rated for 8w and the 6BL7 for 10w. Sound will be very good - it's a good sounding tube, and dirt cheap. There's also the 12A4 but the sound isn't so good. The 12b4 is an option at mu of 6.

there is another 9 pin solution - use a 6EW7. You have a small triode and a bigger output triode in one bottle, and the beefy section is 10w, though mu is only 6. The small triode is mu of 17.5. So you'd lose input sensitivity but this would be a space saving solution. Sounds good too. If you want to go down this road in octal there's the well used and respected 6EM7, where the small triode is mu of 60 so you get the sensitivity back. It's getting more expensive, and is cheaper in its 10EM7 and 13EM7 versions.
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Old 15th November 2005, 09:56 AM   #3
plovati is offline plovati  Italy
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I think the only similar double triode in noval pin is 7044.

For solution with dissimilar triodes in noval socket, the 6GF7 is the best candidate.

Can You post detail of your actual amplifier? Thank You.
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Old 15th November 2005, 01:41 PM   #4
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It is doubtful that you will find a 9 pin tube with the dissipation ratings of the larger 6BL7. There are however many twin triodes that can drive a speaker, depending on the power level required. Many of the twin triodes that were designed for early digital computers are also useful for amplifiers.

I have also used the 7044 with good success, but I have been using as a driver, not an output device. The 6463 may also work.

There are also (as previously mentioned) several dissimilar twin triodes designed for vertical output use. The high Mu triode drives the low Mu triode. This design works well and has been done by many different designers. Tubes that work well here are the 6DR7, 6DE7 and others.
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Old 15th November 2005, 04:29 PM   #5
nafunga is offline nafunga  United Kingdom
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Thanks to all who replied - I have some 7044's so will give them a go.

Schematic attached - nothing unusual! 6BL7 cathode R is 3.3K and 200uF cap. Did try using a doubled up 6BL7 but didn't sound so musical.

Thanks again.

David.
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