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Old 13th November 2012, 09:45 PM   #1
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Default 5751 Mil spec tubes vrs 12AX7

Just wondering if anyone has used a 5751 Large mica spacers and extra support rods Mil spec tube as an replacement for a 12AX7-I understand that they have only a 70% gain instead of 100%. Plate resistance is diffrent also. Just wondering on how they sound or perform?
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Old 13th November 2012, 10:00 PM   #2
freax is offline freax  Australia
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dunno but I do know I love the sound of the 12AU7 which has lower gain than the 12AX7...

I know my unique red top (painted) sounds warm and soft and cuddly like a warm baby blanket.

has a lower noise floor too.

Come to think of it I can imagine that everything sounds better than a 12AX7...
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Last edited by freax; 13th November 2012 at 10:06 PM.
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Old 13th November 2012, 10:29 PM   #3
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Default 5751 vs 12AX7

The 5751 are nice... I like them better in many circuits than the 12AX7.
I just swap em in, theyre close enough so it just works fine.
This is any easy tweak well worth trying.
Max gain is 70 as opposed to 100 for 12AX7.
Of course actual gain depends on the circiut.
Its never 100 in real life, thats a theoretical value which you get with a perfect current source etc. A practical value is probably something like 50-70
Also current capability with 12AX7 is around 1mA and for 5751 about 0,7mA.

Bottom line is, if you can live with a little bit less amplification (turn up the volume a notch or two) and its not supposed to drive a difficult load, you will probably end up with a somewhat better sound with a little less distortion.
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Old 14th November 2012, 12:55 AM   #4
kmaier is online now kmaier  United States
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5751 is a premium tube, commercial grade and not identical to the 12AX7, slightly different spec and heater current is a bit higher. Some manufacturers made broadcast grade versions, RCA had their Command Series and GE had their 5-star series, etc. They usually employed more robust construction, additional support wires and/or heavier support wires, thicker micas and in some cases a third mica support on top. The 5814(a) is the commercial grade 12AU7 (with higher heater current), the 6201 for the 12AT7 and the 6072 (also higher heater current) for the 12AY7 (albeit not identical)... and no, not anything sounds better than a 12AX7, unless you're not using it right (goes for all tubes actually). Perhaps a 7025 or 6681 would sound better ;-)

Regards, KM
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Old 14th November 2012, 03:49 AM   #5
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I've used them. There ok. Sometimes they work really well in 12at7 and 12ax7 circuits. Sometimes there harsh in the phase splitter position. But it does depend on the circuit, and how well they have been treated during storage.
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