6SL7 in place of a 12AX7? - diyAudio
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Old 20th June 2007, 01:39 AM   #1
jarthel is offline jarthel  Australia
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Default 6SL7 in place of a 12AX7?

Eli Duttman has said that the 6SL7 is very close to 12AX7 except for the mu.

Also, people has used a 5751 in placed of a 1AX7 and this website (http://www.audiophool.cjb.net/Equiv_similar.htm) says 5751 is an equivalent to 6SL7.

any thoughts?

Thank you.
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Old 20th June 2007, 06:40 AM   #2
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My own opinion: 12AX7 is insufficiently linear. I would never use it. 6SL7 is a much better choice and the fact that its mu is only 70 compared to the 12AX7's mu of 100 should not matter in most applications.
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Old 20th June 2007, 07:19 AM   #3
cerrem is offline cerrem  United States
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12AX7 is not electricaly close to a 6SL7 spec wise...but can be substituted provided you watch your operating point and re-adjust the cathode bias only if need be..
Even though the mu is about 70 , the plate resistance is lower, thus making up for the lower mu when trying to achive similair gain...
For example lets say you have a 12AX7 100K plate resistor and 1K at the cathode bypassed... 300V B+.....
Plate resistance 65K...
Gain will be roughly 60 .....

Now replace with 6SL7....
Gain is about 49....
Replace load resistor with 150K and Gain = 54 ....
Adjust cathode bias accordingly if needed....
So in some cases it is close substitute.....
6SL7 sounds better anyway

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Old 20th June 2007, 08:06 AM   #4
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Originally posted by ray_moth
My own opinion: 12AX7 is insufficiently linear. I would never use it. 6SL7 is a much better choice and the fact that its mu is only 70 compared to the 12AX7's mu of 100 should not matter in most applications.
SY posted about a recent distortion measurement he did on ECC83's, and concluded that loaded with a CCS they were very linear.

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Old 20th June 2007, 11:30 AM   #5
Klimon is offline Klimon  Belgium
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Besides being thrashed by almost everyone except the generic amp producing companies I kinda like the 12ax7 (or atleast did as driver for el84 se and as two stage MM amp). 5751 and 6072 are variaties with higher esteem (and much higher price-card). I have no experience with 6sl7 though (I'll be using it in a amp that's being built).

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Old 20th June 2007, 11:35 PM   #6
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I also concur with 6SL7 choice - have used both 6SL7 and 12AX7.

But one needs to keep in mind that in low level use the 12AX7 is superior regarding hum (from heater) and noise, simply because of later design, also especially for audio use. Being smaller, it is less microphonic, more sturdy. I do not use 12AX7 in power amplifier designs, especially as (popular) input stages because of Miller-C effect on NFB stability. [Not asked, but in power amplifier use I really prefer the 12AT7 (ECC81) - but this expands the subject somewhat to include still other tubes.]
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