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6SN7/6SL7 swap...

knockbill

knockbill

Lunatic Member
Are there any wiring/component changes that should/need to be made in a Maggy 175 circuit to use 6SN7's instead of 6SL7's? I realize N7s need twice the heater current....
Tried them both, and other than gain, they both sound pretty good, and the amp doesn't need the extra gain...
Just curious, as I've read that just because certain families of drivers are similar, they aren't necessarily compatible with the same circuit....
Plus, I have a lot more 6SN7's!!

Regards,
John
 
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6SN7 is equivalent to a 12AU7

6SL7 is equivalent to a 12AX7

If your 6SN7's have enough gain for you, then use them. The 6SL7 is the one I would use because they are the equivalent to the original 12AX7 (or 6EU7) that the amplifier normally uses.
 
cademan said:
6SN7 is equivalent to a 12AU7

6SL7 is equivalent to a 12AX7

If your 6SN7's have enough gain for you, then use them. The 6SL7 is the one I would use because they are the equivalent to the original 12AX7 (or 6EU7) that the amplifier normally uses.
Click to expand...

Understood...
Question is, do any components need to be changed/adjusted to make teh swap?
Or more specifically, to optimize teh swap....

Regards,
John
 
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The 6SN7 have less gain , and you might have to change the feedback resistors to have enough for the paraphase inverter to work right .
 
cademan said:
Except for heater current, no!
Click to expand...

Can you get away with it? Probably.

Will it be optimally biased? Surely not.

Typical operating points for a 6SL7:

Grid -2V
µ = 70
Plate resistance: 44K

For the 6SN7:

Grid: -12V
µ = 20
Plate resistance = 7K

Surely more distortion, different FB characteristics, etc.

Not recommended without circuit redesign!
 
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BuzzK said:
Can you get away with it? Probably.

Will it be optimally biased? Surely not.

Typical operating points for a 6SL7:

Grid -2V
µ = 70
Plate resistance: 44K

For the 6SN7:

Grid: -12V
µ = 20
Plate resistance = 7K

Surely more distortion, different FB characteristics, etc.

Not recommended!
Click to expand...

Thanks!
I saw the differences in the RCA manual, just didn't know how to apply them... I figured they couldn't just swap....
Didn't hear the distortion, guess it would show up on a scope, tho...
Just trying to find a use for the 6SN7's...

Regards,
John
 
BuzzK said:
Can you get away with it? Probably.

Will it be optimally biased? Surely not.

Typical operating points for a 6SL7:

Grid -2V
µ = 70
Plate resistance: 44K

For the 6SN7:

Grid: -12V
µ = 20
Plate resistance = 7K

Surely more distortion, different FB characteristics, etc.

Not recommended!
Click to expand...

I also know the numbers so this is why I said it would work. He'll be fine! I have always used a 6SL7 in place of a 12AX7 with no distortion and the voltage is only off by about 2 volts plate voltage. May not be optimum, but it will work. I recommend it.

You might call me a an experimental type but I would definitely not recommend anything that was going to smoke or damage your amp. I would buy it first.

Operating characteristics are only guide lines.
 
knockbill said:
Didn't hear the distortion, guess it would show up on a scope, tho...
Just trying to find a use for the 6SN7's...

Regards,
John
Click to expand...

See, you didn't hear anything. These amplifier are bullet proof! Now I would not say this for some other type of amplifier such as one which had manual bias.
 
battradio said:
you might have to change the feedback resistors to have enough for the paraphase inverter to work right .
Click to expand...

That wouldn't be a deal breaker,,, Up or down in resistance?
Is it worth experimenting with, or will it cause more harm than good?
Just thinking out loud...
I got a lot more to learn, I think!!

Regards,
John
 
OP says he is substituting a 6SN7 for a 6SL7. That's sort of like swapping a 12AU7 in place of a 5751. 6SL7 and 12AX7 are closer in characteristics compared to 6SN7/6SL7.

Operating characteristics are only guidelines? Yes, I and I suppose that physics is only a guideline for describing how the universe works?
 
BuzzK said:
OP says he is substituting a 6SN7 for a 6SL7. That's sort of like swapping a 12AU7 in place of a 5751. 6SL7 and 12AX7 are closer in characteristics compared to 6SN7/6SL7.

Operating characteristics are only guidelines? Yes, I and I suppose that physics is only a guideline for describing how the universe works?
Click to expand...

That's not what I meant! Hey, I use those guidelines as much as you do so it doesn't do you much good..............whatever! Guess I'll have to be very specific in terms of characteristics and guidelines in the future.
 
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cademan said:
See, you didn't hear anything. These amplifier are bullet proof!
Click to expand...

You might not "hear anything" at a normal (low) listening level, but that doesn't mean the distortion figures aren't changed.

Where it might really show up is if you drove the driver / splitter stage at a high level.

For example, I had an Eico ST-70 that I used constant current sources on the LTP drivers, set to 6.6 mA. Worked great until I drove it, and then the driver tubes CUT OUT at the higher levels.

Since you say you use plate curves as much as I do (interesting: I don't believe we've met or even corresponded much), take an operating point for the 6SL7 and try plotting the 6SN7 at a comparable operating current and share what you get.

Buzz
 
I have to weigh in here! Those Maggie amps have been modified more than any other amplifier, plus they are a dime a dozen. I think a 6SL7 would be ok in place of a 12AX7 on this particular amplifier but not with something else that has an extensive feedback circuit. Put a scope on it and yeah, big difference. Your ear, not so big difference and it won't harm anything.

I have maybe 5 different papers which give different operating characteristics for many tubes, but I use my 1958 RCA tube manual because this is when a lot of the Magnavox tube amps were made.
 
I don't want to beat this horse to death, but I don't think the issue is subbing a 6SL7 for a 12AX7 -- it's subbing a 6SN7 for the 6SL7.
 
BuzzK said:
(interesting: I don't believe we've met or even corresponded much)
Click to expand...

You're right, we haven't met! I am only going by what numbers you posted in post #6. I go by those too but as a rough guideline. We're talking a cheap magnavox amp here, not a Carver or Fisher tube amplifier and such.
 
BuzzK said:
I don't want to beat this horse to death, but I don't think the issue is subbing a 6SL7 for a 12AX7 -- it's subbing a 6SN7 for the 6SL7.
Click to expand...

There ya go! I was talking subbing a 6SL7 for a 12AX7. We're just on the wrong page or maybe you didn't see it the same way I did!
 
Thanks for the info...
I built (building)a Maggy 175, with 6SL7 drivers, on an old Moto chassis, so, its a new build, but other than the drivers, a modified Moto PS & balanced channels, I stayed with the Mag schem...
I did try it with 6SN7's, and other than less gain, I can't hear any difference either, distortion is nil,,, I actually like it better with the 6SN7's, and less gain... That's what the preamp is for!
Just don't want to have problems or burn something up down the road...

Seems there's a disagreement here, and, I don't have enough experience to sort it out...
Regards,
John
 
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