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Old 20th September 2011, 01:59 PM   #1
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Default Tube Newb: 12sn7 and 12sl7 interchangeable?

picked up what appears to be a custom made tube amp from an estate sale, tubes were in a separate box, the tube complement is (4) 6bg4, (2) 12sn7, (2) 12sl7, (1) 5u4g... unfortunately there are no markings on the amp indicating which tube is associated with which socket, I know enough to understand where the rectifier and 6bg4s fit but I'm not sure about the12sn7 and 12sl7 tubes (input stage vs. driver stage). I understand from doing some research that the plate resistance is different on these tubes. Given that, I'm assuming they are not interchangeable? or are they? if not, what is the best way to determine which goes where?

appreciate any guidance...
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Old 20th September 2011, 02:06 PM   #2
DF96 is offline DF96  England
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I think they have the same pinout, and although different they are not so different that harm will come from getting them wrong. It just won't work anything like so well. My guess is that SL will be input and SN will be driver.
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Old 20th September 2011, 02:35 PM   #3
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Based on my reading they do appear to have the same pin layout... so the suggestion is to use trial and error and use the configuration that sounds better? I wanted to be sure before I brought the amp up on a borrowed variac...
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Old 20th September 2011, 02:47 PM   #4
DF96 is offline DF96  England
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Is this hi-fi or guitar? With two pairs of sockets and two pairs of valves there is one way of getting it right, and five ways of getting it wrong. Fortunately, four of the five wrong ways will probably leave the two channels sounding different from each other or having different gain.
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Old 20th September 2011, 08:08 PM   #5
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Quote:
what is the best way to determine which goes where?
It should be quite simple task. Measure or look the size of anode resistors.
For 12SL7 this should be typically in 100 k...220k range and for 12SN7 some 18k...33 k.
Anyhow, those which have bigger values are for 12SL7.
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Old 20th September 2011, 09:51 PM   #6
DF96 is offline DF96  England
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Good idea!
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Old 20th September 2011, 10:02 PM   #7
cjkpkg is offline cjkpkg  United States
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On that same vein...the SN7 would probably have a smaller value cathode resistor Pin 3/6 as well. Maybe around 1K or more for the SL7 and around 470R for the SN7...The last amp I built that used both had the SL7 closest to the input and the SN7 connected to the driver stage.
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Old 21st September 2011, 03:56 AM   #8
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thanks for all the feedback... checked the cap values and it looks like the higher value caps are associated with the input tube socket so that should be the SL7... I'm in the process of cleaning out all the sockets and tube pins... will put it together a little later tonight with that configuration and start it up on the variac...
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Old 21st September 2011, 05:34 AM   #9
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SL on the input (its primarily a voltage amp), SN on the driver ( it can supply more current to swing the grid).

Done.
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Old 21st September 2011, 05:15 PM   #10
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depends on the circuit. sl7 have a higher gain (70) compared to sn7 (20)

electrically the are interchangeable, pin for pin, but typically the sl7 are used more in the input stages and sn7 are more down the line as line stages, phase splitters and some output circuits.

for example : a two tube stereo headphone amp with a 6sl7 as its input, and a 6sn7 as its output tube.


a more complex example: 8 tube audiophile amp, (4) 6bg4, (2) 12sn7, (2) 12sl7, one half of the tubes per channel: (1) 12sl7 pre-amp , (1) 12sn7 as the phase splitter, (2) 6bg4 as outputs.
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