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Old 24th March 2007, 04:26 AM   #1
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Default What's a 6SL7 Good For?

I've been given a few of these. Any suggestions on what to do with them? Driver tube? Headphone amp? How do they distort in a guitar amp? Thoughts?
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Old 24th March 2007, 04:33 AM   #2
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Default Re: What's a 6SL7 Good For?

Quote:
Originally posted by dsavitsk
I've been given a few of these. Any suggestions on what to do with them? Driver tube? Headphone amp? How do they distort in a guitar amp? Thoughts?
They're quite popular as driver tubes for both audio and guitar amps, you see them used a lot, are you sure you need to ask?

I bought some to make a driver stage for a 6L6 amp, and Blackie's amp looks quite cool:

hey guitar playas, try this one!
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Old 24th March 2007, 05:31 AM   #3
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6SL7 is a high mu (70), high impedance (45k nominal) double triode, used similarly to 12AX7/ECC83, i.e. as a low current voltage amplification stage that is not required to drive a difficult load. It is more linear than a 12AX7 but suffers similarly from rather high Miller capacitance, which means it should be fed from a low impedance source, preferably 100k or less.

6SL7 is typically run at 2mA plate current or less. It makes a great voltage amplification stage but is generally not suitable as a driver, particularly if the output stage is a beam tetrode or triode, because such output stages have a relatively low input impedance which makes them difficult to drive. Even driving a pentode with fixed bias might be too demanding for a 6SL7.

A popular combination in a power amp is 6SL7 for the first stage, 6SN7 for the second (driver) stage and whatever tube you want to use as the output stage. (The 6SN7 is also very linear but has a much lower plate resistance (7.7k) and can drive most OP tubes easily.)
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Old 24th March 2007, 07:31 AM   #4
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Hmmm, how about with both sides of the tube in parallel to increase power and lower plate resistance, as a driver for triode strapped EL34's?
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Old 24th March 2007, 09:22 AM   #5
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Quote:
Originally posted by dsavitsk
Hmmm, how about with both sides of the tube in parallel to increase power and lower plate resistance, as a driver for triode strapped EL34's?
No. If you want to use 6SL7 as a driver (and it can cope easily with the required voltage swing) it needs to be followed with a cathode or source follower to drive the input capacitance of the output stage.
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Old 24th March 2007, 10:42 AM   #6
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Quote:
I've been given a few of these. Any suggestions on what to do with them?
I will send you my address along with postage.
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Old 24th March 2007, 01:39 PM   #7
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I wouldn't use a 6SL7 as a driver either, for the reasons mentioned above. However if you dig through the old Fender schematics you will find the 6SL7 used as an input stage and as a driver/phase splitter. In their later designs they switched to a 6SC7. They are also found as the input tube in a lot of HiFi amplifiers, usually followed by a 6SN7.

The 5B2 amp used the 6SL7 for the input stage.
The 5A3 and 5B6 used it for the driver/phase splitter.

The schematics are here:


http://www.schematicheaven.com/fender.htm

I have a bunch of these tubes also. They can often work in a 12AX7 type circuit with slightly less gain. Some are seriously microphonic while others are dead quiet.
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Old 24th March 2007, 05:47 PM   #8
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Quote:
Originally posted by EC8010


No. If you want to use 6SL7 as a driver (and it can cope easily with the required voltage swing) it needs to be followed with a cathode or source follower to drive the input capacitance of the output stage.
Is the second triode in the envelope sufficient as a cathode follower (or maybe as the top triode in a SRPP), or is a different type of tube really in order?
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Old 24th March 2007, 05:54 PM   #9
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Generally, a different type is more suitable for CF. The 6SL7 types don't swing much current and have a rather low transconductance.
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Old 24th March 2007, 08:06 PM   #10
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As SY says, no. You would also get into all sorts of Vhk problems. Cathode followers need plenty of gm.
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