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Thursday, May 26, 2016

Tube of the Month : The 6SL7


This month I am presenting a high mu dual triode. The 6SL7.

The 6SL7 could be regarded as the high mu companion to the medium mu 6SN7. Like the 6SN7 it became quite popular during the 1990ies among audio amplifier builders. It went through a similar popularity cycle as the 6SN7. After being overused and often in applications for which it is not well suited, it went a bit of fashion.

It shares the same octal base and pinout with the 6SN7. To some tube rollers this made it seem compatible and it was often tried as an alternative to get more gain. While the 6SL7 might work in some circuits designed for the 6SN7 and provide a bit higher gain, it really is a different animal and should only be used in circuits which are designed for it. The heater operates at the same 6.3V as the 6SN7 but at only half the current (300mA). As mentioned above it is a high mu triode with an amplification factor of 70 vs the 20 of the 6SN7. But that high amplification factor comes at a cost. The 6SL7 has a plate resistance of 44kOhms which is more than 6 times higher than the rp of the 6SN7. It also operates at a lower current, typically around 2ma. For complete technical data please see the data sheet.
This high plate resistance which comes with a rather low transconductance probably is the reason why it lost popularity a bit when higher transconductance tubes became more fashionable. But this does not make it a bad tube. If the circuit is designed for it's parameters it is a great performer and it works well in unison with the 6SN7. For example as first stage in a phono preamplifier. Correctly implemented the 6SL7 gives a nice warm and smooth sound. As it does for example in my Octal Preamplifier Mk1. The 6SL7 has also been used as driver tube for small triodes like the 45 or 71A. With both sections wired in parallel to halve the plate resistance this can work well and offer the possibility to build a simple two stage amplifier. I wouldn't use it for larger triodes though. I prefer it in small signal applications like phono stages. The reason for it's deserved popularity in the 1990ies was it's good linearity:

A cross check with measurements of a tube sample confirms the very good linearity shown in the data sheet:

Due to it's popularity prices soared at some point especially for certain hyped versions. But it can still be found at reasonable prices if you look around a bit. The tube has been made by all the major manufacturers and NOS specimens are still around in large numbers.

Above two 6SL7 tubes made by Tung-Sol (the old Tung-Sol, not the currently made in Russia tubes, which only carry the Tung-Sol brand name).

These have a nice low loss micanol base.

Another version which only has a different color printing on the base.

Tung-Sol made these in different variations:

A close up of the one with black base:

Compared to the micanol base version:

Some close ups:

Another one with black base but shorter glass:

General Electric 6SL7:

Some close ups:

PhilipsECG 6SL7WGT made for the military:

RCA also made various 6SL7 tubes:

This is a military version with the VT-229 designation

A consumer variant of the RCA 6SL7:

Close Ups:

A later RCA version with the so called 'coin base':

Coin base tubes are often frowned upon by audiophiles.

But that is unjustified. They can perform just as well as the full base versions if the tube is healthy and measures well.

Like the 6SN7, the 6SL7 has an improved industrial 4 digit version, the 5691:

This is probably the most sought after variant.

Simply stunning looking with the ox blood coloured base.

A lot of extra bracing was added to improve shock resistance.

This was done to make the tube usable in aircraft and space applications.

Wether this is of any advantage in audio is of course questionable.

The 5691 certainly looks good though.

And lastly some 6SL7GT made by Sylvania

Some close ups:

A Sylvania 6SL7WGT:

Brown micanol base:

Close up to the plate structure:

Shots from different angles:

That's all about the 6SL7. I hope you enjoyed it.

Best regards