Tube of the Month : The 6SL7

Hi!

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

Thomas

Stereo 300B Amplifier in Tower Chassis

Hi!

Here is a newly finished project. A new implementation of the Stereo 300B amp.

This is a similar implementation as the stereo 300B amp in portrait style chassis which served as the main amp at last years High End demo.

Like the portrait version it is split into 2 chassis.

I always prefer to go for a separate power supply rather than mono blocks.

This allows for a healthily over designed power supply rather than two wimpy ones which need to be sized to fit into the chassis with the amp.

Since the tower chassis offers more space this version received some upgrades compared to the portrait style amp.

It uses the same output transformers but larger interstage transformers.

The power supply uses a hybrid rectifier set up with two 836 and two 6AU4 tubes.

The driver tubes are 6SN7 as in the portrait version.

The large chassis allowed for a design with just the tubes on top and all other components inside.

For this version a special type of veneer was chosen.

It is called 'maple carbon effect'.

It makes a nice contrast with the gold coloured top plates.

The veneer has an almost silver like touch which is nicely picked up by using silver anodised handles.

The upgraded interstage transformers and beefier power supply improved the sound quality by a distinctive step over the smaller portrait style amp (which is already very good sounding).

Best regards

Thomas

High End 2016 in Munich : Retrospective

Hi!

The show is over. All the equipment is back home safely. Time to write down some final thoughts and impressions.

It has been a great show. Lot's of fun. But only after it's over one realises that it is quite an exhausting task to pull it off, both physically and mentally. Hauling all the equipment over, setting it up, making sure it sounds right. And then of course also to fill 4 days with music which shows the quality of the equipment, but which is at the same time fun and entertaining.

Based on the feedback we got and the mood in our room we had the impression that we managed to do a good job. Of course there is some nervousness before the show if all will go well. Especially since we did not have a chance to try the configuration which we demoed up front.

What was well known and proven was the electronics together with Wolf's SON speakers, since I use them myself. New to the show was Bernd's Kinea turntable.

It did a great job and I will get one for my system, equipped with Bernd's Arrow arm.

This was the version we used for playing stereo records. For monos Bernd set up a separate turntable with his reference arm and the Tedeska mono cartridge, which was a highlight for me. Here a video of this set up playing:

We decided on relatively short notice to also team up with Ze'ev Schlik of PureAudioProject with his Trio15 speaker. During set up in Munich was the first time we could listen to this speakers driven by my amps.

They turned out to play very well together:

Such events are also always great to discover new music. Some people bring their own records and we are always happy to play visitor's vinyl. My personal highlight was a record of a genre completely new to me. At one day the producer of a label which publishes contemporary classical chinese music visited our room. He brought one of his records.

The whole room listened in awe when we payed this record and applauded the producer. This record deserves it's own post. Stay tuned for that.

It was a great experience as in the previous years and I am looking forward to the High End 2017. I would like to thank Ze'ev, Bernd and Wolf for the phantastic team work. A special thanks to Hyun Lee of Tedeska for providing his excellent mono cartridge and to Thomas Fast of fastaudio for the acoustic room treatment.

Thank you to the High End Society for keeping up the tradition with this show and for providing such a great venue to present our products to the public.

And last but not least a big big Thank You to the visitors for all the feedback and good discussions. Without you the show would be nothing.

See you all again next year!

Thomas

Live from Munich : High End 2016, Part 12 - The Speakers

Hi!

Today I will present the speakers of our system at the High End

This year we had two pairs of speakers which got used in turns.

One of them is the Wolf von Langa SON which i am using myself in my system at home.

I also used the SON at the Cool Gales AudioFest, where they worked very well even in a rather large room. The SON combines a field coil chassis made by Wolf with a Air motion tweeter which is specially manufactured for Wolf by Mundorf.

The other pair is a joint project between Wolf and PureAudioProject.

It is a model from the Trio15 range. It uses a A5000 wide range driver made by Wolf.

It consists of a original Lowther cone with some mods combined with a modern field coil assembly.

This driver is assisted by two 15 inch open-baffle. woofers made by Eminence for PureAudioProject.

The crossover is located at the back and is easily accessible for adapting by the end user to specific tastes or room requirements.

For more information refer to the open-baffle website.

A quick sound impression in these two video clips.

Come and visit us in room F231c to listen to these speakers.


Best regards

Thomas

Live from Munich : High End 2016, Part 11 - The Power Amps

Hi!

Here are the power amps of our system at the show.

They are single ended 211 monos with separate power supplies.

This set was built last year.

They use 801A driver tubes, transformer coupled into the 211 output triodes.

Interstage and output transformers are made by Tango of Japan.

Here a small video clip with the amps playing:

Best regards

Thomas