Tube of the Month : The SPHERIA

November 24, 2015, 2:46 am

Hi!

In the last post about the European Triode Festival, I showed a photo of the two very old directly heated triodes which I bought at the auction in Denmark. After returning from the ETF I tried to gather some information about them and it turns out that these are very interesting tubes and very little is known about them. So let me share the little information I have about the Spheria.

What a cool name, no? The name seems totally out of time for a tube which was manufactured about 90 years ago. Could be a name for a product of our time.

The Spheria is a small directly heated triode. It has a European 4 Pin base. The base connections are shown at the left. This base was the european counterpart of the american UX4. All information I found indicated that it is an equivalent or very similar to the Telefunken RE074 or Philips Miniwatt A409. These have a filament voltage of 4 at a very low current of only 60mA. If the tube is similar to those it should have a mu of about 10 with a plate resistance of 10k which would mean a transconductance of 1mS (1000 micromohs). So I measured those values to see if they are really the same. I started with a low filament voltage of 3V and the power supply set to current limiting. Then increased the voltage until I reached 50mA. The plate curves would saturate quite early with filament voltages below 3.5V. At 3.8V they start to look good. and the filament current settled at 60mA. The 4V/60mA filament spec seems to be correct. The the other values however are quite different from the RE074 and A409. I measured an amplification factor very close to 7 coupled with a plate resistance of 15kOhms and about 0.5mS transconductance. These values turn out to be very close to those of the Philips A406. Below the set of plate curves which I generated:

Nicely linear curves as expected from a directly heated triode! Now let's have a look at some photo shots of this beauty:

Isn't that gorgeous?

All information I could find about the origins of these tubes was that they were made in Belgium in the 1920ies. I talked to many old tube heads and most never even heard of the Spheria.

No markings of manufacturer or information about the country of origin can be found on the tubes.

And they came without any packaging which probably disintegrated decades ago.

After some digging around and talking to people I assume that the tube was probably made in a small batch and was most likely a custom development for a specific application.

The manufacturer was most likely the company M.B.L.E. in Belgium which started as a light bulb company and later produced tubes under the Mazda and Adzam brands.

Let's try to get a glimpse of the internal structure.

Since the top is mostly covered with getter material, the internals can only be seen when looking into the tube from the base side

As in many other early triodes the internals are mounted horizontally rather than vertically as in more modern tubes.

The plate is cylindrical and the grid is a nicely wound spiral inside

The filament consists of a single very thin wire which is connected and supported at both sides. Theoretically the perfect arrangement. Let's see how the tube looks with the filament lit up.

As can be seen it has a tungsten filament which is most likely thoriated.

View from the side:

Close ups:

Some more photos:

What a great little tube and marvellous piece of early vacuum tube engineering. If anybody has more information about this tube and it's history please contact me and I will share it. Also if anybody has some of these to sell or swap, let me know. I will see what I can build with them, maybe for next years ETF.

Best regards

Thomas

P.S.: A reader of my blog provided a link with some information about the Spheria. It was used in a radio made by the french manufacturer Gerard Pericaud.

↧

Power Supplies Tower Style

December 3, 2015, 6:17 am

≫ Next: The Ultimate Line Preamplifier, Part 2

≪ Previous: Tube of the Month : The SPHERIA

Hi!

The recently presented 211 amps in tower style received some good feedback for their design. Someone commissioned a set of 300B monos and a 10Y preamp in the same chassis style. The power supplies are already finished.

This time in a light coloured wood body. The power supplies use a full tube Graetz bridge with a mix of 1616 and 6AX4 tubes. Similar to the PSUs for a previous 300B amp and also similar to the PSUs of the 211/211 amps.

Like those the new PSUs can also operate 866A tubes:

The signal chassis will use silver finemet transformers for which I am still waiting. They will share the same circuit as the silver 300B amps and silver 10Y line stage built earlier this year.

Top view, showing the tubes and the rotary type on off switch:

The back side showing the connection terminal:

There is also a version of the Stereo 300B amp planned in the same tower style.

Best regards

Thomas

↧

The Ultimate Line Preamplifier, Part 2

December 10, 2015, 1:16 pm

≫ Next: Tube of the Month : The EC8010

≪ Previous: Power Supplies Tower Style

Hi!

The all silver differential line preamplifier which I announced in October, has been finished since a while. I finally found the time to take some photos and test it.

The unit got finished in a dark wood (smoked eucalyptus) with graphite grey metallic transformer covers.

As with all 10Y line stages the power supply is completely separate:

It houses separate choke filtered filament supplies for both channels and a choke filtered B+ supply using the well proven full wave bridge arrangement with 4 6AX4 TV Damper tubes.

The signal section is completely free of any capacitors.

There are 4 filament chokes under the transformer covers. Inside the chassis there are 14 (!) silver transformers and chokes.

Silver volume control AVCs, silver line output transformers, separate silver wound plate chokes for each tube and a separate pair of silver input transformers for each of the 3 inputs.

This line stage is equipped with a quad of 1602 tubes, a special low microphonic variant of the 10Y.

The big question now, was it worth the effort and expense?

Does the all silver version improve over the differential line stage with copper transformers or the single ended version with silver transformers?

Short answer: Yes absolutely!

When I was challenged with the task to build a line stage which out classes the silver 10Y line stage, I was skeptical if any significant advance in sound quality could be achieved. After all I am very happy with the base copper version of it and happily use one every day. The silver version puts some icing on the cake of that. What more could you wish?

I am normally hesitant to write about the sound of my creations. I have written a lot about the subjectivity of such sound descriptions and of course I am biased. Still I understand that you want some sound impressions.

This new preamp took me by surprise. I first listened to it with some regular 10Y tubes, not even carefully matched ones. I was shocked by the sheer realism which I heard. This preamp sounds incredibly warm. By that I don't mean a 'tubey' warm sound, but a very realistic and natural reproduction.

Today I plugged in a matched quad of the fabulous 1602 and got blown away. Each tone seems to be more textured than before. Each little detail is plainly audible. Yet the representation does not fall into pieces what can often happen with systems which try to be analytical. But everything stays integrated and is easy to listen to without any fatigue. New details pop up on well know records.

The single ended line stage is not really a slouch in terms of realism and resolution, it actually really excels in that and I was surprised to realise that even more is possible. All that combined with the incredible warmth. Normally I am not much into sound staging but I also noticed a added realism in that regard. Again a property in which the standard line stage is already extremely good. Especially with atmospheric electronic music this preamp seems to paint the tones into the room. Also the bass representation is wonderful. Each bass note is more present than before with all the detailing of the textures of the sounds.

The conversion from copper to silver has brought more improvement in the differential line stage than it did in the single ended version.

So this preamp deserves the title of my ultimate line preamplifier.

Please forgive the long rant which is not usually my style. But this preamp deserved it.

Of course this new preamp won't make the standard version obsolete. It comes at considerable cost after all.

As I have written before: The secret to achieving good sound is to be happy with what you can afford or simply to enjoy what you have.

But if budget is not an issue, I can only recommend to consider such a preamplifier. I am trying to imagine how it would sound in combination with a fully differential and all silver phono preamplifier, which I still have to build.

I will be able to enjoy this preamp for a little while before it will be shipped to it's new owner.

Best regards

Thomas

↧

Tube of the Month : The EC8010

December 19, 2015, 4:58 am

≫ Next: Music : Freddy Quinn, Auf hoher See

≪ Previous: The Ultimate Line Preamplifier, Part 2

Hi!

During the last half year or so, all tubes which got presented in this series had been quite old developments from the 1920ies to 1930ies. So I decided to pick a more modern one for the last Tube of the Month post in 2015. A small high frequency triode, developed in the 1960ies by Telefunken. Meet the EC8010.

Readers of my blog are aware of my fondness for the EC8020, which got covered twice already as Tube of the month. The EC8010 is the small sister of the EC8020 and was most likely developed first.

Both tubes share the same 9 pin noval base and pinout. They have similar amplification factors. The EC8010 is a bit higher with 60 vs the 55 of the EC8020. The big difference is in the transconductance which is about double in the EC8020. Still the EC8010 comes with an impressive gm of 28mS. Due to that the EC8010 is often referred to as 'half a EC8020'. In fact a EC8020 could be emulated by using two EC8010 in parallel. Maximum plate dissipation is about 4W which is again half of the EC8020s. In terms of dimensions, the EC8010 has about half the diameter at about the same height. Due to the extreme scarcity of the EC8020 using two EC8010s could indeed be an approach if the transconductance of the former is needed.

The EC8010 is much easier to find at lower prices. Two of them would only need more heater current than a single EC8020. For all technical details of the tube see the Telefunken data sheet. I never compared two EC8010s in parallel to a EC8020 in an actual circuit so can't comment about any degradation in sound with such an approach. If paralleling is attempted the tubes should be carefully matched. Such high transconductance tubes often vary a lot from sample to sample. And in case of the EC8010 the data sheet gives the amplification factor as 'about 60'. While in the EC8020 data sheet it is listed as '= 55'. This was certainly done for a reason and I would expect some variation in amplification factor between tubes. Since I have more stock of EC8020s than EC8010, I never was tempted to use the EC8010 as substitute and also do not have enough samples to verify considerable differences between samples. I have one same which differs from the others a bit. More about that below. Let's first have a look at the plate curves:

A nice set of curves with good linearity for such a high transconductance tube. Here the measurement of a tube on the curve tracer:

Here two sets of plate curves of the two tubes from my stock which differ the most between each other. As can bee seen there is not only a difference in transconductance but also in the amplification factor:

As far as I am aware, EC8010s were made by Telefunken, Siemens and Valvo. I have also seen some with the National brand name which is owned By Richardson Electronics. Not sure if they manufactured these themselves or rebranded them. Interestingly those National tubes carried both tube designations, EC8010 and EC8020. From the physical appearance and measurements however they had been EC8010. In the 1970ies and 1980ies it was common practice to sell one tube with several designations of types which are similar. However in this case this was going a bit too far.
The EC8010 also has an american 4 digit designation, 8556.

Above some Siemens EC8010 tubes in the characteristic blue/orange packaging.

Close up to the tube:

The boxes:

A few more photos of the Siemens tube:

Valvo EC8010:

Some close ups:

Siemens and Valvo in comparison. Very similar construction with only subtle differences:

The EC8010 has a very unique construction characteristic which I have not seen in any other tube. While the cathode and grid assembly is usually completely surrounded by the plate, only one side of the cathode and grid face the plate in this tube:

A close up shows the fine grid wires:

Unfortunately I don't have any Telefubnken EC8010 to show. So let's proceed with detail photos of the internals. For this I cracked open a dead tube:

Removing the glass envelope gives a much better view to the internals:

Here we can see the cathode and grid which are facing towards the top into empty space:

A close up to the grid:

The top showing the ring which held the getter material before flashing:

Some more views:

The silver cage like structure around the outside was probably only there for mechanical stabilisation.

A view from the side opposite to the grid, with cooling fins of the plate:

Another view to the grid and close up:

The top mica disc and getter holder removed:

Cathode extending out of the second mica disc:

Removing the heater wire:

Removing the outer 'cage' gives yet a better view to the cathode grid and plate:

This is taken from the side so that the gap between grid and plate is nicely visible.

Shot from the grid side:

Here the entire plate structure is nicely visible:

Grid and cathode detached from the rest:

Close up:

The plate:

For size comparison with a finger in the picture:

Close up:

Some more views of the grid and cathode:

Some photos with a match for size comparison:

What a marvellous piece of engineering! Now some photos of a tube in operation.

As expected the entire cathode can be seen from one side.

Close up with the grid wires visible:

I hope you enjoyed this last Tube of the Month post for 2015. Stay tuned for more tubes to be shown in 2016!

Best regards

Thomas

↧

Music : Freddy Quinn, Auf hoher See

December 28, 2015, 1:39 am

≫ Next: Happy New Year!

≪ Previous: Tube of the Month : The EC8010

Hi!

At events like the ETF or the High End fair I occasionally like to play some fun records. It is interesting to watch the reactions. While some seem to take things too serious, others really enjoy them. This is one of those records. 'Auf hoher See' by Freddy Quinn.

Freddy Quinn was very popular in Germany during the 50ies and 60ies and produced a lot of records during that time and also played in some movies.

Most of his songs are about the sea and sailors. This is also the theme of this record as is evident from the title and cover. The album was released in 1962 on Polydor. Like most of the Polydor albums of this era, the recording quality is exceptional. Also the vinyl quality of these early Polydor discs leaves nothing to wish for. They can be recognised by the orange label with the stars at the top. Later releases have a red label and are of lesser quality but still mostly very good. The first song on the album is probably the best and also the one which often leaves people with their mouths open if they haven't heard such a record before. It starts with water waves which almost give the feeling as if the water is splashing over your feet. Then a choir slowly sets in which is unbelievably realistic. And finally Freddy starts to sing. His particular accent and the way he rolls the 'rrrrr' together with the hilarious lyrics is really funny. Freddy Quinn probably didn't take himself too serious when he recorded this.

And this is how to listen to such a record. Don't expect anything serious or artistically challenging. Just have fun listening to it and be amazed by the truly exceptional recording quality.

Best regards

Thomas

↧

Happy New Year!

December 31, 2015, 2:01 am

≫ Next: The Tale of Two Preamplifiers - Part 1

≪ Previous: Music : Freddy Quinn, Auf hoher See

Hi!

It is Sylvester and time again to recap what was going on during 2015 and what is planned for 2016.

2015 was the best year for VinylSavor so far. Many exciting projects and some significant improvements in sound quality.

First there was the new generation of LCR phono stages with a new improved biasing scheme introduced in a EC8020 phono stage and later also used in new builds of D3a phono stages in both landscape and portrait chassis styles.

The 211 had a significant come back for me with 4 newly build amplifiers. A new incarnation of the 6HS5/211 amps, two new versions of the 801A/211 concept with Tango transformers. Both in square chassis, one of them with less height and more floorspace and the other introducing the new tower style chassis.

While the former were commissioned by a customer, I built the tower style version as demo amps which I plan to show at next years High End fair. And then there were the totally over the top 211/211 amps with all silver transformers.

Silver was a big topic this year, even more than 2014. Besides the silver 211/211 amps there were the all silver 300B amps with power supplies using mercury vapour tubes. Those two amps represent the best sounding power amplifiers I have built so far.

On the preamp side there was also another build of the single ended 10Y line stage with silver finemet OPTVCs.

And then there was the silver version of the differential line stage. This project was the biggest surprise for me in 2015 and held the biggest step in sound quality I achieved in a component so far. It deserves the title 'ultimate line preamplifier'. A special thanks to those who commissioned these over the top projects and helped to push the envelope!

Although the 10Y and especially it's specially selected variant 1602 remain my favourite line stage tube, there are other interesting candidates as well. For example the 26 as used in a 26 line stage built this year. There was an interesting project going on this year about which I have not written anything yet. This involved building a test setup which enabled the use of a whole variety of different tubes in a line stage and culminated in the build of two new preamps. The tale of these two preamps will be told in 2016. Thanks a lot to the originator of this project which enabled new insights into different sound worlds.

So what is planned for 2016? The ultimate line preamplifier definitely needs a companion phono stage. I finally need to proceed with the all differential EC8020 phono which I already mentioned in last years Sylvester post. Initially as a copper version and hopefully there will also be a chance to build it all silver in similar fashion to the ultimate line preamp.

The topic of push pull power amps is also not forgotten yet and maybe 2016 will be the time for a VinylSavor PP amp.

And finally I decided to look into digital playback. Although vinyl will remain my preferred source, I understand that there are a lot of people who simply prefer the convenience of digital and the possibility of downloading music. And finally not everything is available on vinyl.

All the best for 2016 for all of you and your families!

Thomas

↧

The Tale of Two Preamplifiers - Part 1

January 3, 2016, 9:34 am

≫ Next: Custom 300B Amplifiers

≪ Previous: Happy New Year!

Hi!

In the previous post I already mentioned the project which evaluated the sound qualities and usefulness of a variety of tubes in line preamplifiers. Two preamplifiers resulted from this evaluation and I will present them in a few separate posts.

Although I am very fond of the 10Y in line stages, and also as driver tube for DHT power amps, I occasionally get asked for alternatives. Also some people prefer to not use the same tube twice in a system. So if the 10Y is already in use as driver this means to look at some other tubes.

For this project I built a test set up which can operate a variety of directly heated tubes with UX4 base:

Tubes with other bases can be plugged in via a socket adaptor as seen on the left in the photo above. The test set up has two pairs of different line output transformers, selectable via switch. One is for tubes with lower plate resistance and the other for those with higher rp. The biasing can be switched between conventional cathode bias and filament bias. Different bias resistors can be selected via a set of switches. Filament and plate voltages can be monitored via jacks at the top plate. Plate and filament voltages were taken from an adjustable supply which was shown in a post about a 300B amp. This way suitable operating conditions can be selected for a wide variety of tubes. When used with a passive transformer volume control this set up is a fully functional line preamplifier.

Stay tuned for part 2 in which I will mention all the tubes tested.

Best regards

Thomas

↧

Custom 300B Amplifiers

January 14, 2016, 3:37 am

≫ Next: The Tale of Two Preamplifiers - Part 2

≪ Previous: The Tale of Two Preamplifiers - Part 1

Hi!

Pretty much all my amplifiers are built to order and can be tailored to specific requirements. These latest amps are an example how far customisation can go.

In this case the customer provided the interstage and output transformers and also participated in the chassis layout. The transformers are made by Monolith Magnetics in Belgium. The layout and chassis is based on my portrait style but adapted to fit the different output transformers.

The circuit is based on my earlier 300B/300B amps but with some circuit modifications requested by the customer. Shown above one channel of the amp equipped with EML tubes.

The power supply uses a classic rectifier instead of my favourite full wave bridge with TV damper tubes. Switches are placed on top which allow adaption of the filament voltage to different brands of 300B tubes which draw different filament current. The power transformer was also provided by the customer.

One channel, amplifier and power supply.

The amplifier chassis with Electro Harmonix tubes:

With Elrog ER300B:

The full amplifier set:

Best regards

Thomas

↧

The Tale of Two Preamplifiers - Part 2

January 18, 2016, 2:38 am

≫ Next: Tube of the Month : The 31

≪ Previous: Custom 300B Amplifiers

Hi!

In part 1 the story of these two preamplifier was already introduced. In this post I will reveal which tubes got tested and which were the favourites in terms of sound.

The test set up which was used for evaluation was already shown in part 1. An adjustable power supply provided all the necessary voltages for the different tubes.

Operating points are chosen with a couple of switches which selected from an array of resistors for cathode or filament bias.

Pretty much all directly heated triodes with an UX4 base are usable, for example: UX201A, UX112A, 26, 30, 31, 45, 50, 183/483 and 71A. But even indirectly heated triodes like the 27, 37, 56 or 76 could be used with socket adaptor.

Also the 46 was tested with a socket adaptor which tied grid 2 to the plate.

And in addition two european DHTs. A Siemens Da:

Again to be plugged in via an adaptor:

And the Telefunken RE074:

Also with an adaptor:

The 10Y or 801A were not included since I have separate preamps with these tubes which could be used for comparison. Hopes were very high for the european tubes and also the 45 and 50. But surprisingly none of them turned out to excel in this set up. To be fair, none of the tubes sounded really bad and the differences are rather subtle. After all the overall sound is largely determined by the circuit topology and its execution. One of the winners was not much of a surprise since I was already familiar with it:

The 26 which I already used in a 26 line stage before. It has a very gentle and smooth sound and applies a touch of 'dark chocolate' or 'caramel' flavour to the sound which can be very pleasing. This description is of course an exaggeration for illustration. The effect is rather subtle. The other tube which came out as a favourite and turned out to be even better than the 26 was the 31. A directly heated battery triode.

Shown above as UX231 in tubular shape. The 31 offered a very refined and high resolution sound in this set up. A runner ups in this competition were the 71A and 183/483 which also provided a very clean and impressive sound. Also not bad was the 27. It does not exhibit it the certain magic of indirectly heated triode but is very interesting nonetheless. More about this in an upcoming post.

Since the originator of the project could note decide between the 26 and 31, he commissioned two preamps.

Since both tubes run at similar plate voltages, the two preamps can be used with the same power supply. Both tubes run in filament bias. Additional bypass resistors are used in the 31 unit so it can work with the same power supply as the 26 without any switches.

The powert supply uses a full ave bridge with 4 half wave tube rectifiers. But not with 6AX4 TV dampers as usual but this time with a different tube, the 1V. More about this tube in an upcoming post.

Shown above both preamps with the PSU. The back sides:

An interesting project which gave some valuable insights. Thanks again to the originator of this. It has been an interesting journey. I'll ask for some sound descriptions from the owner of these preamps for another part of this story. Stay tuned!

Best regards

Thomas

↧

Tube of the Month : The 31

January 22, 2016, 3:03 pm

≫ Next: The 27 Line Preamplifier

≪ Previous: The Tale of Two Preamplifiers - Part 2

Hi!

After the presentation of the two 'twin preamplifiers' in recent posts, the choice for the tube of this month is probably no surprise : The 31.

This is a small directly heated power amplifier triode developed for the use in battery operated receivers. Such tubes are also often referred to as battery tubes.

The 31 shares the same pinout as most directly heated triodes which use the UX4 base. But it comes in a much smaller bottle. The coke bottle shaped version has the ST-12 size. This is the same size as the 27 for example. Being designed for battery operation means rather lowish plate voltage and more important very low filament power. This required the development of very efficient filaments. In the 31 the filament operates at 2V and only consumes a tiny 130mA. This translates to just 260mW filament power. Compare this to other types which need more than an Ampere and run on higher voltage. The typical plate voltage for this triode is 135V and the maximum is at 180V. In typical operation it only draws 8 to 10mA. Of course not much power output can be expected when only so little is put in. Still it can produce a whopping 375 milliwatts. Now that would be something to power hyper efficient horn speakers.

Anybody out there who is man enough to join the bragging contest of his audio buddies with that kind of power rating? I am prepared to take the challenge to build a suitable power amp around this tube. The rather highish plate resistance would require an output transformer in the 15k impedance range for single ended operation. Obviously it does not need a huge voltage swing to drive it to it's maximum power output, maybe the 30, another directly heated battery tube would be just right for the job. But that will be another story maybe. What I have used it for was in a line preamplifier only and there it worked extremely well as can be read in the previous post. I could also see this tube to work well in a headphone amplifier or as driver for other small power triodes, like the 71A or even the 45. Many possible uses for this little beauty! As was expected from it's excellent performance in the preamp it has exceptionally linear plate curves. Very typical of a directly heated triode. See the set of curves from the data sheet below:

And as always an actual measurement:

As can be seen in the first photo, the 31 was made in 3 different glass shapes. The early tubes, named UX231 came in the usual globe shape but also in a tubular bulb. Later the tubes moved to the more modern coke bottle.

Here a beautiful sample, made by RCA for Cunningham and named CX-331:

Globe shaped tube made by National Union:

Beautiful old box!

Engraved base:

A later version by National Union:

Triad:

They named it T-231

Nice warranty sticker on the tube still intact:

Although it says T-231 on the box, there is a '31' printed on the glass:

Let's have a closer look at some details of the globe tubes:

Two different globe tubes in comparison:

The one of the right has a little glass stem inside with rods molten into the glass which in turn keep the right distance between grid and plate and one which holds the filament at the top:

Close ups:

The other one has a small mica disc at the top which does this job:

A globe shape 231 from Sylvania:

Some close ups showing the construction details:

The other earlier shape is a tubular glass, here in comparison with the globes:

Two different tubular 231 tubes:

Some close ups:

Here we also have the different variants of support structures on the top.

Next some more modern ST-shape 31 tubes:

Made by RCA

Some different versions

Sylvania:

The top:

Another version made by Sylvania:

This Philco branded 31 is still in a sealed box, so I am not opening it:

Now let's have a look at the glow of the 31:

As expected from the low filament power, the glow is quite dull.

They are also called dull emitter tubes for this reason.

I hope you enjoyed the first tube of the month in 2016. Stay tuned for many more interesting tubes to be presented.

Best regards

Thomas

↧

The 27 Line Preamplifier

January 28, 2016, 2:30 am

≫ Next: The Ultimate Line Preamplifier, Part 3

≪ Previous: Tube of the Month : The 31

Hi!

When I introduced the 27 as tube of the month in June last year, I mentioned that it could make a nice line preamp tube. In the meantime the 27 line stage is finished.

I already experimented with the 27 many years ago and used it in a preamp and as driver for a 45. In both cases it worked very well. So it was time to put it to good use again.

This is the latest addition to my offering of line stages.

Being indirectly heated the 27 requires no fancy filament supply. AC heating is good for hum free operation at line levels.

Many people have space constraints and have difficulties to place the two chassis of a 10Y line stage.

So I squeezed everything into a single chassis, including the power supply.

Of course the proper rectifier to use with it is the 84.

Top view:

The two 27 tubes on vibration damped socket mounts at the front and the rectifier in a corner at the back. The 3 white covers are housings for chokes. All the rest went inside:

Quite tightly packed. Two separate power transformers, one for the high voltage and a heater transformer for the 27s. Another choke, 4 more capacitors, the line out transformers and two autoformer volume controls.

A neat little package. In fact with all the iron it is quite heavy. The backside:

The on/off switch had to move to the back. I didn't want to disturb the look of the front with the two knobs. Of course it would be possible to have it on the front as well.

Side views

This time I picked white for the covers and capacitors.

Here the preamp equipped with globe UY-227 tubes:

With ST tubes:

A nice preamp. It has all the advantages of transformer coupling and transformer volume control. A bit less refined as the 10Y line stage, or any of the other DHT line stages. But therefor it needs half the space and comes at about half the cost.

A few more detail photos:

Those globe tubes are really nice

Best regards

Thomas

↧

The Ultimate Line Preamplifier, Part 3

February 7, 2016, 9:32 am

≫ Next: Tube of the Month : The 1-V

≪ Previous: The 27 Line Preamplifier

Hi!

In part 2 about my ultimate statement in terms of line preamplification I already gave an extensive sound description. I usually do not write much about the sound of my amps and preamps since such impressions are rather subjective. Such rants as in the last post are quite unusual for me so I thought another post is due with some follow up.

When listening to a newly developed amp or preamp it is easy to get carried away, especially when the expectations are high as with such a silver laden component. Often the initial enthusiasm wears off after some time. But often also the opposite can be true and only after living with a component for some time it's true strength reveals itself.

So what do I think about the sound of this preamp after some time? In fact I found it even better than when I wrote the last post. The most notable strength is the extremely refined texture of sounds and the way it sculptures even the tiniest detail with ease.

Especially since I had the chance to listen to this preamp in the system of it's owner last week, I can confirm all the statements I made about it. Even though I listend to it in a system with very different speakers and sources, I could hear all the positive aspects of the line stage as I heard them in my own system.

Voices are unbelievably warm an natural. Very pleasing to listen to with excellent articulation.

I am very happy with the sound which I could achieve with this and another one is planned for my own system.

Once exposed to this kind of sound it is difficult to live without.

And of course the matching phono stage is in the planning as well.

And maybe towards the end of the year I will apply the same differential concept to a power amp. Stay tuned for reports about the progress on this.

Best regards

Thomas

↧

Tube of the Month : The 1-V

February 12, 2016, 2:55 am

≫ Next: EC8020 Differential Phono Stage - Part 1

≪ Previous: The Ultimate Line Preamplifier, Part 3

Hi!

Last month I presented the triode which was used in one of the recently built twin preamps. This month I am covering the rectifier which is used in their power supply: The 1-V.

As the name indicates, the 1V was one of the earliest rectifier tubes in commercial use. In the RETMA naming system 6Z3 was assigned to it. The 6 indicates the 6.3V heater voltage. Rectifiers got letters from the end of the alphabet and 3 is the number of electrodes: Plate, cathode and heater.

The tube has a UX4 base. The pinout is shown on the left. The two larger diameter pins for the heater and the others for plate and cathode. The 1-V is an indirectly heated rectifier. The heater only needs 300mA current. This can be quite useful when the available heater current is limited. The tube only contains a single diode. So two would be needed for a full wave rectifier, or 4 for an all tube bridge rectifier. It only delivers up to 50mA DC per tube so the use is limited to preamplifiers or power amps with small triodes like the 71A. All the versions I have seen are in ST-glass of the ST12 size. I don't know if it was ever made in globe shape. The V in the name stands for vacuum. I have read about a mercury vapour version just named '1' but could never find any. The glass is of the same shape and size as the 31. So it was a natural choice for the power supply of the preamp with that tube.

It works perfectly well in that power supply which only has to deliver very little current at a rather low voltage. The data sheet specifies max. 350VAC on the plate. So with a capacitor input filter, 400V is about the maximum DC which can be obtained from it. More like 300V with a choke input filter. Considerably less if you want to stay clear of the limits. But this is sufficient for many preamplifier circuits and also for small amplifier tubes. Since the tube is indirectly heated the output voltage comes up with some delay and rises slowly as the cathode warms up. This tube is a nice alternative to the 6AX4 which I usually use. You will see this tube more often now in my preamplifier power supplies, where the 6AX4 is a bit of an overkill since it can deliver much more current than is needed in preamps. Although this tube is out of production since a very long time, it can still be easily found at reasonable prices. Not as cheap as TV damper tubes but therefor you get the beautiful ST shaped glass. I have built up a small stock of this type. Let's have a look at some 1-V from different manufacturers, starting with Tung-Sol:

A close up to the tube:

The photo at the top of the post is taken from a box of 1V tubes I just received. These are from Canadian Westinghouse.

Beautifully sealed boxes.

The tubes however were made by Tung-Sol. They look the same as the other Tung-Sol tubes above.

They obviously came from the same production line. A Westinghouse logo got stamped on the bases:

1-V made by Hytron:

National Union:

Again National Union.

These are in JAN (Joint Army Navy) military packaging.

RCA Victor:

RCA:

RCA Radiotron.

Cunningham:

Ken-Rad:

Sylvania:

Some close ups.

More Sylvanias:

Philco branded 1V (probably made by Sylvania):

Marconi:

Raytheon:

And lastly Zenith, again probably made by Sylvania:

Now let's have a look inside of this one:

Removing the glass gives a better view on the plate:

Since it is just a diode, not much inside. Two mica discs, one at the top and one at the bottom keep plate and cathode aligned.

The heater is only insulated from the cathode through the white coating.

No additional spiral as seen in TV damper tubes for increased voltage capability.

The top mica disc removed

The cathode:

And finally some 1-V tubes glowing:

Close ups:

Best regards

Thomas

↧

EC8020 Differential Phono Stage - Part 1

February 17, 2016, 8:30 am

≫ Next: Bath AudioFest 2016

≪ Previous: Tube of the Month : The 1-V

Hi!

I have been talking about this phono stage since a long time. Now it finally becomes reality. A differential version of my LCR phono stages.

After the great success with the 10Y differential line stage especially in the over the top implementation with all silver transformers as 'ultimate line preamplifier', it is only natural to extend the same circuit topology to the phono stage as well. The first version of this phono will use the magnificent EC8020, queen of small signal triodes. Versions with other tubes like the D3a will certainly follow. So this one will be the differential pendant to my single ended EC8020 phono stage. The first implementation will mainly use copper transformers and chokes, only the built in MC step up transformer and an intermediate transformer will be silver. An all out all silver version will follow later as the 'ultimate phono preamplifier'.

The photo above shows all parts which go onto the top plate. The basic design will follow the same layout as my latest single ended phono stages. The chassis gets lengthened a bit to make room for the 8 tubes.

Each tube socket gets it's own sub plate with vibration damped mounting to the main plate.

The socket sub assembly mounted to the main plate:

The underside:

I will post updates as the assembly progresses. Stay tuned!

Best regards

Thomas

↧

Bath AudioFest 2016

February 26, 2016, 7:06 am

≫ Next: 10Y Line Stage in Tower Style

≪ Previous: EC8020 Differential Phono Stage - Part 1

Hi!

Cool Gales will be hosting the Bath AudioFest again this year. As in previous years I will participate at this fine audio exhibition in the beautiful city of Bath in the UK. The event will take place on March 18-19.

The AudioFest will be at the Bath and County Club and part of the exhibition is in The Bath Royal Library and Scientific Institution. I will have the pleasure to set up a system with Frank Schröder who will bring one of his turntables and tonearms. I will provide the electronics among which is this D3a LCR phono preamplifier:

This phono will be demoed along with the same 10Y line preamp and 300B stereo power amp as shown at the Munich High End last year. Wolf von Langa will send over a pair of his SON speakers to complete the system.

Hope to see many of you in Bath!

Best regards

Thomas

↧

10Y Line Stage in Tower Style

March 3, 2016, 2:35 am

≫ Next: Tube of the Month : The 6AU4

≪ Previous: Bath AudioFest 2016

Hi!

End of last year I showed some power supplies in tower style. These will be used with a pair of 300B mono blocks and a 10Y line preamplifier. In this post I will show the finished preamp which will be powered by one of those PSUs.

This preamp uses the same circuit and parts as my 10Y line stage in portrait chassis. It is sonly the look which has been changed.

Obviously this chassis style is meant for placement on the floor.

It is 30*30 cm and 40cm high, plus the height of feet and tubes and transformers on the top.

All connections are made to a metal plate on the backside.

The power amps which will go with this preamp will be finished soon. Stay tuned!

Best regards

Thomas

↧

Tube of the Month : The 6AU4

March 7, 2016, 5:48 am

≫ Next: 300B Mono Blocks in Tower Style

≪ Previous: 10Y Line Stage in Tower Style

Hi!

After the presentation of the very early rectifier tube in last months ToM post, let's continue the rectifier theme. This months tube is a TV damper, the 6AU4.

I covered many TV dampers already, the 6BY5, 6BE3, 6CG3, 6CJ3, 6DQ4 and my favourite one, the 6AX4 and a 25 heater variant of it, the 25AX4.

I like to use the 6AX4 due to it's lower heater power. It's current handling capability is sufficient for most audio applications. If very high B+ current is needed, 6CG3 or 6CJ3 are well suited candidates. But they have different sockets. If an octal socket is preferred but more current is needed than the 6AX4 can handle, the 6AU4 might fit the bill. It shares the same pinout with other octal TV dampers. The increased current capability comes at a price, it also needs more heater current. 1.8A versus the moderate 1.2A of the 6AX4. Like the other TV dampers it contains only one diode. The only exception is the 6BY5 which has two diodes in a single bottle. This means two 6AU4 are needed for a full wave rectifier, which requires 3.6A heater current. Or 4 of them in an all tube bridge rectifier which would need a whopping 7.2A of heater current. Like the other TV dampers it has very high voltage capabilities and can also handle large voltage differences between heater and cathode.

This is limited to the heater negative to the cathode however which is allowed to go up to 900V DC. In the other case with the heater positive to the cathode themaiimum allowed is 300V DC. In actual implementations it is advisable to stay well clear of those limits. I typically use such tubes with the heater voltage referenced to ground and for DC output voltages up to 700V DC. Although the tube manufacturers never advised to use TV dampers for mains rectification purposes, they proved to be very well suited for such applications and have been working in the power supplies of my amps reliably for many years. I guess the tube manufacturers preferred to keep selling their more expensive rectifier tubes for power supplies. TV dampers were produced in the millions for TV sets and had to be cheap. Since they got produced in such large volumes they are still available in quantities at very low prices. The 6AU4 can be found as 6AU4GT and as 6AU4GTA. The GT simply stands for glass tube. The A was added after the tube was improved with slightly higher current handling capability.

Many of you are probably wondering where the name 'TV damper' comes from. So let's have a look at the actual application they were developed for. Before flat screens were introduced, TV sets had a cathode ray picture tube. These had an electron emitting cathode like other vacuum tubes. The cathode produced an electron beam which got modulated, accelerated and directed towards the front. In order to create a picture the beam had to move in a zig zag fashion from left to right and top to bottom. This was achieved by coil winnings arranged around the neck of the tube, which was called a yoke. Different yokes were used for the horizontal and vertical scanning. For this purpose the yolk had to be fed with a sawtooth current. This was generated by horizontal and vertical deflection circuits. Shown below is a very simplified drawing of the horizontal circuit. It typically used a beam power tetrode which was fed by a sawtooth voltage and coupled to the horizontal yoke through a transformer.

The problem with this is a parasitic oscillation set up in the transformer and yoke during flyback of the electron beam, caused by the sudden current cut off in the transformer at the end of each cycle when the magnetic fields collapse. This oscillation needed to be damped to avoid non linearities in the picture. This could be achieved with resistors or RC elements, but a more elegant way to do it is the use of a diode shunted across the secondary as shown in the next picture.

In later TV sets, the energy stored in the inductances of transformer and yoke was partially recovered. This was done by generating a boosted B+ voltage. Also the output transformer was usually replaced by an auto former.

The spikes generated by the collapse of the magnetic fields were used to charge a booster capacitor Cb and the voltage was added to the raw B+. Therefor TV dampers are also called booster diodes sometimes. The schematic above only shows the principle and is by no means accurate or complete. It is just meant to show the concept. In actual implementations this spike was also used to generate the very high ultor voltage of the picture tube through an additional winding which fed another very high voltage and very low current rectifier diode (not shown in the picture).

I hope this little excursion into TV sets enlightened the background of these tubes a little. For more detailed information you can refer to the website of the Early Television Museum. Now lets continue with some photos of various 6AU4 tubes, starting with Westinghouse of which I have various versions. One came in the beautiful boxes shown above and the other in rather bland packaging:

While the latter looks rather regular, the other with the getter applied in the top dome is quite different in construction.

A close up photo shows that it has a small glass piece at the top sitting above the mica disc:

I have never seen this in any other TV damper tube. Here a close up:

Probably done to get some extra strengthening. Here the same tube with power applied to the heater:

From another angle:

The top end of the other Westinghouse 6AU4 with more common construction style:

Another Westinghouse version came with the small 'coin base' as also seen on many other tubes. The coin base was introduced quite late and is a more modern construction style:

Here a comparison between coin base and regular style:

This one is in a Box marked Westinghouse Reliatron:

The plate has two extra wings for additional cooling

These are all the different Westinghouse 6AU4GT/A versions I have.

Up next let's have a look at Raytheon.

From this manufacturer I found 6AU4s in 3 different packagings and 6 different construction styles.

The first one looks quite ordinary like many other TV damper tubes. Clear glass top with the getter applied near the base, printing on the base.

The next one has a bit larger plate structure and looks more rugged.

This one is quite similar as the one before but different cutouts on the plate at the side and the glass is a bit taller:

Yet another minor difference in plate shape:

The next one has a very different plate structure with metal extending at 4 sides which should give better heat dissipation.

The getter applied at the top:

And lastly the one which appears to have the most rugged construction:

The top is also quite different with the spiral which isolates the heater from the cathode extending much further.

The spiral is also attached to a metal rod for some extra support.

Another view:

And lastly two tubes branded Lindal / LTT. Apparently a japanese company but the construction is identical to one of the versions above. So quite certainly made by Raytheon and rebranded.

This sums it up for Raytheon.

Moving on to General Electric.

Most GE 6AU4 I have, look very similar with only minor differences.

Sometimes only the color of printing or position of printing is changed.
These two differ in the plate material:

One has copper.

The other is made of a shiny silvery metal.

Both look remarkably similar to some GE 6AX4 I have, only the plates are slightly longer. So I had a closer look and was up for a surprise.

When I applied heater voltage to the copper version, it only drew less than 1.2A! That looks suspiciously like a 6AX4. Did they take some short cuts in production? The other one draws close to the nominal current:

For 6.3V heater tubes the nominal voltage should be close to spec but the current can vary, but this is some 30% below the datasheet value! This should be taken into account if designing for 6AU4. I doubt such a tube has the increased current capability over the 6AX4.

I also have some GE 6AU4 in military packaging. Unsurprisingly these draw the right current.

I am sure the military would not have accepted such a deviation from the data sheet value. But apparently they were a bit sloppy with the commercial tubes. I will examine this also for other brands.

Lastly 6AU4 by Canadian General Electric above, which concludes the GE section.

Next we have Tung-Sol.

This one looks quite regular.

A different version in another box style:

It is a bit longer than the other one.

Let's have a look at their heater current.

Both well in spec.

CBS:

An older version marked CBS Hytron

Later they dropped the Hytron name.

Both rather large bottles with long plates.

Quite different in construction.

Due to the large plates I expected the heater current to be in spec.

That's all I have from this manufacturer.

Up next DuMont which used to buy tubes from other manufacturers and sold them under their label.

The tubes were probably made by RCA.

Nice large construction and heater current in spec.

DuMont used very unique and beautiful gold coloured boxes.

4 different 6AU4 by Motorola:

They changed their company logo and tube packaging a lot.

Philco was another equipment manufacturer who sourced tubes from other companies and relabelled them.

Zenith also sourced their tubes from other manufacturers.

I have three versions of the Zenith brand:

Large base copper plate:

Large base regular plate:

And the same with coin base:

How do the heater currents compare?

The copper plate tube draws current more like a 6AX4:

The others are close to the nominal 1.8A:

That's all from Zenith.

I only have a few tubes from Amperex. Among them two different versions:

The one on the left looks suspiciously like a GE. It's heater current also measures low.

The other one has the famous bugle boy logo on the base.

And also has the larger construction with the proper heater current for a 6AU4.

Up next various brands of which I only have single tubes or very few. Most of these are house labels from store chains or otherwise rebranded tubes.

Lafayette:

Emerson:

Airline, unfortunately this tube is dead, as can be seen by the whitened getter:

United Electric:

BesTest:

'This one is completely unbranded:

Another 'no brand'. Boxes named Quality Brand and a very faint 'Delta' on the base:

Triad:

Rad-Tel:

IEC:

Truetone:

Selectron:

Silvertone:

Magnavox:

Zalytron:

Since I have not enough of any of these to make a quartett, I did not bother measuring them. But they are nice additions to the tube box art collection.

Before we turn to the big boys in tube munaufaturing, let's have a look at the internal construction of a 6AU4 tube.

I picked one of the Westinghouse tubes with the unusual glass disc inside.

Removing the glass bottle gives a better view. There are 4 support rods molten into this glass disc.

Beneath the glass disc there is the usual mica which ensures the spacing between cathode and plate.

At the bottom there is another glass support disc.

Again with 4 rods molten into the glass.

Heater wire pulled out a bit:

The heater wire with the isolation spiral.

This spiral gives the very high voltage rating between heater and cathode.

The tip of the heater which consists of a folded wire:

Clipping the support rods which hold the plate and cathode partially pulled out:

The cathode still attached to the bottom mica:

The rim at the ends of the cathode clips into the mica discs:

The bottom glass disc. A piece broke off when I clipped the metal rods:

The pate:

Glass and mica disc from the top end:

The other side with the metal ring which held the getter during manufacturing:

The mica discs are held in place by small metal strips which are welded to the support rods:

Two of the large manufacturers remain to be covered. Here are the Sylvania 6AU4 tubes:

Let's start with the oldest versions a 6AU4GT (without 'A'):

A quick check of the current again showed 6AX4 type heater behaviour:

So this is clearly not a matter of vintage. At the end of the tube era it was common practice to use the same internals for different tube types which are close in spec, to save cost. But this tube is from the golden tube age.

Here we have another early 6AU4 which draws the right heater current:

And another one:

Next, two versions of 6AU4GTA. Both with the same internal construction but slightly different glass size:

And the same internal construction in a coin base bottle:

All three have heater currents in spec.

These tubes have the heater wires extending well outside the cathode and even the isolation spiral which makes the heater glow nicely visible:

And finally two Sylvania 6AU4GTA in military packaging.

A nice tube with green printing on the base.

As expected with heater current in spec.

A later version with coin base, made in the late 1970ies

Apparently at the end of the tube era even the military had to accept large spec deviations:

This concludes the Sylvania section.

Of course no tube of the month post would be complete without RCA tubes.

All the RCA 6AU4 I have are of the GTA flavour.

They all have tall plate structures.

With slight variations in construction

Next a version with coin base which came in the later box design.

All the RCAs had a measured heater current well in spec ...

... as would be expected from such a premium manufacturer.

And that's all about the 6AU4.

This tube of the month post turns out to be the longest ever with well over 100 photos. I hope you enjoyed it.

Best regards

Thomas

↧

300B Mono Blocks in Tower Style

March 14, 2016, 4:54 am

≫ Next: Live from Bath : Cool Gales AudioFest 2016, Part 1 : Arrival

≪ Previous: Tube of the Month : The 6AU4

Hi!

The 300B monos in the tower style chassis are finished. These will go together with the 10Y line stage shown earlier this month. And will be used with power supplies in the same style.

These amps use the same circuit design and parts as the 300B monos built last summer. Just the chassis style is different. Signal section of one channel:

The interstage and output transformers are made by Tribute

The interstage transformer is placed on the top while the larger output transformer resides inside the chassis.

The tube sockets are on sub plates mounted below the top plate.

The two mono blocks with the matching line stage:

With tubes lit up:

Some more views:

View from the top:

More tube glow:

Best regards

Thomas

↧

Live from Bath : Cool Gales AudioFest 2016, Part 1 : Arrival

March 17, 2016, 4:26 pm

≫ Next: Live from Bath : Cool Gales AudioFest 2016, Part 2

≪ Previous: 300B Mono Blocks in Tower Style

Hi!

Arrived in Bath today for the Cool Gales AudioFest which will start tomorrow around noon.

As in previous years the event is located in two separate venues in short walking distance of each other. Part is at the Bath & County Club, I got a room in the Bath Royal Literary and Institution.