Tube of the Month : The 300B

Hi!

Since this is the last post in the tube of the month series, of course it will be about the tube many of you have been waiting for: The 300B.

The 300B is probably the most famous directly heated tube and enjoys increasing popularity ever since I came across this tube the first time. But more about that later.

The 300B was introduced in 1938 by Western Electric initially as the 300A and later renamed 300B after a minor change which only involved a different location of the socket bayonet pin. It shares the same UX4 pinout with most of the US made directly heated triodes. The pinout is shown on the left. The filament is brought out to the two thicker pins while grid and plate are connected to the others. The traditional 300B has an oxide coated filament which operates at 5V and consumes 1.2A. The plate dissipation is 40W (initially 36W but later uprated). With its ratings the 300B nicely fills the power gap between small DHTs like 45 and 2A3 and transmitting tubes like 211 or 845. The former have obtainable output powers of 2 and 3.5W respectively while the latter fall well into the 2 digit range with up to 20 or 25W (more when pushing them into A2 operation). While the smaller output tubes are easy to use due to the low B+ voltage needed, the transmitting beasts run st voltages of more than 1kV which comes with a whole new set of challenges. There had been some other DHTs like the 50 which could produce 4.5W albeit at higher B+ voltages than 45 or 2A3. On the transmitting side there were the 10Y and 801A but in the same power camp as the 45 and 2A3. The 300B sits in the middle of these two groups with 8W output power. Some push it harder to get into the double digit realm but 8W should be the target if used properly. This way the 300B occupies a unique spot in the field. 

A look at the plate curves shows the impressive linearity which the 300B shares with other directly heated triodes. Below a screen shot of the curves from a real tube.

The data sheet is available on the website of Western Electric who started making 300Bs again to the original specs.

My own audio journey is deeply connected with the 300B tube although we didn't always had the best relationship. In the 1980ies when I started to use and like tubes, pretty much all that was available were EL34 and 6550A push pull amplifiers. Maybe KT88 if you were fancy. While there was a lively scene of directly heated single ended amps in Japan, not much of that was visible in Europe.

In 1991 I heard about the 300B for the first time. Jean Hiraga is probably the most famous European who started to use this tube and offered amplifiers with them.

I found a small german manufacturer who offered Single ended 300B amps as well. After a listening demo of the amps in my home I ordered a pair of mono blocks. This opened up a whole new world of sound and the amplifier came with a pair of Western Electric 300Bs made in 1988. That was just about the time when  they stopped producing them.

Since the tube was no longer in production I wanted to save the originals and started looking around for alternatives. Back then some chinese 300Bs were available. So I got a pair but that did not last very long, after a few weeks the sound became dull and uninvolving. But the search for tubes brought me into a totally new world. While looking for tube suppliers I came across an underground scene of tube amplifier builders. I discovered Joe Robert's Sound Practices magazine.

I was hooked. I wanted to build my own tube based electronics. I read everything I could find about electron tubes, their use and vacuum tube amplifier design. My first humble project was a linestage. It sounded so much better than the Audio Research SP9 which I was using at the time. That fuelled the desire to build my own audio gear even more. So the next project would have to be an amplifier. 

But after having read so much about transmitting tubes and their brightly glowing thoriated tungsten filaments, it had to be something serious: 211 mono blocks! The initial build sounded so good that I forgot about the 300B and explored transmitting tubes further. After the 845, the 801A followed and would become my favourite tube for years to come.

I initially built amplifiers and preamplifiers for my own use only, I received more and more requests  from friends to build something for them. During that time the 300B and other DHTs would become increasingly popular and they swapped from the DIY underground into the commercial scene with more and more manufacturers getting on the single ended DHT band wagon. In 2008 I decided to start my own audio business with building tube electronics. By that time the 300B was a bit overhyped in my view and I still preferred to explore alternative tube types with the thoriated tungsten filamentary variety still being my favourite. Due to its popularity more and more companies started to make 300Bs again. Among them also Electro Harmonix:

Since 2013 I am doing audio as a full time profession. In the same year I got introduced to Dr. Schaffernicht, owner of ELROG Elektronenröhren GmbH & Co KG who just started to make 211 and 845 tubes in Germany. During my first visit in his factory he asked me which other tubes besides 211 and 845 would be worthwhile making. It wouldn't have made sense to suggest my favourite tube, the 801A, since that hardly saw any use in commercial amplifiers, so of course I mentioned the 300B. And since Dr. Schaffernicht had manufacturing experience with thoriated tungsten I mentioned that a 300B, not with oxide coated filament but with thoriated tungsten, now that would be something! Basically a 801A upscaled to meet 300B specs or a 845 downscaled, whichever way you want to look at it. The promise of that thoriated tungsten sound in a tube which fills that power gap just under 10W. But little did I know how difficult it would be to get a thoriated tungsten DHT close to 300B specs.

The chief engineer at ELROG back then had the same idea of a thoriated tungsten 300B and took the challenge to develop it. About a year and a half later ELROG introduced the ER300B.

The initial sound impressions were so good, that it brought 300B amps back on my radar and I started building and selling them as well. 

But unfortunately Dr. Schaffernicht rushed to market too early. The extremely thin (50 micron) thoriated tungsten filaments were very sensitive and would fail in amplifiers without or insufficient current limiting in their filament supplies. 

This would lead to an immense failure rate in the initial batches. Due to that I stopped offering the ER300Bs except for use in my own amps which had controlled filament start up. But Dr. Schaffernicht and his other distributors kept pushing the tube in the market.

In 2016 this resulted in the demise of ELROG Elektronenröhren GmbH & Co KG and the company filed for insolvency. But fortunately the chief engineer of ELROG, who loves tubes as much as I do, contacted me and asked if the both of us should rescue german tube production. We agreed on the way forward and I acquired the company assets from the insolvency advisor and founded a new company, Deutsche Elektronenröhren Manufaktur GmbH which would continue to produce tubes under the ELROG brand.

Of course this required an overhaul of the production line, improvement of the design of the 300B to rid the filament failures and a fresh up of the logo. Above the initial ER300B (left) produced under the new company compared got the old one. Also the uniformity of the glass bulb was vastly improved.

Over the years we continued to refine the design by switching to a black aluminum base with teflon insert since the older brass bases tended to get a lot of patina from handling with bare fingers. The plate material was also changed several times until we arrived at the latest version which uses bare polished nickel which does not require coating anymore.

And that's how we still produce the ER300B today, now in the 8th year.

But things did not end there and we wanted to explore how the sound quality could be improved even further. In 2020 we released a version of the 300B with molybdenum plate, instead of nickel. We started to experiment with molybdenum with the ER242 which was a variant of the ER211 with this plate material. After the success of this material with its promise of reduced secondary emission from the plate, we also decided to offer it in a 300B variant which we named ER300B-Mo.

Now two variants of the 300B should be all you can hope for, but there is always that itch to try to push the envelope even further. While I was preferring the neutral, detailed and dynamic sound of thoriated tungsten tubes over those with oxide filaments, there was one tube type which would make me forget the shortcomings of oxide filaments if it had more output power, the 46. While it is not able to reproduce with the resolution and dynamics I'd like, the tone of that tube is from another world. Getting that tone combined with the advantages of thoriated tungsten in a package that can deliver 8W would be a big step.

Fortunately I have the pleasure to work with a gifted electron tube engineer who turned this idea into reality and we introduced our third 300B variant later in 2020. Now this was everything I could ever have hoped for and this tube deserved to get my initials in the tube type designation. I can comfortably say this is my all time favourite tube and does everything in the way I want it. 

But of course there always is that itch to explore further. We briefly toyed with the idea of a molybdenum version of the TM300B. The molybdenum does not seem to bring the same advantage in this tube due to its different internal construction. So we did not follow this path further and there are only a few prototypes. But that doesn't keep us from trying other new things like gold plated anodes.

But that turned out to not work too well and the idea is dropped. So the TM300B remains the king of our line up.

At least for now, let's see what the future might bring....

New electrode geometry maybe ?

I hope you enjoyed reading about my journey with the 300B. Now let's have a more detailed look at some of them.

Since I have my own tube production and did not care too much about oxide coated DHTs before that I do not have many 300B types. Of course there are far more 300B types available now than I am showing.

Since the 300B was originally introduced by Western Electric, they get to be shown first.

A beauty in the classic ST shape glass. Initially with the tube type etched into the base and later with the characteristic yellow printing.

A few close ups:

A timeless classic which deserves it's place in the tube audio world.

Next we have the Electro-Harmonix 300B made in Russia.

 

Some close ups:

And of course ELROG ER300B:

Our tubes have the characteristic tip since they get evacuated through the top.

Thoriated tungsten glow!

The ER300B-Mo

And the TM300B:

This time branded with my name for use in amplifiers built by me.

Now I would normally open up a tube to show the internal construction. But having a tube factory at hand I can do better and give a little tour through the production process of an ELROG ER300B made at Deutsche Elektronenröhren Manufaktur GmbH.

It starts with the raw glass.

Which gets shaped into the characteristic form of our tubes.

The foots which will later be sealed to the bottom of the bulbs has to have all the through wires to make connection between the base and the electrodes.

We melt the glass in graphite moulds for them:

The assembly of the internal structure is done by hand.

The individual components are pre assembled and pre shaped.

And then mounted together piece by piece.

The filaments are mounted. Grids need to be wound before the filaments can get inserted.

Grids are made in a two step process. Each winding is welded to the support rod.

Now grid and filament assembly can be mounted together.

The grid/filament assembly then gets attached to the foot.

Almost finished internal systems, but some parts are still missing.

Serial number added.

The getter ring which will be flashed after the tube is evacuated:

And of course the plates still need to be added.

But first everything is washed in an ultrasonic bath.

Final assembly of the internal system:

Ready to be sealed within the glass!

Prepared glass tubes waiting to be married to a system:

The marriage:

Everything is carefully pre heated.

Then the outer glass gets slowly pressed towards the foot.

And makes an air tight seal.

Sealing process finished:

And here all in a video:

The sealing process from an other angle:

This is probably the most spectacular part of the tube making process.

Cooling down:

Sealed tubes waiting to go onto the vacuum pump for evacuation:

But first they must pass a test to check if the base is air tight.

This is done with a helium sniffer. The tube gets temporarily evacuated and helium is blown around the base. The detector will beep if it even measures a single helium atom getting through.

Pass! The tubes get mounted on  the pump for the next step.

Evacuation is typically run over night while the pump stand gets closed and heated up.

The internal system gets heated with RF induction  to drive out trapped gasses while evacuation continues. Here shown with a 845:

After evacuation is complete the stem through which everything is pumped out is melted shut and the tip rounded off.

Now only the getter needs to be flashed before the tubes can go through the activation process.

This is also done through RF induction which heats up the getter material which evaporates through the heat and then condenses on the glass to form the getter mirror.

The tube is ready to be activated and measured for the first time.

This is done without the base. The connections are done with clip leads.

After initial test, the base is glued on and the pins soldered.

Finished tube ready to go through 24h burn.

After burn in each tube is tested again.

The tubes are finished and ready to receive the printing on the glass.

After the printing is applied the tubes are tested again and matched into pairs.

The printing on the glass of course also has the tube type to distinguish the different 300B variants.

We also do custom printing for amplifier manufacturers.

I hope you enjoyed reading about my history with the 300B and about the making of ELROG tubes.

As mentioned above this post is the last in the tube of the month series.

After 13 years and covering more than 150 tubes it is time to end the tube of the month series. It has been fun and I hope you enjoyed it.

Maybe there will still be an occasional tube presentation but more like tube of the year. We will see. Stay tuned!

Happy New Year!

Thomas