Tuesday, September 6, 2011

Tube of the Month: The 45

Hi!

So far mainly indirectly heated tubes and rectifiers have been covered in this series. The only directly heated tube presented was the 46. But that is not a 'real' triode but a dual grid amplifier tube. Today I will write about the first directly heated triode in this series, the 45.



The 45 is an absolutely magnificent tube. It is among the 2 favorite directly heated triodes of mine. When it comes to colors, emotionality of reproduction and sheer beauty of sound, the 45 is unbeatable. It is the queen of tone. When the 45 was introduced, it was quite a step in achievable output power. Back then it required high plate voltages to get more than 1W out of a single triode with tubes like the 10. At lower plate voltages tubes like the 71A would yield a mere 0.7W. With the 45 up to 2W can be obtained with plate voltage below 300V, making the power supply easier. Compared to nowadays standards 2W seem like a joke. The 45 requires a suitable speaker to be able to show it's full capability. Speakers with sensitivities of 100dB or more should be used. It is possible to use less efficient speakers if you don't need to play at very loud levels or if you have a small room. Many people are surprised how much volume they can get from those 2W even with moderatly sensitive speakers, when they listen the first time to a single ended 45 amp. As was once written in the Sound Practices magazine: Don't think two Watts, think two thousand milliwatts!

The 45 shares the UX4 socket and same pinout with most of the famous directly heated triodes for audio frequency amplification. It has an oxide coated filament which requires only 2.5V. Due to this rather low filament voltage the 45 can be operated almost hum free even with AC heating. This together with the low plate voltage requirement of 250V typical, 275V max makes the 45 quite easy to use. For further details refer to the datasheet. So why does this tube sound so good? In fact I prefer it to all other directly heated tubes with oxide coated filament. Only when it comes to absolute resolution and neutrality, I prefer thoriated tungsten filamentry triodes. But as mentioned above, no other tube I know can beat the tone of the 45. This makes it easy to forget about the last bit of resolution and neutrality. Back to the question, which properties of this tube are responsible for it's sound quality? Have a look at the 45s plate curves which show their exceptional linearity:


The load lines drawn into the curves represent the manufacturers suggested loads which are more optimized towards maximazing power output. I like to use the 45 with higher impedance plate loads like 5k or even 7k Ohms. This improves linearity further and gives a better damping factor. With such an output transformer the 45 will not only have beautiful midrange and smooth highs but also a well defined and solid bass. No woolly lows as are sometimes heard from SE DHT amps. The next photo is a screen shot from an oscilloscope showing the plate curves of a real 45 tube taken with a curve tracer:


I have measured many 45s and when the tubes have no defect and the emission is still ok, they all show this exceptional linearity.

The 45 has been manufactured by many companies. Here is just a small selection of different 45s in the coke bottle shape:



And a selection of boxes from manufacturers like RCA, Sylvania, Raytheon, National Union, Fivre and many others:





Not all of these brands had their own manufacturing. As with many tube types, cross branding was a common practice. So you will find 45s with identical internal construction but with different brand names on them.

I'm often asked which brand I prefer in certain tube types. I'm not really a tube roller. As long as the tube is physically intact and measures ok, sonic performance is on the same level. Electrodes can sometimes be misaligned from transport or careless handling. This can lead to non linear plate curves. Especially very old tube samples can have this problem. This is why I use a curve tracer to check all tubes and match them to create perfect pairs. Although many brands just relabelled tubes from the large manufacturers, also many companies built their own tubes. So the 45 can be found with different internal construction, like plate profile and internal support. The next picture shows the different upper support structures which align the electrodes with a mica plate:



However as mentioned above I do not think the differences between various ST shape 45s are significant. It is more important to find tubes which measure well.

I do care for the sound difference between the ST or coke bottle shape and the earlier globe shape of the 45. These can reveal even more of the magic the 45 has and have a smoother even more involving sound. The globe 45, was actually named UX245 (RCA) or CX345 (Cunningham). While the ST shape 45s are still fairly easy to find in good quality, it gets increasingly difficult to find globe shapes which are still good. But I think worth the trouble finding them. The coke bottles can still even be found as NOS samples. This is almost impossible with the older globe shapes. Even if they come in seemingly original boxes, I have yet to come across a genuinely NOS globe shape UX245. Used ones can still have a lot of life left in them. With the globes it is important to be able to thoroughly check them. I think a curve tracer is mandatory to make sure the tube is ok. I have seen tubes which would measure ok on a static tube tester but reveal distorted plate curves on a tracer. Especially the globe tubes are prone to electrode misalignment. A reason for this is the lack of any physical support of the internals at the top of the tube.


They lack the mica plate which is used in the ST shape tubes to align the electrodes and give them support in the top end of the bottle. On the other side there have been speculations that exactly the lack of mica support in the globe shape tubes are one reason for their good sound. The mica plates can release gases over time and have a negative impact on the quality of the vacuum. Indeed most of the old globe shapes I have come across had excellent vacuum.

The globes are quite a bit larger compared to the coke bottles:



As mentioned above, the linearity of the tube is probably one of the most important factors which contribute to the good sound. Another one is the low filament voltage which makes the use of AC heating possible. DC heating can have a very negative impact on the sound if not done right. I have used AC with 45s succesfully, without any hum issues. Another and in my opinion also very important point is the modest drive requirement. The grid of a 45 is a fairly easy load to the driver. It needs a modest 35-40V RMS to drive it to full power. This makes 2 stage amplifier concepts feasable. A good driver tube for the 45 is the 6N7 which I introduced already in previous posts. Below are some pictures of an amp with the 6A6, the predecessor of the 6N7, as driver. The first one shows the amp equipped with globe shape UX245:



The next photo shows the amp with ST 45s plugged in and together with the external power supply:



I hope you enjoyed this first Tube of the Month article about a directly heated triodes. Stay tuned for more DHTs to be covered in upcoming posts.


Best regards

Thomas

27 comments:

  1. Hallo Thomas,

    das ist ein schöner und informativer Artiekl über die 45!

    Bezüglich des Treibers ist mir jetzt die Frage gekommen, ob schon mal jemand auf die Idee gekommen ist mit einem entsprechenden Eingangsrafo a la Sakuma 45 (Treiber) auf 45 zu realisieren? Man bräuchte da wohl ca. 1:8 oder 1:10 am Eingang, soweit man mit dem Interstage nicht auch noch hochziehen möchte.

    Gruß,
    Bernd

    ReplyDelete
  2. Hallo Bernd,

    hinter dem Ansatz von Sakuma steckt mehr als einfach nur die gleiche Röhre als Treiber zu nehmen. Gerade die 45 ist ja recht leicht zu treiben. Wenn es ein direkt geheizter Treiber sein soll, dann ist die 26 eine gute Wahl. Die hat einen höheren Verstärkungsfaktor und man braucht kein allzu grosses Übersetzungsverhältnis im Eingangsübertrager. Ich habe hier auf meinem blog ja schon eine Endstufe vorgestellt mit wahlweise 26 oder 10Y als Treiber. Die 10Y passt noch besser zu 45 als die 26. Meiner Meinung nach eine Traumkombi.

    Viele Grüße

    Thomas

    ReplyDelete
  3. Hallo Thomas,

    ja, den Bericht über die Endstufe habe ich selbstverständlich auch gelesen ;-) Ich dachte, dass bei 45 auf 45 dann der unverfälschte 45er Klang zum Tragen kommt und man hätte die gleiche Heizspannung für Treiber und Endröhre. Wenn Du von Traumkombi sprichst, dann scheinen sich ja quasi die "best of both worlds" Eigenschaften zu vereinen. Könnte hier als Treiber nicht auch die von Dir hoch geschätzte 801A zum Einsatz kommen?

    Viele Grüße,
    Bernd

    ReplyDelete
  4. Hallo Bernd,

    die 45 hat den Vorteil, dass sie als Endröhre problemlos mit Wechselspannung geheizt werden kann, ohne Brummprobleme. In der Treiberstufe müsste sie dann DC geheizt werden. Ich habe selbst noch keine 45/45 gebaut. Wohl aber bereits 801A/801A und 211/211. Bei der 45 könnte das etwas zu viel des Guten werden, die 801A und 211 sind da wesentlich neutraler. Bei der Kombination 10Y/45 vereinen sich tatsächlich die besten Eigenschaften aus beiden, Auflösung und neutralität der 10Y und die Magie der 45. Die 801A ist der 10Y klanglich sehr ähnlich und kann genauso zum Einsatz kommen.

    Aber letztlich käme es mal auf einen Versuch an die 45 mit einer 45 zu treiben.

    Viele Grüße ... Thomas

    ReplyDelete
  5. Hallo Thomas,

    Sakuma hat ja mal einen RCA-50 auf RCA-50 gebaut mit Eingangstrafo 150:20k (ca. 1:11,5), allerdings mit Zwischenübertrager 10k:20k (1:1,4). Die 45 bräuchte aber nicht so viel Signal wie die 50 (-63V bei 350V Anodenspannung, 45mA), wenn man das Datenblatt vergleicht. Dann könnte der Zwischenübertrager 1:1 sein. Die fast 1:12 am Eingang könnten dann aber wohl problematisch werden.

    Viele Grüße,
    Bernd

    ReplyDelete
  6. Hallo Bernd,

    bei Sakumas Konzepten ist zu bedenken, dass er eigene Klangvorstellungen verfolgt und nicht unbedingt wert auf neutralste Wiedergabe und auch in den Höhen ausgedehnten Frequenzgang legt.

    Ein Eingangsübertrager erfordert eine Vorstufe, die diesen adäquat treiben kann. Bei einem hohen Übersetzungsverhältnis ist das unabdingar. 1:8 halte ich schon für grenzwertig und versuche bei den Eingangsübertragern bei maximal 1:4 zu bleiben. Mit der 45 als treiber könnte man allerdings problemlos einen 1:2 zwischenübertrager verwenden.

    Viele Grüße

    Thomas

    ReplyDelete
  7. felocitacionesdesde argentina al autor de esta nota tengo mi amplificador 45 ,,que arme,,y la experiencia es espetacular,es real lo quedice¡¡,,no escuchado nada asi¡,arme varios amplis,,y seguire armando pero creo queel 45 esmi amor,si queren les envio fotos ¡saludos.daniel de bahia blanca.

    ReplyDelete
    Replies
    1. Daniel

      Yo estoy de acuerdo con vos, una vez que uno escucha musica, sobre todo vinil (long plays) en 45s se queda enganchado de por vida.
      Mi amplificador integrado fue manufacturado por Jeff Korneff en Pittsburgh, Pensylvania, y produce un fantastico 1.5 watts por canal. El "midrange" es como miel, acaricia el alma; me faltan palabras para describirlo. La separacion de canales y la precision con la que los instrumentos se posicionan en la escena de sonido es fantastica. Al cerrar mis ojos los musicos y cantores estan en frente mio. Enviame fotos de tu sistema a yes@mris.com.
      Norberto (otro Argentino) desde Virginia, USA.

      Delete
  8. Hello Thomas,

    I really enjoy reading your blog and I often read older articles you wrote as well.
    In this article you mention that the linearity of the tube is probably one of the most important factors and that you use a curve tracer to check all tubes.
    Can I ask you which curve tracer you use? I've been looking for one myself, but can't quite find one to my liking.

    Thank you,
    JP

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    Replies
    1. Hi JP,
      I use the Hagerman VacuTrace

      Thomas

      Delete
    2. Hi Thomas,

      I find that the Hagerman VacuTrace is quite limited with it's capabilities, especially when it comes to the larger triodes.
      If you could have a tube tracer capable of measuring most tubes, including larger and exotic tubes, what would you consider a good range? Ie. anode, grid, screen grid & heater range?
      I'm considering of building one and I'm just curious what the best range would be.

      Many thanks,
      JP

      Delete
    3. Hi!
      The only limitation are maximum plate voltage and current. I rigged it up with a lab supply so I can feed any filament/heater voltage needed. So far I could test every tube I had. Larger triodes can only be tested up to 400V but that still gives and indication. I even tested 211, 849 and 851 tubes with it.

      If you want to test at higher voltages, you ill have to build your own.

      Best regards

      Thomas

      Delete
  9. Hello Thomas,

    Great article about the 45 tube.

    I'm planning for a 45 preamp using output transformers. So far I only have a pair of 10K/600R transformers. Would that be fine to use them, considering the 'standard' seems to be 5K. I do notice in case of 211 tubes, Audio Note used 16K ones, instead of the popular 10K.

    Thanks
    Rob

    ReplyDelete
    Replies
    1. Hi Rob,
      more important is the primary inductance of your transformer and of cause the DC current it can handle unless you plan to use it parafeed. I recently tried several different tubes in line stages (see my recent post about this). As much as I like the 45 in power amps, as line stage tube I did not like it very much

      Best regards

      Thomas

      Delete
    2. Hi Thomas,

      Thanks for the quick reply.

      I got quoted from the manufacturer that primary inductance is 40H, maximum DC Bias Current 60 mA. I heard a 45 preamp at my friend's place briefly, and really loved the sound. My hope is I still can make use of these transformers, without having to order new ones :-/

      Thanks
      Ron

      Delete
    3. Hi!
      I'd prefer more inductance in a preamp output transformer, but it should still work.
      Thomas

      Delete
  10. BTW I have another Tube of the month post. In case you have not seen it yet:
    http://vinylsavor.blogspot.de/2014/03/tube-of-month-45-revisited.html

    Thomas

    ReplyDelete
  11. We are building a 45 preamp 2 box unit using dual 6ax4 rectifiers
    Lundahl dual choke on the output and use just a few coupling caps Audyn true Copper Max Caps are a excellent cap very much overlooked also using 60 k of Jensen 4 pole caps bypass with Audyn. I hope this to be my statement preamp.my Builder Who has been excellent a Radu Tara has taught me a lot And Thomas as Radu says you are the Standard .trying to perfect a DHT preamp that is musical and still quiet .why more people don't get involved ? The challenge it presents is usually the answer.Thank you
    For all your hard work and helping guys like me to learn .

    ReplyDelete
    Replies
    1. Hi!
      I tried both 45 and 46 in a test setup to evaluate different tubes as line stage tube. Didn't like both in this application. Be careful with bypassing of caps. I never do that. I avoid caps which need bypassing. It is easy to do more harm than good with cap bypassing

      BR

      Thomas

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  12. Hi Thomas,

    I am just reading your comment above that combining the 45 with the 801A would be a perfect combination...

    ...well, I was just thinking how to drive my push-pull EML520v2/3...

    initially I thought a 6sn7 directly coupled into an 801A transformer coupled to the 520v2...I like the 6sn7, but not sure if a 4p1l in triode would not be a better choice...fully DHT...but now reading your comment makes me think again...45 into 801a to 300B or 801A i to 45 to 300B...? What would you think ?

    Best Regards

    Frank

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    Replies
    1. Hi Frank,
      I never did 45 into the 801A only the other way around. But anyways there is much more to amplifier design than just the tube selection. It is the entire concept that matters.

      BR

      Thomas

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  13. Will you take a request to do a report on the 833-A? I just finished building a 45 driven through transformer 833-A SET and it sounds wonderful. The 833-A has a thoriated tungsten filament, something you esteem in great measure and you can run it on only 1 KV if you want.

    ReplyDelete
    Replies
    1. I do not have any 833 tubes So cannot write a report about them, sorry

      Delete
  14. Hi Thomas - for the longevity of the globe 45, single-ended with a 5K load, where have you found they are happiest from a bias standpoint? The datasheet puts them at 85% of max plate dissipation at 250V 34mA. I have read the ST types are more tolerant of higher voltages, would be interested what your experience has been with the globes.

    Thanks!
    Keenan

    ReplyDelete
    Replies
    1. This would require a lot of statistical analysis with tests running over many years.

      Delete