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