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 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.
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.
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.