As mentioned in the previous post about the recently built single ended amplifier, the 46 deserves more attention. So I selected it as tube of the month.
Since the 46 was already tube of the month way back in 2011, this is a revisit. That first post was a bit short and in the meantime there are a few more 46 tubes in my collection to show.
The 46 is called a dual grid power amplifier tube hence it has a 5 pin base. This is not to be confused with a tetrode since the 46 is only meant to be used as triode. Depending on how the second grid adjacent to the plate is connected, a triode with different parameters is obtained. By tying both grids together we get a triode with high amplification factor which is meant to be used as output triode in a Class B push pull stage. Two tubes in such a configuration deliver 10W output power. Such an amp design is shown in the RCA data sheet. Since the tubes would be operated near zero grid bias, such an output stage would require some significant driving power. Here the other operating mode of the tube comes into play. When the second grid is tied to the plate the resulting triode is very similar to a 45. As transformer coupled driver stage it can deliver 1.25 Watts into the grids of the output tubes.
Of course it can also be used as a single ended output stage this way. But the 1250 milliWatts is all you can get out of one. Even less than the flea power a 45 does provide, which can be pushed to deliver almost 2 Watts. But those 1250 mW are simply stunning. As I have written in the last post: The quality of the 46 is actually not really a secret. It has been repeatedly reported from different people that they prefer the 46 over the 45. I myself used the 46 almost 20 years ago and found it to be a great performer. But still it stayed below the radar of most amplifier builders. Maybe it is the extremely low power which prevented people from using it or the fact that it is not a 'real' triode with the two grids. But that is nonsense. Maybe it is the second grid which has something to do with the special sound of this tube. Many people are fond of tubes with mesh plates. It could be the combination of second grid and plate tied together which mimics some of the characteristics of a mesh plate. Or maybe it is simply the extremely low output power which means that an amp with this tube is almost always operated near the max and produces a pleasantly sounding amount of second harmonic distortion. Whatever the reason is, when you listen to this tube you will not worry about it and simply enjoy. After all it is a directly heated tube and when connected as Class A output triode it has nicely linear plate curves as can be seen in the data sheet curve diagrams and a photo of the screen shot of some actual curves taken with the tracer:
Let's have a look at some 46 tubes.
Here we have the ST shape variety made by Sylvania which is a bit larger in size than the 45.
Nice green leaf logo printed on the base.
Three different varieties from this manufacturer:
These, like almost all 46 tubes I saw, have an unusually large amount of getter in them. Almost covering the whole bulb up to the shoulder.
A Tung-Sol VT-63, the military designation of the 46:
Here we have a nice row of General Electric 46s:
Similarly large getter area as seen in other tubes:
Two different varieties:
Close up to the plate:
View from the top:
Close Up showing the electrodes:
here a quite unusual 46 made in France by TSF:
These have less getter than the US-made tubes we saw above:
The tubes are branded 'Dario':
Some close ups:
And of course there were plenty 46 made by RCA:
Cunningham branded 46 made by RCA:
And finally some globe 46 tubes:
Interestingly the globe 46 don't have the 3 letter designations like the 45 which was named 245 or 345 before introduction of the ST-shape glass.
These globes are simply beautiful.
Some details of globe 46s:
These have a combination of mica disc and glass piece for alignment of the electrodes.
Cunningham C-46 with beaten up box:
RCA-46 with spec sheet:
Being a tube manufacturer myself now, of course the ELROG 300B, 211 and 845 are among my very favourite directly heated tubes along with 10Y, 801A and 45 and now also the 46.
If you are up for a new amplifier and if your speakers are very sensitive, the 46 might be an excellent alternative outside of the mainstream!
I hope that I will get the chance to build more amps with this tube and a 46 squared (46 drives 46) would be something worthwhile to explore!