Here we have another 'sleeper' tube which is largely ignored by the audio crowd but very well useable as an output tube. The 6HJ5:
The 6HJ5 is a beam power tube which was intended for use as horizontal deflection amplifier in TV sets. Like many of those it can be triode connected and is useable as 'pseudo triode'.
It has a 12 pin duodecar base aka compactron. The pinout is shown on the left. With grid nr 2 connected to the plate and grid 3 (the beam plate) connected to the cathode it makes a nice triode with an amplification factor of 4.2. This puts it in a similar ball park as a 2A3. But easier to use due to the indirectly heated cathode and it comes at much lower cost. With a maximum plate dissipation of 24W it should be good for 4-5W in Single ended mode. These TV tubes are usually very sturdy and can be operated at their max rating and still last long. The heater runs at 6.3V and consumes 2.25A. Quite a lot of heater power which is needed to warm up the cathode which can emit up to 1A of peak current. The high peak current capability of such TV tubes is probably one of the reasons why the perform so well as audio amplifiers. For complete technical parameters see the RCA data sheet
. The data sheet does not show any plate curves. So I took some in triode mode with a curve tracer:
Makes me want to use this tube in a project. But so many tubes and so little time...
Here we have 6HJ5 made by RCA.
6HJ5 made by Raytheon:
I only have 6HJ5 from these two brands.
Let's open one up to see the internal construction.
Through the holes in the sides of the plate we can have a peek at the grids:
Pulling out the heater:
The two grids. Grid nr 2 is carbon coated while grid nr 1 is gold plated.
The beam forming plate :
A 6HJ5 in operation:
What a marvellous tube!