This months tube is a very special type with some extraordinary properties, the 6HS5. This is a quite 'modern' tube, developed in the 1960ies. It's application was high voltage regulation for the acceleration voltage of color TV picture tubes. It was intended as a pulse type high voltage regulator.
This tube is a beam triode. This means it has another electrode between grid and plate. Similar to beam power tetrodes, it has a beam forming plate. What makes this tube so unusual are it's extremely high amplification factor of 300 together with a transconductance of 65.000 micromhos! This results in a plate resistance of about 5kOhm which makes the tube usable for LC or transformer coupling. Thus it's full amplification can be utilized.
Well, nothing comes for free. While the tube has exceptional transconductance paired with a very high mu, it requires a rather high plate voltage to operate. In single ended Class A, over 1000V B+ is needed to achieve sensible results. This is probably the main reason why it is not so commonly used, besides the 'TV tube stigma' which causes many very interesting tubes to be ignored by audio designers.
The curves in the datasheet compare well with my measured results. The picture below shows the plate curves of a actual tube, taken with a curve tracer. This is only a small section of the curves since the tracer only reaches about 350V. As mentioned above plate voltages beyond 1000V are needed for reasonable results. Still this small part of the curves shows that it is very well usable for audio.
Plate voltages of 1200-1300V are still quite reasonable and DIYers who are used to build with transmitting tubes like 211 or 845 can handle this. Another difficulty however is the highish plate resistance. Measurements showed plate resistances above the 5kOhms mentioned above. So an output transformer with high primary impedance would be needed. Very few such transformers exist. Suitable would be the Tango FW20-14S with 14k primary impedance, or the Lundahl LL2735B with 16k primary impedance.
This circuit can be easily scaled up to achieve more power by paralleling tubes. A Parallel Single Ended verison with two tubes per channel delivers twice the power:
This scheme can be even scaled further. Below a photo of Quad Parallel Single Ended mono blocks. These deliver solid 30W single ended pure Class A:
Close up photos, showing the glow of the tubes. The blue glow is normal and does not indicate bad tubes:
This tube would actually also be quite suitable for a multi channel amp or for an active system. Multiple 6HS5 amps which share the same PSU can be easily integrated into a single chassis. I did build something like that for the European Triode Festival 2004, 4 stereo amps (8 channels) in a single chassis for a 4 way active speaker. This consisted of 4 SE amps using one 6HS5 each, two PSE amps with 2 tubes each and 2 parallel push pull amps with 4 tubes each. Even the linestage was integrated using yet another 6HS5 per channel. So 18 6HS5 tubes in a single chassis. The photo on the left shows an impression how that looked like.