Some years ago my friend Paolo brought up this lovely valve which looking at the curves seemed to be a great candidate for a driver which required large voltage swing. This indeed is needed for 300B, 211/GM70, 813 SE Amps. I build it and tested with the 300B, which is great.
The immediate challenge (and probably the reason why it hasn’t been used extensively in the past) is that has a high anode resistance as well as very low current capability. These 2 things are of course a killer for driving these demanding output valves. However, with a gyrator load and a follower stage, the reality is different
The circuit is dead simple. The triode is biased with SiC Schottky to minimise the impact on the reflected anode impedance due to their low dynamic resistance. A pair is sufficient to set the bias to provide large voltage swing. The triode is very linear so no need to add degeneration in the cathode by replacing these SiC diodes with a resistor instead. You can do that if you want.
The output load is minimised by adding the SiC follower which presents a light load to the delicate 6SF5 whilst providing the current to handle any grid of either a 300B or any transmitting valve. In order to add the grid bias to the output stage, this one is DC-coupled to the SiC FET and instead we add a 100nF capacitor between the gyrator mu output and the gate stopper (R10). Here we insert a 1-4M7 resistor with the bias voltage. The T5 and R12 instead of returning to ground they do return to the negative voltage allowing the swing needed. That will vary depending of the output valve.
I hope this is clear enough, the above circuit represents the test rig, not the output stage.
With a mu of 100, be prepared to have a source which can handle a 250pF capacitance due to the Miller effect on this triode. Not impossible, but worth considering when building this stage.
Here is a sample 6SF5GT I used:
The circuit behaves as expected:The
The distortion is very good. At 52Vrms (circa 150Vpp) you get 0.35%. very nice indeed.
Here is the frequency response test, which is the cherry of the cake!
With 40dB of gain you can get a bandwidth of nearly 50kHz. This has sufficient gain to drive any demanding valve to full output without any preamp when driven from a CD player or DAC.
Very simple circuit which performs great. I will have to test it the GM70/813 next.