300b lovers

Another thing that drove me mad, my SET sounded too analytical and sterile. It took me 6 month to find the source of this issue. It was a Vishay Z-foil resistor in input stage 6sn7 cathode. When I changed it to a Shinkoh 2W resistor all this sound sterility was gone! Z-foil worked OK when I used RC coupling between input and driver stage. RC coupling softened sound. But  when I changed it to more transparent IT coupling, the extreme sharpness of Z-foil resistors showed up.

The cathode circuit is quite sensitive to (subjective) parts coloration ... not surprising, because both grid and cathode are the two input nodes for vacuum tubes. The difference is the grid circuit has very low current flow (but not zero) while the current flow through the cathode is nearly the same as plate current (the full audio signal). This means the full audio signal flows through the cathode resistor and the bypass capacitor, and the tube amplifies any errors in the cathode circuit the same way it amplifies any errors in the grid circuit.

Designers have been assuming for a long time that the grid current in normal Class A or AB operation is negligible, but I don’t think that is true for DHT triodes. They demand very high performance drivers with very low distortion into a complex load, which is where RC-coupling falls short.

The primary merit of transformer coupling is its efficiency, with 95% to 97% of the driver plate current available to the grid of the DHT triode. This is NOT true of RC coupling, where 30% to 50% of the driver plate current disappears into a plate-load resistor, where all it does is heat up the resistor. A dynamic load like a current source is more linear, but the transfer efficiency (between tubes) is no better than RC-coupling, so the unused current goes into a transistor heat sink instead of a resistor. Dynamic loads are also more complex if good performance is desired, with cascoded stacked MOSFETs, with secondary protection diodes, as the most reliable and best option.

Transformer coupling is absurdly simple, with no need for a grid-protection resistor, no coupling cap, no plate load resistor, and no circuit board for the cascoded MOSFETs of a current source. Just wires going to tube sockets.

I suspect the 95% to 97% transfer efficiency of transformers is the reason for the vivid tone colors that are the hallmarks of any IT-coupled amplifier. You hear it immediately, which why Don and I hope more people can hear the Raven preamp with the matching Blackbird amplifier.

The takeaway is that is impossible to "overdrive" the 300B. By contrast, a charmer like an EL84 can be driven with a whisper ... even a 12AX7 biased at 1 mA will sound good as a driver (which doesn’t work with any other power tube). A classic Mullard circuit is ideal for a pair of EL84’s since they are so easy to drive. 6L6's take a bit more muscle, so 6SN7's are a better choice.

The 300B is the opposite. A high voltage, high current, and ultra low distortion driver is mandatory, otherwise you never hear the 300B.

A simple and very effective way to drive 300bs is to use a cathode follower driver, direct-coupled to the grid of the 300b as it fulfills the requirements listed above. This requires a B- supply but you can control the grid so well that it can be driven class A2 (grid current) and you can easily overdrive the tube using a single 6SN7 section. This also allows for a much smaller coupling capacitor; 0.1uf (at the grid of the 6SN7) will allow -3dB bandwidth at 2Hz. This frees up the Voltage amplifier/driver of conventional design from a highly capacitive load. The downside might be that the power tube has to have its bias set correctly (so a provision to measure current is needed), which is done by setting the bias of the 6SN7. 

Doing this I've been able to overdrive 300bs (even multiples!) quite easily. The CF circuit, without the typically large coupling cap that often gives CF circuits a bad rep, has a tight grip on the grid of the 300b; so much so that driving class A2 with the substantial grid current that tube needs is no problem. You can easily drive the grid +15V WRT to the cathode with good linearity. 

The cost of a B- power supply is insubstantial when compared to the cost of a good inter-stage transformer and you get less distortion with greater bandwidth. You also don't have to introduce a power tetrode or pentode into the circuit.

A nice feature of this approach is the bias setting is very stable so might only need checking once or twice a year.