In the Tube of the Month Article about the UX201A I already mentioned it's unique sound qualities. This is the first post of a series of articles about a device which I call a 'Sound Processor'. It uses a single UX201A triode per channel as active element. The purpose of the device is to present the sound of the tube itself in it's purest form with as little coloration as possible by other components in the signal path. This is the extremely simple circuit that does this:
As you might have noticed this circuit has no capacitor in the signal path at all. There is also no hidden one in the power supply since that is decoupled through a choke from the circuit.
To achieve this the circuit uses an enhanced version of filament bias. Filament bias effectively removes the need of a cathode bypass capacitor which would be in the signal path. It pulls the entire filament current through a resistor to ground which thus becomes very small in value (22 Ohm in this case). So small compared to the plate resistance of the tube that it can be neglected and does not need a bypass capacitor.
But the usual filament circuit still has a capacitor in the signal path on the B+ side. In case of a transformer coupled stage this is the capacitor from B+ to ground (or B+ to cathode in case of the ultra path connection).
In case of the circuit above this capacitor is replaced by a resistor, the 500 Ohm resistor from B+ to the filament. 500 Ohm is quite small compared to the plate resistance of the tube to which it is basically connected in series. I coined the name 'DirectPath' for this circuit since it DC couples the B+ side of the transformer primary to the filament. It also sounds very direct, removing the last capacitor from the circuit and thus revealing the tube sound in full.
This 500 Ohm resistor serves another purpose at the same time. It derives the filament voltage for the UX201A from the B+. Since the UX201A only requires 250mA filament current, this is doable in this way. Nothing is free however and this approach has a big disadvantage. The 500 Ohm resistor dissipates about 30W. This requires a massive resistor on a heat sink. Also the power supply needs to be quite substantial. It has to deliver 500mA for a stereo unit. Luckily the UX201A runs at low plate voltages of 135V max. In this circuit the plate voltage of the tube is about 125V. The B+ which is fed to the circuit needs to be somewhat higher to allow for bias voltage and the drop within the choke.
The 100k resistor in the circuit serves as grid to ground resistor to ensure the grid always has a defined potential. The cold end of the secondary of the output transformer is connected to ground through a 1M resistor to avoid any possible static charge build up in case the output is unconnected.
I have tested and used this circuit successfully many years ago. I am now working on a new implementation using Tango NP8 line out transformers. The assembly process and power supply will be covered in upcoming articles. Stay tuned.