The PRC-3 audio choke from One Electron was designed as a plate load for the output of either a high quality line amplifier or a phono preamp. Since the plate can swing up to double the B+ value, a high output voltage can be attained with relatively low B+ values. Since the tube is presented with essentially an infinite load resistance at audio frequencies, the maximum gain is obtained and the tube is run in its most linear mode. The sonic improvement over a conventional resistor load is substantial.
The PRC-3 choke was originally designed to be driven by a 5687 triode, but other low to medium-mu triodes, such as 6J5, 6BL7GT, 6S4, 6SN7GT, 12B4, 7044, or 7119, will work well, as will most triode-connected output pentodes, such as 6AQ5, 6BF5, 6BQ5/EL84, 6CW5/EL86, 6V6GT, 6W6GT, or 6Y6G. Generally, the lower the plate resistance, the better the frequency response. With the 5687, the response is from 40 KHz. The PRC-3 choke has an inductance of about 225 Henries (measured at 60 Hz), a DC resistance of 1850 ohms, and a maximum recommended average plate current of 15 mA. The plate current can go higher than this, but the choke will start to saturate and the low frequency response will suffer. The maximum recommended audio output voltage is 25 volts rms. At higher voltages, distortion at low frequencies can occur.