5879

The 5879 is a miniature sharp-cutoff pentode intended for audio amplification where microphonics, leakage noise and hum require particular attention. RCA lists input stages in medium-gain public-address systems, home sound recorders and audio equipment; Tung-Sol presents it as a preamplifier valve. Its main interest therefore lies in handling small signals before subsequent stages. The sources describe these technical requirements without assigning it a particular sonic character.

The cathode is indirectly heated, and the heater requires 6.3 V AC or DC at a nominal 150 mA. At the pentode operating point with 250 V on the plate, 100 V on the screen grid and −3 V on the control grid, the data sheets specify about 1.8 mA plate current, 0.4 mA screen current, 1 mA/V transconductance and 2 MΩ plate resistance. These parameters apply to those conditions: actual stage gain also depends on resistors, the supply and the following load. Tung-Sol's tables accordingly distinguish combinations for maximum gain from those for maximum output voltage.

A practical detail concerns pins 2 and 6: although marked as unconnected in the base drawing, RCA recommends grounding them in all applications. The note explains that this effectively shields the control grid and plate from the heater, helping to reduce hum with AC heating. The base is nine-contact Noval: control grid on pin 1, cathode on 3, heater on 4 and 5, screen grid on 7, plate on 8 and suppressor grid on 9. In pentode operation, the suppressor is connected to the cathode at the socket.

The documentation also provides for triode connection, joining both the screen and suppressor grids to the plate. At 100 V plate voltage and −3 V bias, RCA specifies 2.2 mA, an amplification factor of 21, about 17 kΩ plate resistance and 1.24 mA/V transconductance. This is a documented configuration of the same valve, with its own characteristics and ratings. The March 1961 RCA sheet gives design-maximum plate ratings of 330 V and 1.25 W in pentode operation, versus 275 V and 1.7 W in triode operation; screen-grid voltage must also comply with the conditions in its rating chart. These maxima must not be confused with typical operating points or mixed with earlier design-center ratings.

The curves and tables thus show the 5879 as a valve for small-signal audio stages, whose application also involves careful connections and heater supply arrangements. Operation in any mounting position is permitted. Documentation consulted: RCA 5879 data sheets, including the March 1961 edition; Tung-Sol 5879 sheet dated September 1, 1963; extract from the RCA Receiving Tube Manual, pages 475–476.