6AT8

The 6AT8 is a miniature triode-pentode frequency converter. Sylvania describes a medium-amplification-factor triode and sharp-cutoff pentode for VHF oscillator and mixer service. The sections have complementary roles, a common cathode and separate grids and plates. For TV and FM receivers, the document recommends no more oscillator power than necessary, to avoid over-excitation and excessive radiation.

The 6AT8 heater is 6.3 V, 0.45 A; any mounting position is allowed. The same sheet includes the 5AT8, identical except for heater data: 4.7 V, 0.6 A and controlled warm-up for series strings. Its 11-second value belongs to the 5AT8, not the 6AT8. Heater-to-cathode design-center limits are 200 V total DC plus peak with heater negative; with heater positive, 100 V DC component and 200 V total DC plus peak.

Triode static characteristics at 100 V plate and 100 Ω cathode resistor are plate current 8.5 mA, amplification factor 40, approximate plate resistance 6.9 kΩ and transconductance 5.8 mA/V. The separate pentode point uses 250 V plate, 150 V screen, suppressor connected to cathode at the socket and 200 Ω cathode resistor: plate current 7.7 mA, screen 1.6 mA, approximately 750 kΩ plate resistance and 4.6 mA/V transconductance. Approximately −10 V bias for 10 µA plate current is near cutoff, not the bias of these points.

The 250 MHz oscillator table gives 150 V plate, 2.7 kΩ grid resistor, 13 mA plate and 3.6 mA grid current, approximately 0.5 W under the specified shielding conditions. Pentode mixer testing instead uses separate excitation and grounded triode: plate and screen 150 V, grid supply −3.5 V, grid resistor 120 kΩ and oscillator injection 2.6 V RMS. Conversion transconductance is 2.1 mA/V, plate current 6.2 mA, screen 1.8 mA. Triode-connecting the pentode, screen to plate and suppressor to cathode, gives a separate point of 2.8 mA/V conversion and 7.8 mA plate current. These are not audio power measurements.

Converter-service design-center ratings include 250 V plate for both sections, plate dissipation 1.5 W triode and 2 W pentode; triode connection of the pentode has a separate 2.4 W limit. Screen supply maximum 250 V differs from the 0.4 W screen-input rating, subject to the curve reducing permitted input as actual screen voltage rises. Thus 250 V is not always permitted at the screen under any load. Capacitances also depend on shielding: maximum pentode grid-to-plate is 0.025 pF unshielded and 0.016 pF with the prescribed external shield.

The noval B9A base, diagram 9DW, has triode grid 1, plate 2, common cathode 3, heater 4 and 5, pentode plate 6, screen 7, suppressor 8, pentode control grid 9, bottom view. The external suppressor requires the intended circuit connection. Sylvania gives maximum diameter 7/8 inch, 22.225 mm, overall height 2 3/16 inches, 55.5625 mm, seated height 1 15/16 inches, 49.2125 mm, and height to bulb top excluding tip 1 21/32 inches, 42.06875 mm. Documentation: Sylvania Engineering Data Service, 6AT8–5AT8, July 1955, eight pages with characteristics and curves; RCA, 6AT8 Triode-Pentode Converter, March 1, 1955, capacitances and connections.