12AU6A

The 12AU6A is a sharp-cutoff pentode for RF and intermediate-frequency signal amplification, with one specific feature relative to the 12AU6: controlled heater warm-up for 150 mA series strings. Sylvania’s 1961 note states that the listed variants are otherwise identical to their respective basic types. It lists 12AU6 as the 12AU6A counterpart and cites registration 547; the controlled-warm-up collection refers to JEDEC registration 3300 of June 12, 1961.

Heating is indirect, 12.6 V and 0.15 A. The A variant’s characteristic figure is approximately 17 seconds, but the original definition matters: time for heater voltage to reach 80% of rated value in the prescribed test. Sylvania applies four times nominal voltage to the circuit with a series resistor equal to three times rated voltage divided by rated current. This does not mean normally feeding the heater at four times its voltage or certifying that a whole receiver is ready in exactly 17 seconds.

The reference 12AU6 sheet helps describe electrical function: Tung-Sol presents a high-transconductance miniature pentode with low grid–anode capacitance, suitable for both RF and audio-frequency amplification. Its seven-pin base has first grid 1, third grid and internal shield 2, heater 3 and 4, anode 5, second grid 6 and cathode 7. In pentode connection the third grid joins the cathode at the socket. This is the electrical family to which Sylvania refers, while its A-specific sheet chiefly specifies heater behavior.

As reference 12AU6 examples, Tung-Sol lists class A1 with 100 V anode and screen, 150 Ω cathode resistor, 5 mA anode current and 3.9 mA/V transconductance. Another point uses 250 V anode, 125 V screen and 100 Ω cathode resistor, giving 7.6 mA and 4.5 mA/V. Maximum pentode grid–anode capacitance is only 0.0035 pF. Curves show sharp cutoff as bias changes, not the variable-mu characteristic of other pentodes intended for automatic gain control.

Family documentation also covers triode use, connecting second and third grids to the anode: Tung-Sol gives 250 V anode, 330 Ω cathode resistor, 12.2 mA, 4.8 mA/V transconductance and amplification factor 36. This differs from normal pentode connection. Archived 12AU6A photographs include a specimen bearing a Tektronix checked-tube label: they document that sample without identifying an instrument model or implying special selection for all 12AU6As. The documented distinguishing feature remains controlled warm-up for a 150 mA string.