6U7

The designation 6U7 requires attention to the suffix. Sources for the base designation refer to 6U7G and state that no corresponding metal 6U7 version exists: a search omitting G does not identify a metal envelope. Available original documents describe the glass 6U7G, a variable-mu pentode for RF and IF amplification. Tung-Sol calls it remote cut-off and specifies use with automatic volume control, emphasizing reduced cross-modulation. Grid bias thus allows gain regulation in receiver stages designed for this type.

In the documented 6U7G version, the cathode is indirectly heated and the heater requires 6.3 V at 0.3 A, AC or DC. The octal base has seven fitted pins: heater on 2 and 7, anode on 3, screen on 4, suppressor on 5 and cathode on 8; pin 1 has no connection and 6 is absent. The control grid goes to the top cap. A useful detail is the internal shield in the bulb’s upper portion, already connected to the cathode inside the tube. The suppressor, however, must be connected to the cathode at the socket for the published operating points: the internal shield connection does not replace the external third-grid connection.

RCA and Tung-Sol sheets give limits of 300 V anode voltage, 100 V actual screen-grid voltage, 2.25 W anode dissipation and 0.25 W screen dissipation. The screen supply source may be rated up to 300 V, but this does not mean applying 300 V directly to the screen grid. A typical point with 250 V anode, 100 V screen and −3 V control grid gives about 8.2 mA anode and 2 mA screen current. RCA and Tung-Sol specify 1.6 mA/V transconductance and about 0.8 MΩ anode resistance there; the consulted Raytheon sheet instead gives 1.3 mA/V. Such document differences must remain attributed to their manufacturers, without making a characteristic point a universal limit.

Beyond amplification, Tung-Sol includes mixer service in a superheterodyne circuit, with 100 or 250 V anode voltage and 100 V screen voltage. It gives approximately −10 V grid bias and specifies that this minimum applies with 7 V peak oscillator voltage. It must therefore not be transferred automatically to amplifier operation. Sylvania adds an interesting technical and historical comparison: it refers 6U7G characteristics and applications to 6D6 while distinguishing the base and interelectrode capacitances. This is a manufacturer-documented relationship; identification and replacement still require retaining the suffix and checking the actual tube’s connections.