6SU7

The abbreviated designation 6SU7 refers to the high-mu twin triode documented by Tung-Sol as 6SU7GTY. The manufacturer describes the general characteristics of the 6SL7GT with a low-loss phenolic base, minimized gas and leakage currents, and a specific test for anode-current balance between the triodes. Selection and testing of the two systems in one envelope are therefore part of its distinction. The figures here refer to the documented GTY version, without automatically transferring them to every similar designation.

The two indirectly heated triodes share a 6.3 V, 0.3 A heater suitable for AC or DC. The envelope is glass with an eight-pin octal base; Tung-Sol’s drawing specifies Micanol. Pins 1, 2 and 3 carry grid, anode and cathode of one system respectively, and 4, 5 and 6 serve the other; the heater is on 7 and 8. Separate cathodes give independent access to both systems within the same valve.

For each triode in class A1, Tung-Sol gives a typical point of 250 V anode, −2 V grid and 2.3 mA anode current. At these conditions it lists 44 kΩ anode resistance, 1.6 mA/V transconductance and amplification factor 70. The latter is a valve parameter, not a promise that any circuit will amplify exactly seventyfold. Maximum stated anode voltage is 250 V and maximum anode dissipation 1 W per section. For heater-to-cathode voltage the sheet requires keeping it as low as possible, without supplying a numerical limit there to be added by analogy with other valves.

The balance test deserves retaining when comparing the 6SU7GTY with the 6SL7GT. The manufacturer describes the sections as selected for anode-current balance, but the consulted sheet gives no percentage tolerance: it therefore cannot establish a numerical matching grade or a particular guarantee of gain equality in two circuits. Page two shows anode-curve families for one triode at 6.3 V heater and various grid biases. The document thus distinguishes general electrical behavior, illustrated by curves, from the additional testing characterizing the selected version.