8056
The 8056 is a low-voltage Nuvistor triode with a 6.3 V heater.
It is used in the 10A1, 422 and the 5T3. It can sometimes be replaced with a MPF102 FET.
The 8056 has a maximum plate-to-cathode voltage of 50 V. Contrast this with another Nuvistor triode, the 7586, which has a maximum plate-to-cathode voltage of 110 V.
The 7586 and 8056 have the same pinout, but the two tubes are not interchangable since they have different voltage ratings.
For example, in the front-end cathode followers of the 3A1, the plate voltage of the 7586 tubes, V113 and V123, is 55 V, and possibly a bit higher during warm-up. This exceeds the maximum plate voltage of the 8056, unless the 3A1 is modified for lower front-end plate voltages.