307A (VT-225) National Union
307A (VT-225) is Moderate-power, filamentary suppressor-grid pentode for audio-frequency amplifier or modulator
Class A audio amplifier characteristics
| Plate Voltage | Screen Voltage | Suppressor Voltage | Control Bias | Plate Current | Screen Current | Load Resistance | Input Voltage | Power Output | Second Harmonic | Third Harmonic |
| Volts | Volts | Volts | Volts | mA | mA | Ohms | Peak Volts | Watts | db | db |
| 200 | 200 | 0 | -15 | 36.5 | 3.4 | 4000 | 0-15 | 2.9 | 23.5 | 24 |
| 250 | 250 | 0 | -20 | 50 | 4.0 | 4000 | 0-20 | 5.7 | 24 | 23.5 |
| 300 | 300 | 0 | -30 | 43 | 2.6 | 4500 | 0-30 | 9.1 | 21 | 17.5 |
Limiting Conditions for safe use: Maximum plate dissipation is 15 watts, screen-grid dissipation is 6 watts,
| 500 | 300 | 60 | 25 |
Filament voltage 5.5 volts, A.C. or D.C. Nominal filament current 1.0 ampere
This tube works really nice in SE or PP
One of my favorite audio tubes, specially triode strapped and a bit overrated. The National Union version is exact Western Electric equivalent, with four holes at both ends of plate.