Class C isn't linear, and you need linearity in the amplifier for SSB, i.e. the output power is directly proportional to signal level. In Class C, you need a certain level of signal at the grid before you get output. Class C normally has a ' duty cycle (or 'device conduction angle') of around 120 degrees. Higher efficiencies and more harmonic come from shorter duty cycles. Some 58 years ago, I built an amplifier with an 807, running with 1700 volts on the plate, 500 on the screen and -300 on the control grid. Driven hard with a 6L6G, on 40m it ran at about 220 watts input and 170 out, or about 77% efficiency. If you held the key down, for long, the plate glowed red! But 807s were cheap and readily available back then.....In the UK, they were much cheaper than comparatively rarer 1625, which was a 12 volt heater 807. That 807 lasted a long time and it wasn't bust when the amplifier got broken up....