Many, many years ago when I was very enthusiastic about Amateur Radio 144 MHz transmissions (amplitude modulation), I found a cct. diagram in a 'Short Wave Magazine' (March 1956, I think) for a 120-watt output modulator. It used a pair of 807 with fixed bias, the rest on the valves were 6SN7. Of course, the O/P transformer had a relatively high-impedance secondary (mine was a Woden UM3), to modulate the HT for the RFPA: a QQV06-40A, but the cct. would tolerate a different transformer with the appropriate secondary winding for a 3 ~ 15 ohm speaker load. The PSU for it was a separate affair - and it was massive! Although that modulator vanished many years ago, I do still have the mains transformer (somewhere). I seem to recall it has a H.V. winding of something like 500 - 0 - 500 v. at about 500 mA: it is large - and heavy - and at those voltages and currents, positively lethal ! 
Al.
Al.
