...on a 1,000-watt tube amp driving a smaller amp as a load:
Same amp, with a 25Hz signal running through it to show how the tubes react:
Update: Here are the specs for the EL6471 amp. The 6 mercury-vapor rectifiers are there to handle some pretty hefty juice flow in the power supply--a 3-phase (220/380V) AC supply that gets kicked up to a 3,200 V anode voltage to feed the two big output tubes up top.
The two smaller tubes in front of and to the right of the big rectifiers are part of the power supply for the amp's third stage tubes.
The amp was intended for stadium sound.
The 1kW model is the baby: Philips also listed 2kW, 10kW and 20kW amps.