The 500W Radio Power Amplifier

For 160, 80 and 40 meter bands. With 58N20s it will deliver 400W on 40 and 700 on 160/80 meter band. The amp needs 25/35W of drive for full output.

The input transformer T1 is wound on a ferrite ring with about six turns pri. and sec. and the 9:1 step-down (T2) on a receiver type ferrite rod with about six turns trifilar wound, but a ferrite ring would do equally well. The 3.3 Ohm gate load resistors are 20 Watt TO220 power resistors (Farnell 551-533) and the attenuator resistors are 2 Watt metal film. The 1N4148 bias diodes should be in thermal contact with the heatsink and well decoupled to prevent RF pickup.