Above the Chassis Amplifiers
Most PC boards can be mounted ABOVE THE CHASSIS. This method is usually the easiest to implement. The PC board is mounted above a (typically flat) chassis using small spacers. The transformers and other components are mounted above the chassis also. Spacers may be required for transformers and other components with leads protruding from the bottom. The resulting amplifier will have exposed circuitry on the top which MUST be protected in a way that prevents contact by a user.
Below the Chassis Amplifiers
Some PC boards can be mounted BELOW THE CHASSIS. This method is often harder to implement since large holes must be punched or drilled for the tubes. The PC board is mounted below a (typically flat) chassis using spacers. The transformers and other components are mounted above the chassis with the leads passing through holes in the chassis to the PC board underneath. The resulting amplifier will usually not have exposed circuitry on the top since the wiring and components are beneath the chassis. A bottom cover must be used to keep fingers away from the electricity.