Have you got the amp working yet? Those 4 diodes are the rectifying circuit, and are the weak point of the amplifier, IMO. I have replaced mine several times over the years, and so far, that's fixed it every time.
BTW, I built mine in 1967 (8th grade), and cold solder joints were my biggest problem at the time.
BTW, I built mine in 1967 (8th grade), and cold solder joints were my biggest problem at the time.