Early SVT D1-D2 Power Amp

[AEB-1]

I have an early 70's SVT and troubleshooting preamp hum. There are two diodes D1 and D2 on the input of the phase inverter. They appear to be glass. Does anyone know what type diode they are and what function they serve?

[Hangman]

Those are very unlikely to be contributing to a hum. Those are basically clamping diodes. If a signal applied to to the external amp input is too large, the diodes will clamp it down, and potentially fail in order to protect the amplifier.

have you started pulling preamp tubes starting with V1 and V2 to see if the hum disappears with the tubes?