Bias Transistor Substitution and Replacement
The original Q6 bias transistors are the 2N3403 devices on the back wall mounted in a small D shaped metal thermal bracket. Substitute matched 2N5088 devices as they are better than the original parts and have higher beta which is important for this Q6 location. The band clamps from the original 2N3403s are saved and reused. Once removed from the amp, the 2N3403s are relatively easy to tap out from the open top hat end of the metal band clamp. First slightly spread the overlapping metal portions of the band clamp in the area that the mounting hole is in using the blade of a small flat bladed jewelers screwdriver.
Then clamp the part of the 2N3403 metal band clamp sideways in your vise so that you are holding it but not putting closing force on the band clamp (transistor leads facing away from you and parallel to the closing screw of your vise), you can gently tap the transistor out of the bottom of the strap using a small wooden dowel.
The 2N5088 devices fit the band clamp opening that remains nicely. Put a small dab of instant glue on the face of the 2N5088 and slide it into the band clamp such that the base of the transistor where the leads exit is flush with the edge of the band clamp.
If applicable, rewire the original wires, heat shrink the connections and you are back in business. Note that the pinout of the 2N3403 is different from the 2N5088 so take that into account. The 2N5088 has a pinout of E-B-C when looking at the flat face of the transistor with the 3 leads pointing downward. The 2N3403 has a pinout of E-C-B when looking at the flat face of the transistor with the 3 leads pointing downward. The red wire connects to the transistor Collector, the brown wire connects to the transistor Emitter, the black wire connects to the transistor Base.