The diodes have low current and should match the low current of the base-emitter diodes of the output transistors. I would use little 1N914 or 1N4148 diodes but they are difficult to fasten to the heatsink (to sense the temperature) of the output transistors.

Power diodes will probably cause crossover distortion because they have a lower voltage drop at the low current the diodes pass.

 

hmmm, i guess i'll use 1n4007 and put TIP31c/32c to a heatsink together with the diodes...
by the way, do i still need to put an insulator between the output transistor and the heatsink?
last time i separated the heatsinks of the two transistors and i didn't mount the diode into a heatsink..

 

do i still need to put an insulator between the output transistor and the heatsink?

The datasheets for the output transistors show that their metal tabs are connected to their collectors that MUST be separated.

Use insulators and a thin layer of thermal grease that are made for the output transistors.

 

I couldn't attach this photo of a heatsink for the output transistors and diodes in my reply to your Personal Message: