It's a 1N3754.
Electronic parts outlet in the Netherlands has some for E7.50 each.
Do not operate the amp without atleast a 1n1004 or 7 in its place .
E7.50, Ouch!
A long time ago I started to test other diodes to use as a replacement for these diodes. Generally little glass diodes like 1N914 or 1N4148s seemed to work the best. The problem comes in mounting them in a way to get them to react to the heat of the output transistors.
If you don't want to buy the expensive NOS one, get a 1N914 type and solder two thin wires to the ends. Heat shrink the connections so that there is no chance of shorting out to the heat sink. Then using as thin a bead of silicon glue as you can, glue the diode to the center of the heat sink near the clip that held the original diode. Then run the wires to the pc board making sure that you keep the correct polarity.
This diode string ties the lower half of the power amp to the upper half. If the diode string breaks circuit, the upper half of the circuit turns fully on and will cause the power amp transistors to fail. So don't run the amp without the diode in place.