You do not have to buy 4 Cree diodes, just one. They cost dollars not cents.
Put it in series with the B+ (high voltage) anywhere in the power supply. That is after the + of the first four diodes, before/after the first cast resistor, before/after the second cast resistor or as the B+ leaves the terminal strip going to the 6080.
It removes the Reverse Recovery Spike that the stock diode string puts into the power supply.
I went for this upgrade after reading this post by Paul Joppa where he says that it probably would make more difference than bypassing a filter capacitor. Please, keep in mind that this post applies specifically to the Smack headphone amplifier that is now a legacy product.
First, parts connection is the last place you want to purchase cree schottky diodes, I usually use Mouser or Digi-Key, last purchase was $2.08 for 2A 1200V. I used 1 amp 600V in my crack (Paul (PB) said the 600V cree was OK in the Crack), but normally use 1 or 2 amp 1200V. The parts connections PCB are really easy to install, I did not use standoffs just 20 or 18 gage teflon wire and let the PCB float.
To expand on PB's terse note, the 600v diode should work in the SEX voltage doubler, since it works in the higher-voltage Paramount doubler. Whether you can physically connect it may be a more complicated and difficult question, since the Crees have short PC board mounting leads and the SEX uses the long leads on the UF4007s to connect between different terminal strips.
I can confirm the Cree's dont physically fit into the S.E.X. as the terminal strips are too far apart. Having said that i did manage to fit them into mine by extending the legs and insulating both the legs and main body to prevent shorts, but i dont recommend that setup to anybody.
The best solution is to have some small PCB's made up for voltage doubler configuration. Unfortunately i dont have any left before anyone asks.
I didn't expect the changes because the cap is on PS side but wow, i'am liking it with the HD 650 but not so much for the DT880 600 ohms it's a bit bright.Now i can't wait for the diodes that i ordered what that thing would bring up.
Can anyone please tell me if the way i'am installing the CREE BOARD is right.
Looking at Richard's Cree Board picture i can't tell where the + and - leads from the Cree board connected to but i check the crack manual and i think + from the Cree board connect to T21 and - from Cree Board connect to T20 is this correct?
I will remove all of the Diodes from the CRACK and T18(Black Wire) connect to Cree Board Solder Pod bellow D3 and T19(Red Wire) connect to Solder Pod bellow D2 is this right(Referred to Richard's Picture,Thanks Richard).
I'd wait for one of the big kids to chime in but I just did this and your analysis of the wiring is correct. I posted some pictures over in the Crack forum.