When I built my Loftin-White amp, I originally used the 1200V 5A Cree diodes instead of a tube rectifier. Over time & making other mods, etc., I still notice the sound seems to be slightly harsh. I installed Harris FRED 1200V diodes (that I've had good luck with, in other projects) & the sound is noticably warmer / better sounding. The Cree's drop maybe 5 B+ volts, but this is not likely to account for the difference in sound. I've done this swap twice to make sure it wasn't my imagination or whatever. To me, the Cree's sound more like an ordinary normal recovery diode & the Harris more like a tube. Wondering what other peoples thoughts are?
Cree Diodes
I like them . Even against GZ37 Mullard . I dont know about FREDs . I m sure they dont sound harsh in my TT22 SE , Black Gat(ed ), Auricap(ed) , tantalum resistor(ed) , silver wired etc. amp . To me harsh cames mainly from bad components ,or bad audio sources . I put them in a Sonic Frontiers D1 DAC , an AudioNote PS2 AMP and the sound quality from them is superior , certified from different ears of many individuals .
I like them . Even against GZ37 Mullard . I dont know about FREDs . I m sure they dont sound harsh in my TT22 SE , Black Gat(ed ), Auricap(ed) , tantalum resistor(ed) , silver wired etc. amp . To me harsh cames mainly from bad components ,or bad audio sources . I put them in a Sonic Frontiers D1 DAC , an AudioNote PS2 AMP and the sound quality from them is superior , certified from different ears of many individuals .
TugaTweaker:
I took your advise that something else besides the Cree's may be causing a very slight harshness in the highs. I've tweeked everything I could inside the amp, all voltages are right on, so what could be going on? I noticed it seemed to be mainly in the right channel.
Then it dawned on me, the schematic calls for 50VDC to be taken from the right channel 2A3 cathode resistor string to place bias on the 6SL7 SRPP driver AC filament. Maybe this is somehow mucking up the sound? I removed it & the harshness is gone! Problem solved, Thanks.
Now there's more hum & buzz as the driver stage filament is floating AC...... now I'm looking at DC filament for the driver stage.
I took your advise that something else besides the Cree's may be causing a very slight harshness in the highs. I've tweeked everything I could inside the amp, all voltages are right on, so what could be going on? I noticed it seemed to be mainly in the right channel.
Then it dawned on me, the schematic calls for 50VDC to be taken from the right channel 2A3 cathode resistor string to place bias on the 6SL7 SRPP driver AC filament. Maybe this is somehow mucking up the sound? I removed it & the harshness is gone! Problem solved, Thanks.
Now there's more hum & buzz as the driver stage filament is floating AC...... now I'm looking at DC filament for the driver stage.
Instead of using DC filament - try to bias AC filament to cathode potential+20-30V.
To do this - make the resistor divider from the plate power supply, to obtain the voltage 20-30V higher then cahode voltage.
Put electrolitic capacitor in parallel with the bottom resistor, and connect this divider o the filament - one side or central point with 2 resistors.
To do this - make the resistor divider from the plate power supply, to obtain the voltage 20-30V higher then cahode voltage.
Put electrolitic capacitor in parallel with the bottom resistor, and connect this divider o the filament - one side or central point with 2 resistors.
Way late reply. Seems you found the issue. 
PIV for 350-0-350 fullwave is about 1,000V.
Cree 5A 1200V SiC Schottky diode.
Reverse current peaks at ~6uA here.
PIV for 350-0-350 fullwave is about 1,000V.Cree 5A 1200V SiC Schottky diode.
Reverse current peaks at ~6uA here.
The Cree's are rated at 1200V, as well are the Fred's. After finding the major source of the harshness (DC bias for 6.3V filament taken from 2A3 cathode resistor string) I still prefer the sound of the Fred's over the Cree's.
Got rid of the hum issue with the 6.3V filament supply, by biasing it with what I had on hand, a 9V battery. Totally kills the hum.
On another note: I notice Transcendar has a new line of OPT's on e-bay.
Got rid of the hum issue with the 6.3V filament supply, by biasing it with what I had on hand, a 9V battery. Totally kills the hum.
On another note: I notice Transcendar has a new line of OPT's on e-bay.
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