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Did the SiC Schottky Cathode Bias Mod Tonight!!

And all I can say is "time well spent"!

First of all, let me say I'm reporting on an August 2006 build Supratek Chenin preamp. I was bored with the 6SN7 sound, so I had previously modded it to run 2C22 tubes as single triodes in place of the 6SN7 as a directly coupled triode pair (which is the stock setup). I did this by fabricating adapters for the tube sockets, so the 2C22 was still running at the set points for the 1st 1/2 6SN7, namely 100V plate and -3V grid bias (cathode resistor and 330uF cap). I much preferred the sound of the 2C22, so I decided to do further mods to bring the plate voltage up to a more typical range for a single 2C22 triode.

After reading about LED bias here, I decided to try it while I was in there - I had some Cree SiC Schottky diodes lying around so I investigated those further and found out that their properties were favorable for use as cathode bias diodes. There are other posts here about that experiment. Also, using fixed bias eliminated the need for me to play around with cathode resistors and caps, and took an electrolytic cap out of the signal path, always a good thing.

Since the Vf of the Schottkys was around .85V for the current I wanted, I used 3 of them in series. Since they are in a TO-220 package, it was very easy to stack them and secure them together with a drop of Crazy Glue between each of them - they ain't comin' apart! I then tied together the appropriate leads and I had a nice neat package. Out came the resistor and cap (generic carbon film and Sanyo Oscon), in went the diodes. Note: when stacking these diodes together, be aware that the heat sink is connected to the ground leg, so be careful of shorting things out.

I then disconnected the stock 66K plate resistors and wired in 35K, 2W carbon pots so I could easily vary the plate voltage while playing music. I set it at 15K for starters - the plate curves say I should get about 135V at that resistance. I hooked up the power supply and made some measurements:

Voltage to the regulator tubes: was 292V, should be 300V, so I tweaked the adjustment pots up to 300.0V

Cathode Bias: was 2.60V dead on for both sides! Those Cree diodes are VERY consistent!

RCA 2C22 tubes: one had 133.5V on the plate, the other 145V. The tubes were obviously unmatched! That could be a problem with my new fixed bias, so I went through my 2C22 box and found a match for the lower tube, now they are 133.9 and 133.5 plate volts.

I then hooked it all back up in my system and did some listening. The first thing I noticed was the bass - more yes, but also much more nuanced. The bass lines on familiar songs were there, but I realized that what I once thought was one note was really 2 or 3 notes! Excellent! I think the substitution of the diodes for the resistor and cap was the reason for this improvement, since it was consistent at whatever plate voltage I dialed in.

The other big difference was in the midrange; it was much more meaty than before - I think that was due to the tubes operating at higher voltage and current - they were a bit starved before. This effect was more noticable at lower plate resistor values. I tried everything from 35K down to 10K ohms, and I think I prefer 15K the best. Everything just seemed to come together there. I will re-check it all tomorrow, and if I get the same result, I'll order the best damn plate resistor I can find!

Any plate resistor recommendations???

Edits: 07/08/09

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Topic - Did the SiC Schottky Cathode Bias Mod Tonight!! - Maxamillion 21:04:15 07/07/09 (9)


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