Update on my 7193/2C22 Chenin Modification:
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). This modification was detailed in this thread, several pages back. 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 biasing as a replacement for cathode biasing with a resistor and cap (which the Chenin uses), I decided to try it while I was in there. Instead of using LEDs, however, I had some 600V, 1A Cree Silicon Carbide (SiC) Schottky diodes lying around so I investigated those further and found out that their properties were favorable for use as cathode bias diodes. They have a much higher forward voltage (Vf) than a Si Schottky Diode and a very low dynamic resistance, meaning they don't need to be bypassed with an electrolytic cap. The stock Chenin setup uses a resistor bypassed with an electrolytic cap to set the cathode bias voltage.
Since the Vf of the Schottkys was around .85V for the current I wanted, and I needed 2.5-3V of bias voltage, 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 soldered together the appropriate leads and I had a nice neat package. Out came the cathode resistor and bypass cap, 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 pots for each channel 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 behind the regulator tubes up to 300.0V. The difference was most likely due to my use of a GZ33 rectifier instead of the stock 5AR4; that is now compensated for.
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.
Cathode Bias: was 2.60V dead on for both sides! Those Cree diodes are VERY consistent!
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. That also happens to place the load line in the most linear region of the plate curves - no coincidence, I guess. 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! Or maybe I'll check out constant current sources...
I liked the sound I had before, but with the diode biasing and the optimized voltage, it has improved beyond my expectations. The crystal clear floating highs I had before are still there, but the midrange has blossomed and the bass is to die for. Well worth the time and effort.
