Preamp Deal of the Century

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.

My Chenin just arrived today! Got the call from my wife that the box is waiting for me at home - now how can I skip out of work early...
I feel like a kid at Christmas!

It took 2 months, 1 week to get the Chenin. Shipping only took 2 days from Australia to NJ. No power cord was in the box, though - good thing I have a few spares.

Just finished an extended listening session with the Chenin, and I'm very impressed, although I've only listened to the linestage so far. Very tight bass, lush mids, and extended highs. I wasn't expecting better bass from a tube preamp, but I certainly did get it! I'm hearing things I haven't heard before in recordings, such as musicians breathing! All this using a computer power cord attached to a zip cord extension while I wait for the Belden cable and Wattgate ends to arrive from Parts Express. Should only get better!
I'm running modified Flying Mole class D monoblocks, a Sony CD player, and Zu Druid speakers.

Has anyone here replaced the umbilical cable on the Chenin/Syrah with an upgraded one? The stock cable is not shielded, and I wonder if shielding might lower the noise floor some, since it is routed in close proximity to a lot of other equipment (especially my class D amps which have been known to emit RFI). Revelation Audio Labs sells a shielded cryoed silver cable that is supposed to be a big upgrade - anyone here have it? If so, could you describe the effects on your system?

Any further Malbec Impressions? I'm dying to find out how it sounds, especially paired with a Chenin Preamp.

Still no further Malbec Impressions?

Outlier, if you look a couple posts above yours, you'll see my review of the Mullard GZ33. It's approved by Mick, and I think it sounds fantastic.

Just to clarify, no holes need to be drilled in the preamp! I was asked this question in an off-line email, so I thought I should post a clarification. I specifically designed this mod to make the fewest possible changes to my precious Chenin (they don't make these any more, ya know!), especially irreversible changes such as hole drilling. The wires from the top caps go into the tube base of the adapter and connect to two of the tube base pins, which then make the appropriate connections with the Chenin's internal wiring through the Chenin's 6SN7 tube socket. The ONLY mod to the preamp itself is to move the two output wires from the connections for triode II to the connections for triode I.

I'm preparing to do some more mods to my Chenin. As you know I have converted it to 2C22/7193 tubes. Next I will be replacing the PS electrolytic caps with Solen PP caps. I have purchased a 100uF 630V Solen (radial leads in a metal outer can) and a 33uF 630V Solen (axial leads) and will be doing this after I finish a couple other projects. This should lower the ESR and result in quicker response to transients (or so the theory goes). I'll post on how it turns out.

I originally posted this as a separate thread but am re-posting it here at Slowhand's request.

I have been intrigued by the 2C22/7193 tubes since reading about them in the "Supratek Tube Swappin'" thread, especially the comments from Audiogon member Ecclectique, who described them as "magnificent". I was not, however, interested in totally rewiring my Chenin in order to use them. So, after thinking about it for a while I came up with the idea to make socket adapters for the 7193s, using new tube bases, ceramic tube sockets, ceramic tube caps, and rubber-coated hookup wire.
How did it work out? In short, GREAT! The 7193s are spectacular in both sound and looks (with their cute dual top caps). The gain of the tube is lower than that of the 6SN7, but I had WAY too much gain anyway - I only ran the Chenin on the low gain setting with the 6SN7s, and I had to switch my Gilmore amp to the low gain setting (20dB gain) to get any usable range on the volume control. With the 7193s, I have the Gilmore set to high gain (27dB) and 12 o'clock on the volume dial is about 85-90dB at my chair. The gain switches behind the 6SN7s on the Chenin no longer do anything after this mod, since they only attenuate the second triode of the 6SN7. The following two links have pictures of the completed adapters and the Chenin with the adapters and the 7193 tubes installed.

Adapters

Chenin

One minor wiring change needs to be made to the Chenin in order for this to work: the two output leads (one has the big Auricap on it) that connect the 6SN7 tube socket to the output transformer on each channel need to be moved from Anode II (pin 2) to Anode I (pin 5) and from Cathode II (pin 3) to Cathode I (pin 6), respectively, since the 7193 is a single triode tube. This is a very easy job - there is plenty of room in the chassis, even for a ham-fisted guy like me. The reward for this minor change - glorious sound!! The 7193 is a spectacular tube, with airy highs that seem to float into the room, a nice warm midrange, and solid, tight bass. Best of all, the National Union tubes I am using retail for $5.00 and change from AES!!! They are pristine new-in-box JAN tubes, dated June 1943, with all the printing intact. I'm getting sound at least as good as the very best 6SN7s almost for free!! I have RCA and Ken-Rad versions of this tube on order so I can compare the sound, but the NUs are certainly no slouch. They kick the hell out of the RCA Grey Glass VT-231s I was using previously.
If you are still uncertain, how about this fact - you can still use your 6SN7 tubes without moving the wires back! Just remove the 7193s and adapters and plug the 6SN7s right back in! You will only be using one of the triodes in the 6SN7, but that still gives plenty of gain to work with, at least in my system. In fact, I think the 6SN7s sound better this way than with both triodes hooked up. If you don't like the sound, simply move the two wires back to where they were.

Details of the adapter wiring:

Anode of 7193 (top cap) to Anode 1 of 6SN7 socket (pin 5)
Grid of 7193 (top cap) to Grid 1 of 6SN7 socket (pin 4)
Cathode of 7193 (pin 8) to Cathode 1 of 6SN7 socket (pin 6)
Heater 1 of 7193 (pin 2) to Heater 1 of 6SN7 socket (pin 8)
Heater 2 of 7193 (pin 7) to Heater 2 of 6SN7 socket (pin7)

Remember, all pinouts are looking at the bottom of the tubes/sockets, with the index pin detent at the top; the pin numbers increase from 1 to 8 in a clockwise direction. You can obtain data sheets for these and many other tubes online from a variety of sources.

Hints:

Drill holes in the tube base to pass the two top-cap wires through. Be certain to actually test-fit the bases into both of your Chenin's tube sockets and mark where you want to drill the holes - the other tubes might interfere with the top cap wires if you don't put the holes in the right places, plus you probably want to make the placement of the wires symmetrical on both sides, and the Chenin sockets may not both be aligned in the same way - it's always better to measure than to do it twice.

Tin the wire ends that go into the tube base to prevent loose strands from accidentally short-circuiting the adapter. This also allows the wire to form a bond with the entire pin after the pin is heated with the soldering iron.

Make the wires between the tube base and the socket as short as they can be while still allowing you enough room to guide the wires into place and solder them. There is not a lot of room inside the tube base when the sockets are pushed into place inside the base, so you need to arrange them so the socket will fit to the proper depth.

Only three of the pins from the socket are used for this adapter, so I snipped off about 2/3 of the length of the unused soldering tabs and bent them over to prevent them from touching any of the other tabs when the socket is pushed in. I didn't snip them off completely since the pin holders on the other side would then fall off.

I coated the socket tabs and the base pin holes with a high dielectric conformal urethane coating (from a local electronics shop) and let it cure until it was no longer tacky before pushing the sockets into the bases to help prevent any accidental short circuits.

I used a two-part epoxy to seal the sockets to the bases - just coat the base on the inside and push the socket right into it. It is a nice tight fit. I then clamped the two together with a C-clamp for 24 hrs until the epoxy was completely dry.

Re-check the pinouts before you use the adapter, just in case! This can be done by checking continuity with a multimeter.

Materials: I sourced the tube bases, ceramic sockets, and small (1/4") tube caps from The Tube Center in Orlando, Fla. The wire used was Belden rubber coated hookup wire #8899 from Newark Electronics in Palatine, Il.

If you try this, have fun and enjoy the results!

Disclaimer: You do this mod at your own risk as I cannot be certain of the setup of all of your Supratek preamps. I know this mod works for my preamp. Please check your preamp thoroughly and verify the correct pin locations before attempting.

I understand, Kg. Even though I thoroughly studied the circuit and went over the mod dozens of times in my head before I did it, I still had quite a bit of anxiety when I pushed in the power button. Happily, there was no factory smoke released.

I just finished an extended listening session, and a few things stood out with the NU 7193s.

The highs are extraordinarily detailed and sparklingly clear. They might be fatiguing if it wasn't for the total lack of glare - these tubes are clean, clean, clean. This probably has to do with the anode and grid connections being through the top caps, a design feature which was adopted specifically to improve high frequency performance in radio frequency applications, according to several on-line sources I have found.

The mids are very natural and warm, but not as warm as those from the RCA Grey Glass 6SN7s, which some might consider overdone. The harmonies on the Beatles "Love" CD were thrilling.

The bass is about the same as I had with the 6SN7s, maybe a bit tighter. I'm looking forward to receiving the Ken-Rad 7193s (which should have been here by now...) as Ken-Rads are well-known for their prodigious bass. Stay tuned.

The anode of the first 6SN7 triode stage is connected to the grid of the second stage in the Chenin. The output wires in the stock version are connected to anode II and cathode II.

I did not change any operating voltages in my Chenin. The only thing I did was move the two output wires from II to I, and plug in the adapter with the 7193s. From what you say, maybe varying the voltage might produce even better results, but my aim was to keep the Chenin as close to stock as possible, yet still able to run 7193 tubes.

Sorry, but I hesitate to pull the Chenin out of my system, open it up and take internal pictures at this point, since the only internal change is moving the two wires.

OK, Phono section tubes:
6688 (E180F - Philips SQ)
6922 (stock EH)
Rectifier (NOS GE 5AR4 with the copper sides - niiiiice)
Regulators (stock Sovtek 5881)
Linestage (well, we've been through that, huh?)

Want to see my latest project (the Chenin project is so last week; yes, I'm Maxamillion over there):
click here:
Awesome Tuner Get One Now
I cannot believe that this tuner I picked up for $15 at a junk store is this good! Un f'in believable!

I'm curious, too. Do we have someone besides me running these tubes?
By the way, they sound fantastic with my "new" Infinity RSIIb speakers - the EMIT tweeters and EMIM midranges really deliver the clarity the 7193s are capable of.

National Union vs RCA vs Ken-Rad 7193 tubes.

Since I have converted my Chenin to 7193s, I have been evaluating the different brands of 7193s available (in addition to enjoying the great sound!). Let me say that they are all excellent tubes, but there are some subtle differences between them, both in sound and in construction. All have black, relatively flat plates, with small boxes in the center.

National Union: The NUs have the smallest bottle of the three 7193s, and the getter is not visible from the exterior of the tubes. They have a round top and bottom mica. These were the first 7193s I ever heard, and they sound very good indeed. These are silky smooth, with a rich midrange. They are a little rolled off on the extremes compared to the other two tubes, but it is a not a huge difference.

RCA: The RCAs have a larger bottle than the NUs, with a bottom getter and round top and bottom mica. They have more extended highs and lows than the NUs, with fuller bass and a little more detail on top. Not quite as smooth as the NUs, and not quite as warm in the midrange.

Ken-Rad: The biggest bottle of the three, slightly larger than the RCA, with bottom getter and non-round top and bottom mica (looks like a round mica with a section cut out in each quadrant). The best bass of the three - the impact of kick drums is sensational, and bass lines can be felt and heard in great detail. The highs are nice and airy, with very good detail, and the midrange is very sweet as well.

So overall, in my opinion, I would rank the 7193s as follows:

Ken-Rad: best overall - it has everything

National Union: smoothest and warmest sounding, but a little less top and bottom than the others

RCA: Similar to the Ken-Rad, but not quite as smooth in the highs.

I've been listening to the Ken-Rad 7193s in my Chenin for about a month and one half now, and I decided to try an experiment last night. After listening to a few songs with the 7193s in place, I switched back to my RCA grey glass 6SN7s for comparison.
The difference was quite startling! The sound I used to think was very warm and full (the RCA 6SN7s) now sounded quite constricted and flat, compared to the 7193s - they just have so much "air" and sparkle at the top end that the 6SN7s just can't match.
It was back to the 7193s in very short order for me! I don't think I'll be repeating that experiment any time soon.

Picked up a pair of NOS Tung-Sol 5881 (6L6WGB) tubes for $25.00 at an antique radio swap meet this weekend. They have light brown bases and look to be 1950s vintage. The seller said they tested new, and they looked perfect, so I went for it. I've seen these online for $210.00/pair, and I know they are very popular with the guitar guys, but I figured I'd give them a whirl as regulators for my Chenin.

Verdict: They are an improvement over the stock Sovtek "5881" regulator tubes, providing a fuller midrange and slightly tighter bass, with no noticable ill effects. They are definitely a good match with the 7193 linestage tubes. I don't think I would pay $210 for them, but for $25 they are a nice upgrade. I was thinking of selling them at a profit when I picked them up, but I think they are staying put in my Chenin. My rectifier is a NOS GE 5AR4 (the one with the copper sides).

The tubes I'm using are also from 1942 and 1943 - World War II vintage. I wouldn't worry about the age, they built them tough in those days - after all they had to survive warfare inside RADAR units - much tougher than surviving in your preamp! In fact I dropped one from a height of ~6 feet yesterday onto a hardwood floor while swapping among the different brands; not only did it not break, it still works perfectly. Even if it was to break, they're cheap to replace compared to good 6SN7s!

Great to hear someone else is enjoying the 7193s! There is something poetic about using a tube designed for warfare to produce something so beautiful, isn't there?

Looking for a fight, Maril?

Personally, I am thrilled with my Supratek Chenin. It was delivered faster than I expected it, it has worked flawlessly since I got it, and I am enjoying doing mods to it to try to wring every last drop of performance from it - the point-to-point wiring some have derided as a rat's nest makes it very easy to mod, so I have no issues there. Plus it's beautiful to look at as well. If Mick was still in business I might even buy another preamp or amp from him in the future.

Maril, This is the quote that made me think you were spoiling for a fight:

This whole "Preamp of the century" thread makes me to re- think value of A-gon posts and integrity of some of it's members.

You went from complaining about Mick, which is your right, to implying that A'gon members who post positive things about Supratek somehow lack integrity. I for one was insulted.

I have made another mod to my Chenin that may interest some. As you know, I have converted it to run 7193 tubes in place of the 6SN7s; that's still humming along fine.

I recently decided to replace the no-name electrolytic filter caps in the power supply with 630V Solen PP film caps. I used the same value caps as the electrolytics - that would be 100uF and 33uF. The 100uF 630V cap comes in a metal can with radial leads, the 33uF is a standard film cap with axial leads. It is a very simple swap, just unglue the old caps from the chassis, remove the wires one at a time, and transfer them to the new caps. I fabricated clamps for the new caps, and also glued both of them down wth "Amazing Goop", a very strong adhesive/sealer. I recommend clamping all power supply caps (even though the stock caps aren't!) as it would be unfortunate to have one come loose and touch a part of the metal chassis.

How does it sound? I noticed a pretty obvious difference immediately. Cymbals and drum thwacks sound sharper, there is more detail in the highs, and the background seems a bit blacker. In general, the sound is more robust and dynamic - I heartily recommend this mod. Keep in mind the rest of my Chenin is not stock, so your mileage may vary. One more bonus: the film caps will last forever, unlike the electrolytics.

If you try this - enjoy!

Those square blue caps are probably Wima caps - OK at best, not what anyone would call high end. My Pre has the Auricaps and I'm really enjoying the sound now; I've though about replacing the Auri's a million times, but still haven't done it yet as I like the warmth they bring to the sound.

Now if I stumbled across some Duelunds, maybe I would change my mind...

I think there is a real synergy between the Solens in the PS and the 7193 tubes. Since I put the Solens in, the highs I'm hearing a really astounding - they seem to float out into the room with me; everything sounds much more 3-dimensional than before. I would suggest that anyone who does the 7193 mod also do the Solen mod, it's that good!

Any by the way, this is all with one of my backup sets of speakers, Dahlquist M905s. My Infinity RSIIb speakers are currently being completely re-worked by me, with outboard x-overs, internal bracing, etc. I can't wait to hear how they sound in this setup!

I'm using a 5AR4 rectifier in my setup, but not the stock one. I'm using a NOS GE 5AR4, the one with the copper sides on the plates. I think it sounds excellent and it is nowhere near as expensive as the Mullard GZ34 (I haven't heard those, however). You can get the GEs for ~$50 at Antique Electronic Supply.

I have been playing around with the regulator tubes since I did the Solen mod. Here are my opinions:

Stock Sovtek 5881/6L6WGC: The worst of the bunch. Sounds glary and brittle, with deficient bass

NOS Tungsol 5881/6L6WGB: I picked up a used pair at a swap meet a year or so ago for ~$30, I think. They immediately replaced the Sovtek 5881 in my setup, and have been there ever since. They sound much better than the Sovtek 5881, with more bass and without the glare on the highs. They only read about 63 on my tube tester, barely in the "good" region, but they still sound very good.

New Sovtek KT-66: I picked up a matched pair of these a couple days ago from Parts Express for $50. I like them! They have a very full sound, with killer bass, yet still retain most of the high-end shimmer and detail. I can see why Mick recommended these to people who want to tube roll. They do not get very good reviews online as power tubes, but they seem to be excellent regulators. Very beefy construction, with copper grid posts and a larger plate structure than the Sovtek 5581/6L6WGC, so they are definitely not just re-badged versions of that tube in a different bottle. The metal base and coke-bottle shape look cool as well. I'm very pleased with this purchase.

Just checked with AES - they're out of the GE 5AR4s, even though they're listed on the website. Bummer. Glad mine still have many years of life in them...

Experiences with GZ33 vs GZ34? I'm looking for anyone who has compared and contrasted these two rectifier tubes in a Supratek preamp. The GZ33 is very cool looking, but how does it sound versus the GZ34/5AR4?

Well, I think my GE 5AR4 with the copper sides on the plates sounds great, but I just bought a Mullard GZ33 on e-pay, and I'll try it out when it arrives. If it doesn't sound as good as the GE, I'll take it out. I'm not running 6SN7s anymore, I'm using the 2C22 single triode, so I've reduced the demand on the power supply already - I guess I'll find out what the GZ33 sounds like soon enough...

So, I tried the Mullard GZ33 tonight, and I like it a lot! I bought a pair, and both had good getter flashing, full lettering, and tested 100%+ on my tube tester (set to 5V4, 5U4 and 5AR4 settings). Compared to the GE 5AR4, the background is cleaner, the soundstage is more focused (I can tell more easily where the instruments are) and the bass is a bit tighter and deeper. Cymbals are cleaner and have more focus. I swapped the two tubes back and forth twice each (I didn't want to stress the preamp with too many rapid on/off cycles). It just sounded more powerful and focused with the GZ33 than the GE 5AR4 - I'm keeping it in.

So my current optimum combo is:

Mullard GZ33 rectifier
Sovtek KT-66 regulator
Ken-Rad 2C22/7193 output tube (sub for 6SN7)
Solen PP Caps (33uF and 100uF, 630V) for power supply

It sounds incredible, IMHO.

No, just GZ33 and GZ34/5AR4. I prefer the GZ33.

If you look at the ratings, the GZ37 should not be used with a first PS cap greater than ~4uF due to too great of a current draw on the tube; the first PS cap in my Chenin is 3uF. Yours is probably close to that. Maybe that's why the GZ37 died?

The GZ33 can handle a lot more (up to ~60uF, I think I read somewhere).

Ooops! I meant to type "the first PS cap on my Chenin is 33uF", hence why I wouldn't use a GZ37 - now my post makes sense!

Sorry if there was any confusion...

I am only reporting back what has worked for me, YMMV. Remember, my unit is not exactly stock anymore, as I stated above. The Mullard GZ33 is actually less expensive than the Mullard GZ34 - I bought two for the price of one GZ34. I am very aware of the larger voltage drop of the GZ33, and was not expecting better sound from it, yet that's exactly what I got.
Mick recommends the GZ37 as an alternate for the 5AR4 in the manual for the Chenin, and it also has 3 amp current draw, so that does not seem to be a problem...

I have to say, I just went back and listened again to the difference between the 5AR4 and the GZ33 in my preamp. It's very clear to me that the Mullard GZ33 is superior to the GE 5AR4. The sound is clearer, cleaner, more focused, and overall just has more guts to it. Not what I expected from lowering the B+, but it's what I got.

Is the OP of the tubes changed? Most certainly! Is that a problem? No, isn't that the whole point of rolling rectifier tubes? It's not like the signal goes through them, you know...

Agreed. My Chenin is not stock anymore. By the way, the Radio Museum site lists the current draw of the GZ33 as 2.8 amps, same as the GZ37, which is why I figured that there would not be a problem since the Chenin is approved by Mick to run GZ37s. They even have a picture of a Mullard GZ33 on the page (looks just like mine, except mine has yellow lettering - only done in 1964 from my understanding).

http://www.radiomuseum.org/tubes/tube_gz33.html

So to settle this once and for all I emailed Mick, to see if he would weigh in on this. His typically succinct reply:

"33's are fine - good tubes actually."

So I would say that everyone should feel free to use the GZ33 if they so desire, since the designer of the preamp says it's OK to do so.

I too used Goop to secure my Solen PP filter caps when I replaced the electrolytics in my Chenin, but I also made a steel bracket that I screwed into the wooden side of the unit just in case. Do I think the Goop will come loose? No, but a little extra safety doesn't hurt.

Mick marks them left and right becuase that'sd where they were when he tweaked the adjuustments. He has told me it is not important to keep them that way since the channels are very close.
The Chard is self-biasing using a cathode resistor and capacitor, so there is no need to adjust bias with different tubes.
I have been deep inside my Chenin and done a lot of mods, but I have not completely traced out the schematic (the Chenin has an added layer of complexity because it includes a phono stage). Don't know where you can get one, either.

Further Chenin Tweaking (7193/2C22)

Well, I took the next step and replaced the plate resistor of the 2C22 tube with a constant current source, using an IXYS IXCP10M45S current regulator chip. A resistor in series with the chip adjusts it to whatever current you need. I'm running at 11mA current, with 3.45V of cathode bias (4 Cree SiC Schottky diodes replacing the cathode resistor and capacitor) and ~150V on the plate. The sound is incredible - this is the best my system has ever sounded!

A CCS gives you a horizontal load line, meaning very little distortion and maximum gain. It is unbelievable how real the music sounds now, even with the simplest possible CCS.

Next step is to try a more complex CCS using cascoded IXYS chips - the word is that the more complex CCS sounds even better, but that's hard for me to imagine.

Just thought I'd give a heads up to anyone who has done or plans to do the 7193 mod. Adding the diode bypass and CCS is easier than doing the adapters, and the payback is HUGE.

Correction, the last paragraph above should read "diode bias" not "diode bypass".

The increased gain comes from the constant current supply (CCS) on the 2C22 tube plate. It allows the tube to operate at maximum efficiency by ensuring a constant current regardless of load - CCS is very popular with the tube DIY crowd, and now I know why! The preamp seems to have unlimited supplies of clean power with the CCS, and vocals are downright startling.

Since I substituted the single triode 2C22 for the dual triode 6SN7, I had much less gain than stock, so I don't have the "too much gain" issue anymore.

CCS update:

I installed the cascoded CCS in my Chenin and experimented to find the best settings. I have found that 15mA plate current sounds optimal, with 3.45V cathode bias. Incredibly clean, clear and lifelike. Plate voltage is ~175V at idle.

I'm planning to stick some self-adhesive sheets of Soundcoat on the bottom plate of my Chenin main unit and power supply while I have them open to do other upgrades. The plate is a bit flimsy. I figure what the hell, it can't hurt!

I have found that the Zu Varial interconnects and Zu Ibis speaker cables work very well with my Chenin. I have no glare whatsoever, but my Chenin is modded to use 2C22 tubes instead of the often microphonic 6SN7. The 2C22s have no detectable microphonics whatsoever, you can tap on them, rattle the shelf ... nothing. I think microphonics may be a big reason for the glare sometimes heard in systems using 6SN7 tubes, hence the reason improved isolation has such an effect.

Well, you could always buy a Chenin and gradually upgrade it. I have done tons of work to mine, changing the output tube type, plate loading scheme, output transformers, bias scheme, power supply, etc. Point-to-point wiring makes it a lot easier to upgrade.

Just now working on an "ultimate" power supply upgrade for the Chenin, which I should have completed within the next week. It is a CLCLCRC design using Lundahl LL1638 low DCR chokes and all film caps. Just fits inside the Chenin PS unit! It simulates very well on PSUD-II, giving a 100x decrease in ripple noise from the stock, while also having a faster settling time. The Lundahls are dual-coil chokes, meaning they can be wired with one coil each in the + and - legs of the circuit, which should give added common mode noise reduction. I'll let you now how it sounds.

Power Supply Upgrade Completed!

Wow, it's so true that you listen to the power supply - the upgrade sounds excellent. Went from the stock values of 33uF/10H/100uF to 6.8uF/10H/47uF/10H/100uF using Mundorf Supreme/Silver oil and Solen caps and Lundahl LL1638 chokes. When I computer modeled the PS, I optimized it so that I would get the same output voltage as stock, and the upgraded unit put out 299.1V versus 300.0 from the stock; a slight tweak of the regulator adjustment pot and I was at 300.0V on both sides. The model showed ~30uV of ripple before the regulator (yes microvolts!), and ripple was undetectable on my oscilloscope, so that was accurate as well.

Way more bass slam, more body through the midrange and smoothness everywhere. The preamp sounded very good before, but it is even better now. I have a attached a snapshot of the new PS unit.

New Chenin Power Supply