Octal Cornet

hagtech · « **on:** 21 Jun 2003, 07:42 am »

Since I've had a number of requests for this, I designed a version of the Cornet that uses 6SL7 and 6SN7 tubes. And it works great. I was able to use the same capacitors (just change resistor values) and still get 38dB gain.

Ok, so my prototype isn't pretty. Wiring the 9-pin layout to 8-pin sockets wasn't fun. Nevertheless, this thing sounded good right from the start. I was also proving the concept of a "chopped" circuit board (to separate power supply section).

Don't try this at home. Please use a real chassis!

You can download the schematic at http://www.hagtech.com/pdf/cornetoctal.pdf. I'll post a parts list soon, but you can figure out the changes just by looking at it. I'm making a real one of these for myself that will be shown at VSAC in October. Please note the schematic is copyrighted.

jh

mgalusha · « **Reply #1 on:** 21 Jun 2003, 12:52 pm »

Oh sure, right after I order my Cornet kit.

Just kidding, I may have to build a second one once you have the board ready.

Mike

mgalusha · « **Reply #2 on:** 25 Jun 2003, 10:50 pm »

Jim,

How does the octal version compare sonically to the 9 pin version? I know this can be a hard question to answer, especially about your own designs but I'm curious as to what the strengths and weaknesses are between the two.

Thanks!

Mike

hagtech · « **Reply #3 on:** 26 Jun 2003, 12:34 am »

I haven't done any comparisons yet. I imagine they would sound very much the same - only real difference being the tubes. The octal seemed a touch quieter, but I've made no measurements.

Being an open-frame unit, it's not fair to compare to one with a real chassis. This prototype was just to prove out the concept and tweak in the EQ values.

I'm building a real unit that I can run comparisons on. Check back in a month.

jh

hagtech · « **Reply #4 on:** 26 Jun 2003, 12:35 am »

Here's the parts list for the octal version. Very few changes, really. Just a pain wiring up the sockets. Schematic & parts lists are now combined into the following document.

http://www.hagtech.com/pdf/cornetoctal.pdf

Schematic is copyrighted. For DIY use only.

jh

hagtech · « **Reply #5 on:** 8 Jul 2003, 03:55 am »

Ok folks, here's my completed chassis. I added step-up trannies internally for a total of 63dB gain. That's 38dB without step-ups.

Measured response showed a bandwidth of 14Hz to 110kHz (-3dB). RIAA has been tweaked so it it no longer bass shy. Driving the inputs with a 10 ohm source totally fixed the bandwidth. With 50 ohm drive treble rolloff was at 25kHz. At 10 ohm (most MCs under this figure) the HF goes up to 110kHz at -3dB. Photos below show response with input BW limited to 30kHz (my test equipment).

Burning it in now. Will try to listen to it this weekend.

Without the input bandwidth limitation, the full 110kHz can be seen in this 10kHz waveform. Not bad for pushing through 26dB of tranny.

jh

hagtech · « **Reply #6 on:** 8 Jul 2003, 09:38 pm »

Tweaked on EQ a bit to get the minor bass tilt flattened out. Changed R9 to 47k. That did the trick. EQ is now +/-0.1dB throughout audio band with -3dB points at 14Hz and 110kHz. Except for a 1dB peak at 30Hz. Here's a plot of measured response. Gain is 63dB (38dB without trannies).

Also changed R21 to a 1.3 ohms to set heater voltage at 6.2V. Heater ripple was measured at 4.5mV. Not bad, and the impedance is still high enough to insure a rather gentle turn on.

Measured noise, REAL good. SNR came out at -73dBA referenced to 500uV. Or -93dBA ref 5mV, if you prefer.

jh

mgalusha · « **Reply #7 on:** 9 Jul 2003, 01:17 am »

Very cool. I really love the red chassis plate. Are you planning on producing a PCB for the octal model?

hagtech · « **Reply #8 on:** 9 Jul 2003, 06:51 am »

Well, I wasn't planning on making a special PCB for this. It was merely an exercise to show that it could be done (per request). Depends on how much interest there is in this, I'd need 15 - 20 orders just to break even.

Really, it wasn't that bad wiring up the sockets. Just takes a bit of time.

Yeah, I like the red too. Had to do something dramatic and unusual. When the real 370BX power tranny comes in, I plan to paint the bells gold.

jh

hagtech · « **Reply #9 on:** 14 Jul 2003, 11:30 pm »

Wow, this machine sounded better than I expected. But it took a bit of tube rolling to get there. The 6SN7 I had in there was cheap and low quality.

But 'oh my god'. After we put in a vintage Raytheon 6SN7 (it even has a Good Housekeeping seal on the box), the amplifier really came alive. Much more weight and dynamics.

This makes sense, in the stock Cornet, the output tube is configured as a cathode follower - known for being a sonically compromised circuit. Without the active current source pulldown, this location greatly benefits from a super tube. The Raytheon provided it. Sound was absolutely delicious with great tone. We listened to some rare LPs well into the night.

jh

JoshK · « **Reply #10 on:** 15 Jul 2003, 01:02 am »

I too love the red chassis! Look pretty! Makes me want to try to build this kit, but I am a novice at best at kits.

djn · « **Reply #11 on:** 6 Jul 2008, 02:44 am »

Hi All, I am new to this forum. Here are pics of my Octal Cornet. I am fairly sure that I did not wire the power side correctly but I will work on that tomorrow. I get a hum and buzz, but the music sounds wonderful. I think it is the best phono pre I've ever heard. Cheers.

rotcoddam · « **Reply #12 on:** 6 Jul 2008, 02:55 am »

DJN,
Too cool. Cooler than cool. What's that made of?

mgalusha · « **Reply #13 on:** 6 Jul 2008, 03:15 am »

Oh that is just too cool, reminds me of a guitar with the curves you have in there.

djn · « **Reply #14 on:** 6 Jul 2008, 05:04 am »

Thanks guys. Here is a pic of what I started with. It is a piece of 1.25" marble. I drilled 64 holes in this thing and shaped with a diamond blade on my high speed hand grinder. The grenade is at the machine shop now getting wacked and milled. It will mount where the power chord sticks out. The grenade houses the power chord, fuse, and switch. If you look at the pic of the butterdish circuit in back, that is the power side and I have the last two caps and the tube heater grounds all grounded to the same ground rod as the upsteam of the power side......could that cause the hum/buzz? Cheers. ps, the red and white twisted pair are going to the three tubes for heat.

amandarae · « **Reply #15 on:** 6 Jul 2008, 06:49 pm »

I like it! I like it!

I feel my soldering iron needs some workout this summer..... hmmm....JH's chassis looks awesome too!

Thanks for sharing.

regards,

Abe

Brinkman · « **Reply #16 on:** 7 Jul 2008, 06:33 pm »

Anyone do any direct Cornet2-to-Octal-Cornet comparisons? I was planning on sticking with my Ripper until I earned enough to purchase a Trumpet (or whatever Jim's top-of-the-line phono pre-amp is at that point in time), but I am suddenly very curious about this Octal Cornet...

djn · « **Reply #17 on:** 7 Jul 2008, 09:39 pm »

Hi Brink, yesterday a friend brought over his Cornet2 and we swapped it with my Octal Cornet. To be fair, mine is just finished and I have a 60hz buzz in the left channel, however, We both agreeded that the Octal had better detail, much better lows, smoother highs, and a little bit more unvailed mids. We where both impressed with the Octal over the 2. My system is a cheap-O Onkyo TT, 300B amp, and Jensen Imperials loaded with EV woof, midhorn and tweethorn. Cheers.

mingles · « **Reply #18 on:** 9 Jul 2008, 04:19 am »

Jim, sign me up as a committed buyer for the new circuit board. I want to encourage you to do this. Any others willing to commit to a PCB?

Brinkman · « **Reply #19 on:** 9 Jul 2008, 06:49 pm »

Mingles,

The recent eight or so posts here have been to a thread that was started five years ago. Since then Jim has put out the Cornet2, which includes the CCS upgrade that was introduced for the original Cornet kit (after it had already been out a while), but after the start of this thread.
The idea with the Octal Cornet is you use either a Cornet or Cornet2 PCB and instead of soldering noval (nine pin) tube sockets to the PCB, you use Octal sockets, mount them to the chassis, and connect their pins to the PCB with wires. Changing from nine-pin to an eight-pin type tubes requires some resistor values to be changed, and info on that can be found with the search function. Also, since the original Cornet is no longer available as a kit, the current Octal variation would more appropriately be called the Octal Cornet2.
According to Jim and others, the overall gain of the Octal Cornet2 is less than the regular Cornet2 and for many this could be problematic. So yet another variation combines the stock front end of the Cornet2 (the two 12AX7s in the RIAA EQ) with an Octal tube (6SN7 or 6FQ7) in place of the nine-pin 12AU7. This has been informally referred to as the Octal Cornet Hybrid, or more appropriately Octal Cornet2 Hybrid.

A lot of folks have experimented with tweaking the Cornet. One popular tweak was to choke-load the plates of the 12AU7. This is what led to Jim to implementing the CCS upgrade (now stock) which accomplishes the same thing (high and low impedance). Others have also choked the power supply. If you use an Octal in place of the 12AU7, you will have to change the value of the emitter resistors from 220ohms to 510ohms, along with other changes, some of which are mentioned above.

When I can get around to it, I'll start a Octal Cornet/Cornet2/Hybrid FAQ where I state the common questions and try to provide the answers as I can dig them up with the search function. In fact, I would love to help Jim have FAQ threads for all his products and their variations. If they were helpful, he could sticky them.

Jim, would you be bothered if I do this?