For the 2011 Russian Line Stage competition, I decided to do two entries. Hey, I'm retired, I've got the time.
I sent off for a bunch of Russian tubes from Sovcom on Ebay. Good service by the way.
The first line stage is based on the 6S17K-V planar triode. It's shaped like a nose cone on a rocket and is quite tiny at only an inch long. 4 for $10 from Sovcom. The data sheet is here.
a - anode
g - grid
k, f1 - heater and cathode
Here is the schematic (done with LTSpice). The amplifying part is taken from Eric Barbour's AudioXPress November 2001 article where he uses this tube as the front end of his phono circuit. My schematic does not show the connection to the tube correctly. The tube is indirectly heated but the heater is connected to the cathode which is unusual. There is no resistor to ground from the cathode. In his article, Eric found that "it generates so much self bias that its cathode could be directly grounded, The grid, with a 47K resistor to ground, then floats at approximately 0.5v below ground and with a B+ of 140v and a 51K resistor for anode loading, the anode idled at approximately 50 - 60 volts, one half of the B+ supply.".
Eric used this tube as the front end of a phono stage with no pot in front so I found that the bias varied as I would change the volume. And of course, one tube measured about 50 volts and the other channel about 35 volts. The other two tubes from my lot of four wouldn't do this self biasing trick at all. Still, sound came out so I left it as it was.
The parafeed transformers are the inexpensive 10K:600 half watt versions from Edcore. I ordered four of them over ten years ago and they are fun to play with and give reasonable sound. I think the yellow tape that covers the bobbin is quite fetching.
The power supply uses two 12.6 vct transformers from All Electronics that are wired back to back. You could also use Radio Shack transformers which are also inexpensive.
The high voltage supply is a copy of Pete Millet's supply from his engineers amp. I've built a high powered version of this and it is a good sounding push pull amp.
Since the 6S17K-V tube is directly heated, I needed a DC supply which is fairly simple. Each tube uses about 0.3 amps at 6.3 volts and I used a separate regulator for each tube. The 47uf final capacitor is a United Chemicon organic type with a very low ESR that I used I used when I did the Lampizator mods. They are reasonably inexpensive compared to the Oscons.
So here is the $1 bowl from Target, looking from the back. Very basic with a switch, an IEC connector, 4 RCA jacks from Radio Shack and a 100K dual volume control.
The guts. From the left you can see two Edcore line transformers with two MBG-O PIO 2uf caps in between. Look closely and you can see the two 6S17K-V planar tubes held by camps attached to european style connectors. At the 12 o'clock position is the high voltage (150v) which I pinched from Pete Millet's Engineer's amp. The Radio Shack board at the 4 oclock position holds the 6.3 volt regulators for the heaters. To the right are the All Electronics 120:12.6 transfomers and finally at the right the fuse, IEC and switch. They all are attached to a circle of polycarbonate that is glued about half way up the bowl.
I've drilled ventilation holes under the volume control knob. I found that my initial holes were too small and that the linestage would shut down from too much heat inside so I increased the size of hole.
So here you can see how I mounted the tubes. I cut off four parts of the european style terminal, cut of the top plastic bits of the barrels on either side and attached the alligator clamps which hold the tubes. The green bits are 12 gauge copper that just rests on the rings. It seems to work. No doubt there are better ways.
So how does it sound? So far it is very promising, but I'm concerned about the amount of bass being a bit low. Eric calculated plate resistance as 13.2K. I suspect the 10K transfomer is probably too little to properly load the tube. I did try putting a 600 ohm resistor across the outputs of the line transformer, but the magic disappeared.
And yes, this is a magic tube. The detail is incredible and I will be very interested to see how it does compared to the other tubes in the competition. For $2.50, it is well worth trying. When the competition is over, I will probably convert the linestage to Eric's phono circuit. I might also try this tube feeding Tubelab's Powerdrive circuit.
There was too much heat build up in the blue bowl so I moved everything to a fruit bowl colander. There was a bit more room and the wire made it easy to atttach the various bits.
More guts. The pot on the right is a 47k dual that lets me vary a resistance across the outputs of the Edcore transformer. It is in series with a 500 ohm resistor so 500 ohms is the minimum. I find that at lower resistances, the life gets sucked out of the music and that I prefer closer to 47k. It's better having something there than nothing.
My other sort of entry is 6N6P based. The top plate is the usual 11 x 14 polycarbonate you can get at Home Depot. The base is a simple plastic box from Target that is pretty close to the 11 x 14 size of the polycarbonate. I just used parts that I had hanging around so I don't know if it comes under $100. The rectifier tubes are 6AX4's that were given to me years ago as part of a few boxes of tubes. The 6N6P is loaded by a Gary Pimm CCS set to about 15 ma that I rescued from another project that I disassembled.
The 6N6P is a lovely sounding tube. To my ears it sounds sweet compared to the 6S17K-V, but doesn't have quite the detail of the nose cone. I'll be really interested to hear how they compare at the meet.
Finally the bucket which uses 4P1L's. Unfortunately, I can't get rid of the hum and I suspect I'm trying to shove too much into the bucket. The tube is a direct heated pentode with a loctal socket and when triode wired, is supposed to sound pretty good as an output tube. Some reckon it sounds better than a 2A3. At about $2 each, I'm looking forward to playing some more with them.