Tube Time
Stage 1
Time For A Change
A while back I decided to go with passive attenuators for the clean sound they put out. So, I disconnected the active circuit in my Zen preamp and never looked back.... Until now. I've finally decided I needed change. But what to do? I wanted something that would bring me closer to that magical sound everybody wishes they had. I'll call it "audio nirvana," or AN. Looking at my system, the only thing that could drastically change the sound at a low cost was a CD player. So I searched the Web, read the reviews, and purchased a new Cambridge Audio D500-SE in hopes of finding AN. No luck yet.
One late night, with ABBA's "Take A Chance On Me" playing in the background, I was falling asleep and I thought I heard whispers of "take a chance, take a chance, take a chance on tubes." Waking up, I paid closer attention to the song, enjoying the music and taking in the lyrics and harmonies only ABBA could provide. "This is close to AN," I thought to myself. The vocals were drawing me in, but yet it wasn't quite real enough to hold me tight. Then I remembered what my friends have been saying about their tube gear all these years: "It brings you closer to the real thing." I was sold!
What To Build?
Money was tight, so I knew I had to do it myself. Tubes. My knees were already shaking. Lethal voltages. Was I making a mistake?
One of my friends (lets call him Tube Guy, or TG) sent me a bunch of links with suggested kits and schematics. At first I thought I would go with the Foreplay preamp from Bottlehead.com. There is a small community of fans supporting this kit, and you get everything you need to build a complete unit. In the back of my mind I knew I wanted to use my old Zen preamp for scrap parts, so I held off while looking at some of the other schematics. The best design is a simple design with as little in the signal path as possible, so I knew I had to keep the parts count low. I decided on a single-ended triode (SET) schematic I found on Vandit Kalia's Web site. Designed by Sridhar Gantimahapatruni, the preamp was built and tested by Vandit before being favorably received by his peers in a blind taste test. OK, he just gave it the thumbs up. Visit his site for more details..
The design uses a 5687 dual triode and TG has suggested it may sound better than the 12AU7 in the Foreplay. If I've learned anything from surfing the tube sites it's that you won't know how any circuit will perform until you've built it.
I hope Vandit doesn't mind, but I've taken his schematic and placed it here on this site for easy access. Since I don't know anything about tube electronics, I've made some notes on my copy of the schematic. Let's take a look:
Schematic 1 - The power supply
Schematic 2 - The line stage
5687 - The driver tube, 9 pins
5AR4 - Rectifier, 8 pin octal - This is the same as the GZ34
272X - Hammond power transformer
Design Considerations
One of the first things I wanted to do was reuse my switch attenuators from the Zen project. Although the schematic calls for a 100K ohm pot (see line stage), I'm thinking I may try using a cheap Radio Shack pot for initial testing and later swapping those out with my 10K attenuators. My cables are already short at 2 feet long so the CD player isn't exactly working very hard. But who knows, I may settle and use a Noble/Alps pot. New mono switches would cost me $155 from Michael Percy and the aim here is to spend as little as possible.
Cost. Let me address that. The key is to spend more on the parts that count, like the grid resistors and coupling caps, and moderately on the associated parts. Make no mistake, cheap parts in the power supply will destroy any SET design. Having a low parts count is what keeps the overall price down, not cheap parts.
Let's now consider the battery situation. Looking at the line stage, you see that 2.4 volts is being fed to the cathodes. The battery setup replaces a 100-ohm resistor and 220uf capacitor tied in parallel to the cathodes. TG says it makes everything sound smoother and that "the inside joke for tube builders is that the preamp is just a fancy NiCad charger" -- an expensive one at that!
Next is parts quality. How much do I want to spend? Well, we already know that the parts in the signal path are going to be important. So, I've decided to go with Hovland MusiCaps for the output and generic tantalum resistors from The Parts Connexion for the input. I will probably not try tantalums from Angela Instruments as they are $8 a piece. However, I will try out Kiwame carbon film in the future. As for the rest of the parts, spend as much as you can afford. TG says that with such a simple circuit, all of the parts will affect the sound in some way. I'm going with 3 watt wire wound resistors from Digikey for cost and convenience, a Mills 6.49K ohm MRA-12 for the important plate resistor, NiCad AA batteries for the bias voltage, an Elna Cerafine dual cap for the output of the B+, Hammond transformer and choke, Alps stereo series attenuator, Solen cap for the power supply (PS), and Panasonic TSHB and TSHA electrolytic caps for the rest of the PS. Tubes will be a 5687WB JAN Philips '70s-'80s N.O.S and a Sovtek 5AR4 rectifier. I am looking around for alternative tube selections now. All of the above should cost you around $400 or so, depending on the quality of your RCA jacks, hookup wire, etc., and whether you already own some tools, such as a soldering iron and chassis punches.
How 'bout cheap alternatives? Go with a cement plate resistor, Panasonic electrolytic caps for the entire PS, Digikey wire wounds all throughout, a polypropylene coupling cap (or metallized poly), and an Alps blue pot or Noble pot. You can't do much about the transformer or choke as there aren't that many parts available to choose from. If you build this low-cost version, please drop me a line and tell me how it sounds and how much you actually spent. I'm curious how such a simple circuit will perform on a budget.
One last word on parts. You will notice that I did not use any metal film resistors. Tube nuts stay as far away from them as possible. I don't know why so I am just going by recommendations that wire wound and carbon resistors will provide a much sweeter, warmer sound. I suppose the metal film resistors must make tube gear sound like solid state. ;-)
Stage 2
Ladies and gentlemen, start your buying! Here is my final parts list and where I purchased from.
Quantity Part
2 5687WB JAN Philips NOS - I may try GE "5 Stars" in the future. Cryogenically frozen tubes sounds fun, too. (although expensive) - Angela Instruments
1 5AR4 rectifier Sovtek - Triode Electronics
2 9 pin sockets - ceramic w/gold pins - Angela
1 8 pin octal - ceramic w/gold pins - Angela
1 Hammond aluminum project box 1444-22 - 12X8X2 - Angela
1 Hammond aluminum panel 1434-22 - 12X8 - I thought I would need this to close the box, but upon reflection, I may not use it at all since I will need to drill holes on the rims and then buy sheet metal screws. Pain in the butt. - Angela
1 Hammond 272X 310-0-310/5v-2a/6.3v-2.4a - I could not find a 330-0-330 tranny like Vandit used in his preamp and I couldn't figure out new values for the voltage drop resistors so I got as close as possible. With AC at the wall at 120v, the unit may put out a bit more than 310 volts, so I will leave the resistor values as they are until I can build the power supply (PS) and measure voltage with a 10K ohm load. - Angela
1 100K series attenuator - I decided to try a factory-made Alps series attenuator. This unit was a custom pot and is no longer produced. - The Parts Connexion
4 100 ohm Tantalum - I am attaching one resistor per grid so four resistors are needed for the dual triode pair. Why tantalum? The Parts Connexion tants are affordable and help keep costs low. Angela's are more than double the price, and I figure a tantalum is a tantalum is a tantalum, etc. I hope they are... - The Parts Connexion
2 4K7 and 2K2 Mills MRA-12 resistors. I could not find 7K 10 watt resistors of high quality. So instead of using metal film or regular carbon film or comps, I knew I wanted to use Mills at this important position so I just used two resistors in series for a total of 6.9K ohms at 12 watts. With the tranny at 310, I may have a lower voltage than 250 anyway. Good thing tubes have large tolerances/margins. - The Parts Connexion
1 0.3 ohm 3 watt - Couldn't find a wire wound at this value so I used a carbon film type. Shhhhhhhh! Don't tell anyone. - Digikey
2 100 ohm 3 watt - Wire wound - Digikey
1 100k ohm 3 watt - Wire wound - Digikey
1 300 ohm 3 watt - Wire wound - Digikey
2 0.47 uf 400v Hovland MusiCap - The Parts Connexion
2 4700 uf 16v - Panasonic TSHA series. I think Bottlehead's Doc likes the TSHB and says it is an acceptable replacement for Elna Cerafines and Black Gates. Your wallet will thank you. - Digikey
1 100 uf 450v - Panasonic TSHB for high voltage - Digikey
1 47 uf 400v Solen - The Parts Connexion
1 220 uf 350v - Elna Cerafine - TG recommended this or LCR. Being that these are dual caps (two caps in one casing) I figure I was getting two for the price of one so the cost was acceptable. Plus I read Herb Reichert's article about his 300B SET amp and he preached about only using Black Gate caps, but for the poor, he recommended Cerafine caps as second best. - Angela
1 8h / 150ma choke (inductor) - Hammond 193D - Angela
All other parts are from my old preamp. I've unsoldered the jacks, wires, etc.
Notes:
Tube Guy (TG) says I will need two sets of AA NiCads, one set per tube. This will kill all my space. And I only purchased one dual-AA holder. I guess I will have to visit Radio Shack... need fuses from them anyway.
Next: Stage 3: Construction
