Line stage schematic suggestions sought:

winters860

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I know these threads get done to death, and become a grab bag of suggestions only tangentially related to the subject matter ("Buy a used Adcom GTP-500II instead!" "Psvane makes the best new-production tubes and I'll CUT anybody that says different!") but that's okay, right?
:whip:

I'll be building a stereo tube preamplifier soon. This will be my first scratch build, to be done concurrently with a SEP EL84 stereo amp.

Objectives:
1) Be able to drive a wide variety of amplifiers
2) Be relatively simple and inexpensive to build
3) Be compatible with 280V-0-280V power transformer secondaries (I have a nice NOS transformer)
4) 9-pin miniature tubes from the 12A*7 family strongly preferred (I have lots)

Suggestions, anyone?
 
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Looking hard at a cathode follower design, like the one from the RCA Receiving Tube Manual or one of the ones from Shine7.

12AU7-Cathode-Follower-Schematic.png 12ax7_schematics.gif

If all the power supply calls for is a single B+ voltage, I'm comfortable at this point designing my own.
 
Check out Vacuum Tube Audio, the Aikida preamps. They sell board kits for both 9 pin and octal tubes. I use one that was built from a complete kit with chassis and all and love it, found it on CL. Lots of great info out there on them. You can even find the schematics and do point to point.

BillWojo
 
Look up glassware audio
They sell a number of kits suited to your needs here
I would go with the CCDA pre kit and one of the power supply kits like the ps1 which will give you nice clean power without trial and error

The manuals are excellent and will teach you a lot as you assemble the parts
 
I would offer up a grounded cathode 12B4. Simple grid to volume pot, and output coupled to the plate with a 2-5 uF capacitor. If you wish to cheap out on filtering B+, use a CCS instead of a resistor for the plate loads. You can get WAAAAAY more carried away too, and I present you with the following evidence:
http://www.emotiveaudio.com/epifania_info.htm
cheers,
Douglas
 
Looking hard at a cathode follower design, like the one from the RCA Receiving Tube Manual or one of the ones from Shine7.

View attachment 1350588 View attachment 1350589

If all the power supply calls for is a single B+ voltage, I'm comfortable at this point designing my own.

My pet preamp is similar topology to the RCA, but with the volume pot between the gain stage and cathode follower, and with lower-impedance plate and cathode resistors. I usually use something like a 47K 2w plate resistor and an 820 ohm cathode resistor on the gain stage, and a 47K cathode resistor on the cathode follower. I put .22uf caps before and after the volume pot (which is 100K ohms), to couple from the plate of the gain stage and to the grid of the cathode follower.

Regards,,
Gordon.
 
I built a "forewatt" as one of my first DIY projects in the vacuum tube realm. I still occasionally use it, and I'm planning to someday add a phonostage to it. You mentioned having some premium iron which you would like to use. Well, when I built my forewatt, I saved a few bucks by designing my own power supply, using what I had laying around, and it turned out great. It sounds fantastic, and uses some of the cheapest tubes out there the 12AU7. The main thing keeping me from using it is the way I designed the chassis, with the tubes coming out the top,I can't stack anything on top of it and it's inconvenient to change the volume.
 
I went back and looked at my notes- and here's the last iteration of the circuit I used.

stupid-simple-buffered.jpg

I usually use a three-stage power supply for this (RCRCRC)- with something like 100uf caps in all three C positions, for a stereo unit. B+ can be anything from 250-300V. I think that higher B+ sounds a little better, IME.

Regards,
Gordon.
 
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I find that simple is better for preamps. My PAS has a 12AU7 stage that Gordon built for me, and I like it just fine. Its basically invisible, which I think should be the goal.
 
Thanks all! Many intriguing variations on the cathode follower pre here. You've given me a lot to chew on.

Further thought:

Thinking seriously about adding a phono section. Does anybody do this any more?

Should I just keep the phono tubes on all the time, like in vintage designs, or do they switch off the B+ and heaters when only the line stage is in use? If it's preferable to power down the phobo tubes when not in use, is there a more elegant solution than a toggle switch for phono B+ and heaters and a seperate toggle switch for inputs? Would I be asking for noise putting power and signal on the same switch? Am I inviting trouble putting B+ and heaters on the same switch?
 
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