Basic tube Pre amp schematic, anyone?

chuckworkb

chuckworkb

Super Member
I am looking for a basic tube preamp that I can build from a schematic. I have a bunch of 12AT, AX, and AU7'S in a bin, or I could go with a 6 volt tube set. I really just want a couple of high level inputs, and a volume control, tone controls aren't required, but they could be used. I will be doing point to point wiring so that's why I am looking to keep it fairly simple.

Thanks guys.
 
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Maybe a Marantz 7C circuit with the tone controls removed. Perhaps not the most simple, but its said to be quite good. If you want really basic, you could clone the Dynaco PAS circuit and add a cathode follower. The Dyna tone controls aren't spectacular, so consider leaving them out.
 
I personally never liked the 12A*7 series much for preamps, just my opinion. I'm not a big fan of cathode followers sonically which most circuits employ.

Preamps need drive, not gain. For a first project, look into the 5687. A very simple circuit will sound great and drive anything nicely. I grabbed the circuit below from the Joelist, probably 10+ years ago. I unfortunately don't know the original designer so can't offer appropriate credit. My son has the preamp currently driving a Dynaco ST70.

pre5687.jpg


linestg.jpg


Part selection except for the .47uf output cap is very non-critical. The transformer in the picture came out of some test gear. Tube in the picture is a Bendix 6900. Ironically an inexpensive TungSol 5687 sounds better. If you decide to built it let me know and I'll send you a 5687 for postage :)

- Gary
 
Clarification

Gary,
I hate to sound like a dummy (which I pretty much am when it comes to this).

Does the second channel just attach to where it says 250V on the schematic? B+ in other words?

And, any special issues about grounding? The never show that much on schematics. Do all those ground indicated connect at a single point?

Would there be an "off the shelf" transformer you'd recommend a part number and source for?

And the 10H chokes?

Thanks in advance. This looks fairly expensive to build with two chokes, probably two transformers (power and filament). But if it sounds great, might be worth the expense.

Mark
 
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All good questions. The 250v supply is common to both sides.

Surplus shopping is key. Lots of vintage tuners and test gear had appropriate power transformers which can be salvaged. The 250v is very non-critical and anything from probably 180-280v would be fine. A simpler power supply with resistors instead of chokes could be designed. Also the chokes can be very low current, hence tiny and low cost. The linestage in the picture uses a 6X5 rectifier as I only had a 6v filament available. You really can build this linestage for under $100.

Grounding is probably a science for some folks, but I've learned by trial and error. A single grounding point helps, as does clean wiring.

I agree with Jay that the 12B4 is a great tube. It's actually what I use on a daily basis. For a first project the 5687 is probably easier to tame. I found the 12B4 to be microphonic and prone to RF oscillation. When you get it all right it's a fabulous tube - but I spent time with grid stopper resistors, ferrite beads etc to get it quiet.

I just made up a small circuit board with the above 5687 circuit. I haven't had a chance to test it out yet. Stay tuned.

- Gary
 
I'm interested to see what is everyone's feedback.

I'm curious; what are you using for a source? Do you need gain or are you looking for a certain tube tone?

I have a new project coming up and my plan was to remove this line stage from the integrated amp and just go with the power amp side. As you can see, the 12AX7 is knotted up with the tone controls.

 
chicagozer said:
I'm interested to see what is everyone's feedback.

I'm curious; what are you using for a source? Do you need gain or are you looking for a certain tube tone?QUOTE]

The tube preamp will be used with a CD player, and a tuner.
It will be feeding two Baldwin Mono amps I recapped, and added speaker outputs and RCA inputs to.
Click to expand...
 
My 6SN7 Preamp thread have a link/path to a few of them,,, You could use a 12AU7, instead, if you wanted...

Regards,
John
 
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Gary Kaufman said:
I personally never liked the 12A*7 series much for preamps, just my opinion. I'm not a big fan of cathode followers sonically which most circuits employ.

I like that preamp...I can't find the 5687 in my tube manual,,, your schem is of one channel, correct? Can you post pics of your amp? Bottom, back...
How do you think it would work, with 6SN7...

Regards,
John
Click to expand...
 
gain usually refers to an increase in voltage. Sometimes you need an increase in current more than voltage to make up for input load impedance on the amplifier. Too little current capacity and the amp input impedance will just load the preamp down to the point that it doesn't have enough output voltage to drive the amplifier to full power. Often you'll see a cathode follower used to provide more current output and a lower output impedance so the preamp doesn't end up loaded down by the power amplifier. Plate output (or anode follower if you like that term better) tends to give more voltage, but with a higher output impedance. That Ohm guy insists that higher impedance has to mean less current.
 
5687 Data is at http://www.mif.pg.gda.pl/homepages/frank/sheets/127/5/5687.pdf

Schematic is one channel, but it's a dual triode, so only a single tube is needed.

It's a great tube, just ask any Ongaku owner.

Pics may be tough - I'd have to kick my son off of his stereo and unscrew the bottom.

If you search 5687 linestage you'll get lots of hits and variants.

I need to get a few hours to build up the circuit board I designed into a linestage.

I'm no engineer - others can probably give a better explanation - but I think of "gain" as voltage and "drive" as ability to deliver current into a load.
 
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