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Old 5th April 2013, 11:21 AM   #1
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Default 12AU7 - 5V4G easy amplifier

Hi there and nice to meet you all.

I just got some kind of old frequency meter and found inside it almost 60 tubes 12AU7, 2x5V4G,some 6CB6 and 6AL5.

First of all I must say that being a young CS student tubes are complete strangers to me and I don't even know how to test them to see if they are of any use.

With your help I would like to build a very easy make tube amplifier and play some music through it knowing that I made it.

So all summed up I am waiting for your advices,schemes,ideas. Thank you!
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Old 5th April 2013, 11:29 AM   #2
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You could build a line stage with your tubes.
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Old 5th April 2013, 11:37 AM   #3
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That is a good idea, thanks for your fast reply. I found a project here 12AU7 (ECC82) Cathode Follower Tube Preamplifier Schematic what do you think about it?
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Old 5th April 2013, 11:38 AM   #4
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Click the image to open in full size. That is a good idea what do you think about this schematic?
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Old 5th April 2013, 11:59 AM   #5
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Build an Aikido line stage
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Old 6th April 2013, 11:42 AM   #6
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Any other ideas?
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Old 6th April 2013, 02:48 PM   #7
GoatGuy is offline GoatGuy  United States
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The 2-stage input amplifier is very classic ... but not very good, I'm afraid. It was designed in an era where the tubes were expensive, capacitors were really expensive and where the main concern was "doing much with little"

You really need to start your new-found hobby by building a nice power supply, if you don't have one. For virtually all your first-round projects, you can re-use the same supply, if it is "nice enough". The voltage almost doesn't matter, within the range of 200 to 400 volts, and making a supply from a classic rectifier tube is oddly satisfying. Go for what we here call a "CLCLC" filter network (capacitor, inductor ("choke"), cap, choke, cap); build it in a sharp, sturdy box, with high-voltage connectors that ensure safety. (You can buy 'instrument jacks' like you find on better multimeters from Mouser, that are 1000 V rated and conveniently "red" and "black" and "green" and so on.

Then - per the advice above - perhaps make an Aikido line stage. They're great little preamps, and there are so many cool "new think" design features in them, they make good projects for a pair of double-triode valves. Again, invest in the woodworking and sheet-metal tools, so that you can build a proper (not necessarily pretty, just functional, and sturdy) box for it.

Since you're probably a stereo fellow ... like all of us ... you should just build it as a double, of course.

Then, stick it between your favorite CD player or other audio source (but not a conventional/retro vinyl record player!) and see the magic it adds. Often it is remarkable.

Let's see ... then in this same vein, I'd probably go on to produce a small class-A type headphone amplifier. You probably have tubes which are more suited to low-power work anyway, and what better than a schweet little headphone amp? More box-work! See why I'm advising tools over toys? Now you can plug your i-Whatever into it at home, with a very nice pair of Grados or other great headphone, and sit drooling all evening, hearing music from your very own recordings ... that you never heard before.

For simplicity, the headphone amp could be another 2-tube wonder: the "LTP" (long tailed pair, utilizing a single dual-triode as a gain-stage and phase inverter), and a nice pair of medium-power triodes (12BZ7 anyone?) in a classic cathode-follower (which gives quite usefully low output impedance), or, as plate-transformer output loaded configuration. Either way... quite different sound, but there are great followers / advocates / evangelists of both strategies.

But do not attempt a classic speaker-power capable amp at the first go, buck. ONCE you have the skill honed by these smaller, more tolerant projects, THEN go on to build a classy (classic, too) class AB push-pull "final". Again, if you've been thinking ahead with the box-work and sheet-metal stuff, you can make yet-another pair of boxes that will be the "mono-blocks" (single channel each) to take the symmetric output of your headphone amp directly, and drive the finals. Its a blast! All 5 of these, chained together (power supply, Aikido, LTP headphone/phase splitter, Right+Left Final) will look great if you sheath all the boxes in the same height hardwood (sides, not top), if you carefully spray-paint or polish the tops, and if you apply some very nice millwork to the whole thing (you know polished brass screws, or black hex nuts, etc.)

After that? Hmm... probably a RIAA phono stage; go buy a used (but very good and reputable) vinyl transport, and have fun listening to music, and I mean MUSIC like you've probably rarely ever heard it from CD/MP3. Almost uncompressed, uncompromised. And it'll fit nicely in your box scheme. Further, by making ALL the stages "modular" like this, you can easily pull any of them out, and substitute another "just to see" whenever you feel like it and the muses bite you behind your ear.

Good luck! TOOLS, time, modularity, persistence. And not too many disappointing ancient designs.

GoatGuy
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Old 6th April 2013, 04:44 PM   #8
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Wow thank you a lot for all your help. I have already found the great sound of uncompressed music and I bought last year a NAD 3020A amp, Magnat speakers a squeezebox and a turntable so that is why I want something tube based to add a self made piece to my system.


Probably a preamp it's what I would love to build and it also suits my needs.
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Old 7th April 2013, 07:06 AM   #9
GoatGuy is offline GoatGuy  United States
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So... nothing of the SEQUENCE of my advice changes.

Power supply
... then ...
Preamp
... then ...
Headphone amp

And remember: you have to get decent tools, but not ridiculously expensive ones. Otherwise you'll be building "amateurish looking" boxes.

GoatGuy
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Old 7th April 2013, 03:28 PM   #10
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I already have almost all the tools I need from some projects in the past and will start next week a power supply as you suggested do you recommend any scheme I should use ?
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