I want to be able to listen to music with high quality and an acceptable volume (no need for blasting windows out of the walls. I would like to have an input selector, volume, bass and treble control. I have read that tube amplifiers have an interesting sound quality. Is that correct?
This is quite a project. Tubes have different kinds of distortion than transistors, are expensive and delicate. I would not bother, really. You can get a nice Sony TA-N55ES power amp on e-bay for $300 usually. You would need a pre-amp too.
Can you still even buy tubes? 
what kind of speakers and audio power do you want?
Some companies used to offer kits. Not sure anybody still does.
Woo hoo. There is a serious slippery slope here.
You really need to set some goals and constraints.
Do you want to save money?
Do you want the satisfaction of building it yourself?
Do you have an interest in the intricacies of audio design?
Do you have an interest in electronics?
Do you have any background in electronics?
If you want to save money, head to eBay. In the end you can build fantastic gear for much less than new, but the cost in time is huge, and you probably won’t win.
If you tick some of the other boxes, there is a lot of fun to be had, but be aware - it is a hobby. You need an investment in tools and time to build experience with them.
There are a range of options, from ready to roll all in one kits, that provide everything you need in one hit, a range of amplifier kit boards, where you will need to source you own case, power supply, switches and connectors, but the amplifier itself comes as a kit with printed circuit board and needed components, down to a a range of printed circuit boards that can be bought, and you need to buy everything else.
Tube based HiFi is a separate little niche area. It is pretty active, but there is an astounding amount of psuedo-science associated with it, and the prices can be very high, as you need a lot more expensive components to get a result. But, some people love the (coloured and innaccurate) sound.
Your profile says Brazil. I can’t say I know much about the DIY scene there, but I have heard murmurings that there is some good stuff going on.
However, you could do worse than the check out diyaudio.com for a reasonably friendly forum. If you look in the commercial sector area you will find a list of group buys, where members sell (at no profit) printed circuit boards, and in the Manufactures area, some of the kit amp companies have a forum. The group itself also has a couple of purchasable amp boards, one a conventional rather high power amp, and some others based upon Nelson Pass’ designs, which are quite low audio output power, but very inefficient - and something of a cult .
Yes. There is a tiny electronics store in San Mateo CA (s. of SF) with a shelf wort of new mfg vacuum tubes.
OP - Don’t forget chassis punches - socket holes start around 1" and easily go to 1 1/2".
Drilling a large hole is dangerous with a good drill press and vise plus about 8 -10 drill bits.
If it needs to be said, using a file to enlarge a hole is neither fun nor accurate enough to get perfectly round.
GreenLee is the go-to name in punches.
And get a good quality soldering station - good tools are a pleasure, and, in radio work, a sloppy connection can introduce noise you will go crazy trying to isolate and fix.
Good luck and have fun.
I’d recommend getting a non working tube amp from Ebay and restoring it. It’s going to take less skill then trying to design an amp from scratch.
Typical restoration will involve replacing the large can filter capacitor and the paper/wax capacitors.
Next turn it on and see if the tube filaments light up. If they don’t, then check each one with a multimeter until you find the bad tube.
There may be other issues to fix. Thats going to require troubleshooting.
Usually replacing the capacitors and burned out tubes will restore the majority of Amps that were working and got stored in the garage for 30 years.
The other option is to buy an old electronics magazine with a stereo amp project in it.
http://www.ebay.com/itm/Cute-little-6973-TUBE-STEREO-Amplifier-you-can-build-Scott-Stereomaster-130-/200877359380?pt=Vintage_Electronics_R2&hash=item2ec5394114
Hopefully you can still find all the parts. At least with this project there would be a circuit diagram and a parts list.
Begin with a high quality victrola as a starting point.
If you simply want an amplifier w/ tone controls, the most economical approach is to buy a receiver. You can buy a 135 watts/channel receiver for $118. You can’t even begin to build one for that amount.
However, if your goal is to build something, I would suggest getting a tube amplifier kit and solid state stereo preamp kit.
With a tube amp, the price/watt is extremely high, even with a kit. Be forewarned, this is strictly hobbyist territory:
8 watt tube amp kit for $204
90 watts/channel tube amp kit for $1995
And then build a preamp from a kit, and you’re good to go.
Ok, so many questions!!! Yay!
I have a good workshop downstairs. Wood (and metal) working are not a problem. I have basic knowledge in electronics, and a good friend of mine will help. I want to have the pleasure of building something from scratch and I would like to save some cash. What is thr better option rather than vacuum tubes? I look for audio quality above volume… And we have some hacker spaces here, and a whole neighbourhood dedicated to electronics (it is even scary to be there - 10 shops blasting music on sound systems that are installed in churches) i think I will have a blast on this project!
If you couldn’t there would be a lot of guitar players crying over it.
Tube amps can give an electric guitar a genuine warmth that is very difficult to create with pure semiconductor electronics. Blues and rock guitarists love to find the sweet spot on a tube where it is just breaking over into distortion.
Ok, no vacuum tubes! Too expensive! 
Quality is a difficult term in HiFi. Be quite clear, from a purely objective point of view, tubes are not high quality. They colour the sound, and have a whole heap of curious issues. But from a subjective point of view, there are some who would listen to nothing else. They are a small but vocal minority, and many have a lot of fun. Most people however buy or build transistor based amplifiers.
For all useful purposes, an accurate, low distortion, well behaved, amplifier is going to mean a conventional transistor based design. Even here there is a lot of room in the options. SOme purists love class A, despite its poor efficiency, there is a following for Nelson Pass’s designs that are very simple, and very inefficient, but have a certain sound many like. There are chipamp designs that can produce very good quality results with not a lot of effort.
But it is a good idea to set some baselines. What speakers do you have? Efficiency and other attributes matter a lot with tube amps. What sort of music do you like, and listen to? How much do you want to spend? You can spend essentially any amount of money on this hobby. What source material do you have? I assume CDs, or other digital sources, but one needs to be sure. The quality of the source matters.
What voltage power do you have? I notice that Brazil has a mix up and down the country. This may affect what designs you can easily source a kit for, if you go that route.
Your enthusiasm is admirable. But to be frank, a number of things don’t compute here.
“I would like to save some cash” is a bit perplexing. Can you elaborate on this? Do you want to build something because you think it will save you money versus buying new? Because that’s simply not the case; you will save a lot more money buying a COTS product than trying to build something.
If, OTOH, your goal is to have fun building something, then “saving some cash” should not be on the radar screen. If you want to build something from scratch, be prepared to spend lots of money on PCB artwork, connectors, parts, heat sinks, etc. Not to mention tools, equipment, and your time.
Here’s my advice: if you’ve never done this before, do not attempt to build from scratch, else you will be quickly overwhelmed by the sheer complexity of it, and it will become a huge time burden, even for something that seems simple. I mean, do you know how long it takes just to lay out a two-sided PCB using free software? That alone will eat up at least a week. It would be much smarter to buy a kit for a solid-state power amplifier that contains everything you need, including the PCB.
If your goal is to listen to music with a dynamite sound system, those are available off-the-rack for not much money.
If your goal is to tinker with something as a hobby, the advice given in this thread is a good start.
I feel that these goals are quite different. The hobby approach will be quite time-consuming, educational, and probably more expensive, but may not be as rewarding in the long run. It’s your choice!
Just wanted to express my admiration that someone out there still wants to tackle a moderately complex project like this instead of going and buying one off Best Buy’s shelf. (I wrote and designed gear for the last viable era of the hobby electronics magazines, and was fortunate to hook up with the Australian hobbyist world that continued to be active for a decade after the US’s. All pretty much one with the wooly mammoth now.)
Yep, you can buy ten times the quality and features in a box for one-tenth the price of a kit or scratch-built. So what?
Depending on how deeply you want to get into the electronics, you might find these books useful:
The standard works on the subject, in my opinion.
You might be surprised at how vibrant it still is - but it has changed. Magazines are not where it is happening. Although Elector is still going strong. Internet forums with wholesale sharing of designs and expertise, along with cooperative designs, with group buys of PCBs. Plus a range of startups providing additional support.
It is still possible to significantly outdo commercial designs in price performance, but not competing with the lower end gear. Cheap amplifiers can be bought for less than the cost of the parts that a hobbyist would pay. But when you get into high quality gear, it becomes possible to compete reasonably well. Partly because the margins on commercial gear are so much higher in this segment, and partly because the volumes are also so much lower, so the hobbyist can do reasonably well. But a big part of the alure is that you can build something that you can’t buy. For me that means fully active, with digital end to end processing. But it is a bit obsessive. Had I simply moonlighted cutting code for cash, instead of devoting endless hours to this pursuit, I would probably have been able to simply buy the entire thing. But that would be working, this isn’t. It is fun.
Any clues as to any of your designs?