Those breadboards you mentioned are very handy indeed and the prototype could become also the final unit.
Yes, those surface mount sockets are very handy. Most breadboard setups are spread out over a fairly large area but that just allows more room for experimentation. Clip leads are typically used to connect things. But the same sockets and screw connectors can be positioned closer together and connected with short wires. The whole thing can be built in a simple box.
I see your point but i have really to study first to feel more comfortable with the all issue. I need a basic text on tubes with calculation examples. For sure given that i am only interested in preamp very basic circuits is what i am interested in. Like a tubes for dummies book.
There's lots of stuff online and I'm sure you can find some books too. You should be aware, however, that much of what you'll find is about guitar amps. Some of the most basic info applies to both guitar and Hi-Fi but guitar amps and preamps are designed to produce distortion so a lot of what you'll find is of limited use with Hi-Fi.
manufacturers tend to use tubes readily available even if other obsolete tubes could be much better fit for a same purpose (what a pity). And often they were tubes not used in audio equipment. Still the same equations apply to triodes of different electrical characteristics i guess.
What scares me most are the plate voltages involved. Very very high and so dangerous.
Yes, there are thousands of tube types out there and very few were designed for audio but they can be often still be used in audio applications. I like to use tubes that are somewhat obscure as long as they are relatively available and cheap.
It's good to respect the dangerous voltages but, as I've said,
nobody works on a circuit when it's powered up. When you're building something it's just parts and wire so there is absolutely no danger involved.
After I put something together, I then double and triple check the connections against the schematic.
When it's powered up the only thing that touches the circuit are the probes of the meter. It's no more dangerous than plugging a lamp into the wall socket. After it's been powered down, caps can remain charged so they must be discharged through a resistor. I then clip wires between the caps and ground to make sure there is no voltage before I continue working on the circuit.
May i ask why the battery ? are there benefits from using it instead of a resistor ? can it be used in any schematic ? i am asking for another preamp i have somewhere this one here
https://www.passdiy.com/projects/images/content/brideofzen_1.png
i mean .... can R108=100 ohm be replaced with a 4V battery ? with which benefits ?
If you look at the schematic of an anode follower preamp you'll see that the tube's cathode is typically connected to ground with a resistor, the cathode resistor. The ground side has 0 volts and the cathode side has some amount of positive voltage.
Bias voltage is measured from the tube's grid to the cathode. The grid is 0v so compared to the, let's say for example, +9v at the cathode the grid is at -9v. So you can accomplish the same bias point by grounding the cathode directly (no resistor) and connecting a negative voltage from a battery to the grid.
The grid, in most cases, draws no current, so the battery does not run down. Batteries have a "shelf life". For a lithium it's roughly 10 years so it won't need to be replaced for a while. Other types may only last a few years.
The only advantage is that, in many cases, the cathode resistor is bypassed with an electrolytic cap. These caps are used because a large capacitance is needed (you'll have to research this on your own) but sonically they aren't that great.
If no cathode bypass cap is used, there is no advantage to using the battery. But if there is a bypass cap, the battery allows you to eliminate it. But batteries only come in certain voltages, so they aren't always practical. If you need a different bias voltage, a battery won't work.
Depending on the circuit, however, you may need to add a cap to block the DC voltage from the battery from getting to whatever you have plugged into the input. But the blocking cap can be a small value, which means you can use a film cap, which is technically much better.
The circuit you've posted is not a tube circuit so I have no idea how it could be modified.
Maybe i am underestimate the building side of the task but i really need to know some basics on tube circuits first
For instance just to understand what to look at in a tube datasheet like you told me ... to understand if a certain tube is the right one for the task.
Again, you're not talking about the "building side of the task" here, you're talking about the "design process". Learn to build someone else's designs first. You will, over time, learn more about how to design things on your own. You don't need to know how to design an car engine in order to change the oil or do minor repairs on your car.
I am attaching as a try a datasheet of a tube that could be also suitable, imho, for a line stage. Am i right ?
At least the version 2 ... mu=5.4; plate resistance=750 ohm
it is a double triode so just one tube for both channel in an anode follower configuration should provide the needed low gain and probably also a decently low Zout.
There are several dissimilar triodes like the 6GF7. I briefly tried one, the 6EM7, on my breadboard. They are typically used to build an amp with the high mu section as input and the low mu as output. One tube per channel. A lot of people have built these and there are several schematics out there. Not much power but they reportedly sound quite nice.
Since the two sections are not the same, you would need one tube per channel for a preamp, just as you need one per channel for an amp. But, yes, you can just ignore the high mu section and use the low mu one.
The only downside to the 6GF7 is that the base is not very common (a large 9 pin like the 7868 output tube). The 6EM7 uses a common octal socket, as does the 6EA7. There are others that use 9 pin miniature bases. (6CY7, 6DE7, 6DR7, 6EW7, 6FD7). I was going to try one of them but I got tired of breadboarding and, since I really liked the 1626, I just built with it.
I haven't read through that thread but it looks interesting. I see that you revived it (it's been dormant for 5 years) so maybe some additional comments will be made.
The tube board here, where the thread above is located, is actually the best place for you to post about your interest in tube preamps / buffers. This board is really supposed to be about discussing new, commercially sold, products, not DIY. And the "DIY" board here is most all about solid state. And much of it is not even truly DIY. The DIY Audio forum is very much about DIY but, as you may have noticed, a lot of the participants have such a high level of technical expertise that it's hard to understand at times. Some of them are helpful, others are only interested in arguing with each other about highly technical issues.
and IMO needs recapping & better tubes
i will read it with attention