I want to share with you the steps on building a little and practical audio amplifier, based on Soviet vacuum tubes. I decided to build this little device in order to use it as an tool in future experimentation on vacuum tube electronics. I have always planned to build a radio using vacuum tubes, but in my modest opinion it would be very useful to have a little and compact amplifier to amplify the signals if I'm able to tune and detect them.
First of all, as this is a log, date of beginning:
The circuit in question is the one which attracted my attention to vacuum tubes about 7 years ago. In that moment I tried to build it but things went improving, knowledge too, and I finally ended building a bigger stereo amplifier which is currently in use at home. I've been a bit distanced from vacuum electronics in the last couple of years due to time availability, but I think that now I can resume my experimentation (and fun) playing with glass and electrons, so I decided to go for it.
The circuit that I want to build I found it on a Russian website (I don't remember which one right now, sorry). It is shown on Picture #1.
Anyway, I will make some variations of this circuit as I want to use a vaccum rectifier. All of the tube-based circuits that I have built, have silicon rectifiers, but I'm interested to use a vacuum full-wave rectifier to experiment with the behaviour of this kind of power supply. Problem is that I also want to use a transformer that I winded for another project (to save time and labour) which has a single HT winding. Then I decided to use the circuit on Picture #2, which appears on The Valve Wizard page about rectifiers. This way I can have the behaviour of a full wave vacuum tube rectifier while I have a single winding transformer. I know, I know.....it will not be an "all-vacuum" set but after all, this is for experimentation.......and fun.
The other thing to have into account is: SPACE. Not outer space, but space in my workshop, my bench and my shelves. My shelves are at full capacity of "probably-I-will-never-use" stuff, as it should be on any DIYER workshop, so space is something very important to have in mind. So, I decided to build the chassis in tower style, in order to save space. This will be a very compact (if I can say so) audio amplifier with a built-in speaker. Oh! I forgot to tell you that it will have a built-in speaker. Well, for the same considerations of space and usefulness, I will not use a separate speaker but I will insert it into the chassis too. As the chassis will be build on steel plate, which is not good for a speaker cabinet, I will try to build a small wooden cabinet for the speaker inside the steel chassis. What I have done in the chassis can be seen on Picture #3.
Will you come with me on this new adventure?
On weekend I build several pieces for the chassis. My intention is ro shield the entire circuit using steel pieces to avoid interference from inner or outer sources. Althought I have used bolts and nuts to have all of this parts together, I will use rivets for finishing. On pictures 4, 5, 6 and 7 are shown details of the construction.
I have prepared the PCB with the circuit for the amplifier. (Picture 8). I want to apply a layer of paint to the chassis before begin to assembling.
The circuit is already mounted and working. I have not tested it with an audio signal, just a 1kHz signal from the oscillator of the previous project supplying a dummy load (10 ohms). The sinewave was very clean, at 3W (+/-) which is more than enough for my purposes.
This weekend I built the rest of the chasis, but I wasn't able to build the built-in speaker cabinet as the speaker that I had at home is too big and there is no room for it inside the chasis, so I have to find a flatter speaker to fit it inside the cabinet.
I found a smaller and flatter elliptical speaker. I also fixed the dront panel to the chassis and on weekend I played music with it. Now I just have to mount the mains switch, the "ON" indicator and apply a layer of paint to the front panel, and this project will be finished. here some pictures of how ir looks. The speaker cover is from an old Soviet TV
The little amplifier is already fully functional. I used it yesterday the whole day. I'm just waiting to have time to give it a layer of paint to finish this project.