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.