Hey guys. This is a preamplifier board but is not made with transistors. As you can see is made out
with vacuum tubes. I’ve designed this PCB and order it from PCBWAY
and also ordered all the needed components in order to mount it, the tubes, the sockets,
the resistors and capacitors and so on. In this video I will show you how a vacuum tube works and
that’s very interesting so stick till the end. Then I want to explain to you the difference
between a preamp and a power amplifier and why some people want to use vacuum tubes instead of
transistors. Finally, I will show you my circuit and how the pre amplifier works,
then we assemble it and test it out. Sounds interesting, so let’s get started.
What’s up my friends, welcome back. This is my pre amplifier PCB made with vacuum tubes.
And this here is a power audio amplifier based on a TDA IC. The basic difference between a preamp
and a power amplifier goes like this. A preamp amplifies a weaker signal such as the output of
an electric guitar or a microphone, to line level, while an amplifier amplifies a line level signal
so that it can be sent to powerful speakers. This here for example is an electret microphone.
It creates a very weak signal when exposed to sound.
If we directly connect that weak signal to an amplifier, this would amplify both the signal
and the noise and since the audio signal is so weak, we would get very bad results. First,
we need to elevate that weak signal to line level. For example, microphone level is around -50 dBV
while the line level is around 0dBV or around 1 volt, something a smartphone would give or any
normal audio jack that we could then amplify. I wanted my preamp to be based on Vacuum tubes
but that doesn’t mean we can’t have preamps based on transistors. For example, this here is
a microphone preamplifier made with transistors. Gets supplied at 5V and once you connect the
microphone, it will give out a line level signal. So why do people want a vacuum tube amplifier.
Well, that’s because for some people it seems these tubes make the music we create sound better,
smoother, warmer and cleaner and they give the audio signal some nice harmonics. And by the way,
we also have power amplifiers made with vacuum tubes but these tend to be a lot bigger. Actually,
I’m preparing such a project for a future video and I already have the tubes, the transformers and
some other components that I need and this is for only 20W. To compare, the amplifier before has 50W
for each channel so a total of 100W and is 5 times smaller than the tube-based amplifier of only 20W.
That’s because with microtechnology we can make everything fit inside of just a few mm
and all we care about is to keep the IC cool so they won’t burn out under high power.
So, let’s check my PCB and the schematic I’ve prepared for today, then the part list,
how a vacuum tube works for amplifying signals and finally we assemble my PCB and we test it.
If you want the same PCB, download my GERBER files from below, go to PCBWAY.com and click
the quote now button. Here insert the size and the amount of PCBs you want to order.
Then select a color, in my case I select purple because I think it looks amazing.
Click save to cart and, on the next page, click the button and upload the GERBER files
and then make the payment. In a few days, I receive my PCBs and they look very good. My PCB
has an AC input for supply and on this other side we have one audio Input and one audio output for
left and right channels. This is the schematic for the entire PCB and we can see these 3 inputs here.
The AC supply is then connected to an on and off switch that will go here on the PCB.
This potentiometer has two channels and also a switch integrated
so we can use it to control volume but also to turn the amplifier on and off.
On the schematic we have this potentiometer connections here, here and here.
Then, on the PCB we can see 4 diodes followed by some capacitors and 4 transistors. And yes,
yes, I know I’ve told you this amplifier is not based on transistors, and is not. We use these
BJTs transistors only for the voltage doubler part of the circuit. Let’s go step by step,
so the AC supply will be connected to these LEDs which will be below the tubes to make them glow.
You can use any color for these LEDs if you want. The AC supply is also connected to the
heating filaments of the vacuum tubes and in a moment, I will explain to you how that works.
These heating filaments are connected on pins 3 and 4 of the tubes.
This R1 resistor will limit the current going to the heaters.
Then using these diodes, capacitors and BJT transistors we make a voltage doubler
so getting this point as a reference we can get out minus 28 and plus 28 volts.
This voltage is connected to the vacuum tubes.
The audio signal input is connected to the potentiometer for volume control and then is
connected to the tube for amplification. And that’s it for the circuit of this PCB. I’ve
tried to make it as compact as possible but using tubes, you can’t really make it smaller than this.
Before we assemble it, we must go over how a vacuum tube works and could amplify audio signals.
You can get all sorts of vacuum tubes but the ones I’m using today are 6J1 and this is a
pentode with 7 pins. Let’s start with how a triode works and then we can understand the pentode.
A triode is called triode because inside the vacuum tube we have 3 elements. We don’t count
the heating filament as an element. We have the cathode, a control grid and then we have a plate.
When we heat the filament, that will heat the cathode which is usually connected to ground.
When this metal is heating up it will start bouncing electrons around it. At the same time,
we connect a positive voltage to the plate. Usually this is a high voltage of more than 50V.
Since the electrons on the cathode have negative polarity, these are attracted by the positive
polarity of the plate here so they would start flowing in that direction creating a current.
But that’s why we have the control grid. If we apply a negative signal to this grid,
this negative polarity will repel the electrons in the other direction so the current flow would
get smaller or near to 0. But when we apply positive, electrons could flow towards the metal
plate above. This is actually an amplifier because with a small signal applied at the control grid,
we can have a bigger current between the cathode and the plate above.
Now we add another screen so this time we have a tetrode.
We apply a positive voltage to this, and because this grid is closer to the cathode, those negative
electrons will be first attracted by this new grid so they are accelerated even more.
But when they get here, they will see the plate above that has a higher voltage,
and in that way, they will continue to flow towards that metal plate, but with more power.
Like this this tube will have a little bit more gain. But this time, the electrons are gaining
so much kinetic energy that sometimes they hit the top plate so hard that some electrons are
bouncing back and go to the screen plate. So, we are losing some efficiency here.
That’s why we then have the pentode with another screen grid that’s called suppressor grid.
This is usually connected to ground or a lower voltage.
So, like this the electrons can’t bounce back anymore, because they don’t have enough energy
and the suppressor grid will repel them back to the positive plate.
Now we get even more gain. So that’s pretty much how we amplify signals with vacuum tubes.
Now let’s assemble my PCB and if you want to make the same amplifier,
you have my design and the components list below in the description.
I start by adding the input connectors. First, I solder the AC input jack. Remember, this PCB
needs 12AC supply. It can’t work with DC. I also solder the audio input and output connectors.
Before we add the vacuum tube sockets, we need to solder some LEDs below. These will
glow through the tubes and give them a cooler look. Now we can solder the vacuum tube sockets.
We use these so we don’t have to solder the tubes directly to the PCB.
Then I also add the volume potentiometer and on and off switch.
I add the power resistor for the heating filaments of the tubes
and this must be a power resistor of at least 2 watts. I add the rest of the small resistors.
Finally, I add the diodes and the BJT transistors and make sure you don’t solder them backwards.
Last components to be soldered are the capacitors
and in my case, they are a little bit too big but they still work.
Everything is soldered so let’s test if it works. I add the tubes to their sockets.
To supply the PCB, I will use a 12V transformer that I have from some halogen lamps and this
has no rectifier at the output so we have AC voltage of 12V and 4 amps. I connect
this transformer at the AC input and we can see the blue LEDs below the tubes are now glowing.
Since the preamp doesn’t have enough power, I add my amplifier between the preamp and the speaker.
Like this we can have decent power but the soft harmonics and effects of the tubes.
I get the music from my smartphone. First thing I’ve noticed is that once you turn it on, it will
take a few seconds for the sound to start because the filaments inside the tube must heat up. Take
a look, I turn it on. And we have to wait and the volume will slowly go up. Let’s hear now the
same song with the preamplifier and then without the preamplifier and let’s see the difference.
I don’t know about you but I can really hear those harmonics as an echo after each sound peak.
The pre amp works. So guys, that’s how I’ve made my own tube based pre amplifier.
What do you think? Is quite interesting, right and it should give your music more harmonics
and a smoother sound. You have all that you need to make this project below in the description.
If you like this video and you have learned something new,
consider giving me a like or comment below. Thanks again and see you later guys.