Hybrid Vacuum Tube Headphones Amplifier (Part 2) The Building Process

This post is continue from my previous post about the photo teaser of hybrid vacuum tube amplifier.

In this post I will share the building process in detail, including the building idea and schematic.

Building this hybrid vacuum tube amplifier require the skill not only on electronic, but skill to work with small parts, including cutting and sanding material. I should also make some improvisations to make all components fit into this small candy tin. More over, there is no PCB available for this amplifier. So I made with an universal board and all components are connected in point-to-point using each components leads.

At the end, this is just like the real vacuum tube amplifier building. I spent about more than 6 hours building this amplifier, with the result I review at then end of this post.

The basic idea of this vacuum tube amplifier is comes when I saw a photo from Tokyo Headphones Show 2011 reported in Headfonia websites.

It is the hybrid headphone amplifier in the small Altroids tin that fit a vacuum tube and only using 2 AA batteries as power source. This is very tempting for me to build one for sure. The vacuum tube on the photo is CV4003 or equivalent with 12AU7. It is required 12 volt heater voltage and of course it will need a voltage converter to increase from 3 volt batteries to 12 volt.

I shared the phot I mentioned in the photo bellow.

Inspecting the components, there is an inductor besides the vaccum tube socket. This could be the voltage inverter. If an alkaline battery can produce 600 - 800mAh, so using 2 battery is 3 volt by 0.8 ampere can produce up to 2.4 watt. It is quite small current I guess.

This photo is taken from Headfonia.com

So I begin searching the schematic and I found this gentlemen sharing the hybrid vacuum tube amplifier in his website. It using the same method as the hybrid vacuum tube amplifier I mentioned before but it is built for desktop amplifier, means it is require an external power supply from the wall adaptor.

Then, to make this possible, I print the schematic and make some changes to suit to the batteries powered hybrid vacuum tube amplifier project.

This is the original schematic, using 6DJ8 vacuum tube that required heater voltage 6.3 volt.

I use this schematic but with slightly changes:

1. I use volume control at the signal input using stereo potentiometer A100k ohm. The green ALPS potentiometer really suits in this project

2. The R4 output resistor is bypassed.

3. The heater supply not as per schematic, I will using the inverter voltage module to increase from 3 volt battery to 6.3 volt.

My analysis using 6N23P similar to ECC88 or 6DJ8 required less current on the heater compares with 12AU7. It is same dual triode and in my opinion it will sound better than 12AU7 in small voltage like in this project.

The op-amp will be use to increase the current at the output. This can be use any type of op-amp from the NE5532, 4556, 4558 or anything else as long it is unity gain stable.

I using the voltage converter module with the IC MT3608. This module capable up to 2 ampere current and it is very efficient up to 98%, means less heat or current loss when it doing the voltage conversion. I will using 2 module, one for the heater that required 6.3 volt and one for the amplifier that required 12 volt.

Since I will put this module inside the small tin, so I need more compact module. I sanding the edges of the module to make it even smaller.

Those module will be put next to the battery holder and the length should be about the same as the battery holder itself.

The next step is preparing the PCB using universal double layer board and I do some fitment inside the this candy tin.

A lot of works for this PCB to fit inside the tin, especially for the corner fitment that required more time to make it fit perfectly.

Once the PCB is done, then I put the headphones socket and the potentiometer into the PCB. This is to make an alignment for input output terminal inside the tin. Then I make some mark to make a small hole on it. Then little by little sanding the outer holes to fit the holes into the socket.

Because there will be a solder join on the bottom of the PCB, so I want to make sure that there is enough room bellow the PCB.

Also the holes height will not exceeding the lid when it closed. Otherwise, when all the socket installed, the lid cannot be closed properly. I take so much time when make this fitment to be correct.

Once all the holes are ready, I can now focusing to put all components into the PCB.

I make a matrix drawing for easy explanation. There are 11 holes in vertical lines and 22 holes in horizontal line. I mark them with alphabets and numbers. This matrix drawing is on the top components view.

As you can see the first components I put is the socket headphones out and socket for line input, then I start installing the output capacitor first on E4 & F4 and the other on E7 & F7. All electrolytic capacitors that I am using in this project are Rubycon ZLH 220uF 35v.

I mention the negative lead of the capacitor F4 is connected to socket lead on I4 & I5, and negative lead on F7 is connected to I8 & I9.

I using Dale RN55 metal film 1/4 watt for all resistors in this project. The bleeder resistor each 2k2 ohm connect I3 to I4 and J3 to I7, and the others resistors lead are joined together at I2 & J2 then connect as ground to socket K6.

The 8 pin op-amp IC socket can be install with pin number 1 is at F10.

The op-amp pin number 1 and 2 join together, F10 and F11, then connect to output capacitor at E7. 

Same with the other channel is IC pin number 6 join together with pin number 7, they are C11 and C12 then connect to other output capacitor at E4.

Finished with above steps, then I can put the battery socket on A3 for positive polarity and A4 for negative polarity. The negative polarity will join to the 3 pin power switch on A10 and A11. I leave the A12 on the switch to connect to the circuit ground later.

Then I return to the input socket. The ground point between K6 and K12 are join together. Then take a route to IC pin number 4 at F13. After that I connect the input resistor value 1 mega ohm on F14 and E14.

The other leads on the input resistor E15 and F15 are connect to the input capacitor. The capacitor are 100 nF Wima located at E16 to E18 and the other channel is located at F16 to F18.

I also put the voltage resistor each 47k ohm on C14 connect to IC pin number 5 at C13 and D14 to IC pin number 3 at F12. The other resistors lead are connect together, the C15 and D15.

From C15 and D15 is connect together with IC pin number 8 at C10 then it connect to the capacitor on C20. The other lead of the capacitor will be ground is C21 connect to heater tube capacitor on F21. The F21 connect to the ground at K12.

The power switch on A12 is now connect to circuit ground at C21.

This is the current progress.

Now continue to the input signal.

There are 2 pin at the potentiometer on the left is for ground at H16 and I16 connect together to H15 and I15. The center 2 pin of the potentiometer is the output signal connect to each input capacitor at E18 and F18.

Take a break on the main PCB.

I am now looking to the voltage converter.

The 2 module at the input voltage converter is connected together. Positive input on first module connect to the positive input on the second module, same as the negative input. Since all ground is common ground so only 1 ground required for this 2 module for both input and output.

Then I set the voltage output at 6.3 volt and 12 volt by simply turn the potentiometer on each module.

At this point you can check the output voltage using volt meter and make sure the color code of the cable are representative to the voltage output.

I choose the red for 12 volt and blue for 6.3 volt.

The other twist cables is the input voltage from battery, red for positive and black for negative.

Back to the main PCB, I am now connecting the 2 pins at right side of the potentiometer to the input socket. H18 to I13 and I18 to I11.

Since this connection will crossing each others, so I need 22 AWG solid wire cable for jumper.

I make some note on the schematic for the 9 pins socket.

I put the code on every socket tube pin from A1 to A9 on bottom side of the socket. I add cables on each lead, each about 2 cm.

The vacuum tube pin A3, A4, A8 and A9 are all connect to the ground.

The photo bellow is for illustration of the pins. I remove all the cables again and changed them with single wire cables to improve the stiffness of the socket when attach to main PCB.

The pin 5 for tube heater, I use the white cable just to make it different.

The rest of the pins, which are pins 1, 2, 6 and 7 using the red cable code.

See the illustration bellow where I put all vacuum tube's pin on the main PCB.

You need very careful to locate all the vacuum tube's pins to the PCB. Back to the matrix coding they are:

D16 - C14

D17 - D14

D18 - E16

D19 - F16

F20 - heater tube on tube socket pin 5

At this stages, the main board is fully completed.

Then back to the tin, I using the cloth tape to cover the bottom side of the tin. This to avoid any short circuit when main PCB attached inside.

I also put the cloth tape on the voltage converter module on each polarity for the same prevention of short circuit to the tin.

This is the cables from voltage converter module that will goes to the PCB.

Vin positive  - A2

Vin negative - A13

12 volt  - C19

6.3 volt - E20

Once all done, I make sure all connection is correct as per schematic. I double check all including the voltage to the heater tube and to the op-amp IC.

After all correct, then I can take my precious vacuum tube and put into the socket.

I put all module together inside the tin and I power on the switch.

Hmn.. the vacuum tube isn't glowing.

Yet, I connect to the cheap earphone for initial test, it doesn't make any sound.

I check again at the voltage converter for the vacuum tube heater, it read only 2 volt instead 6.3 volt. It is dropped so much because not enough current from the batteries to glowing the vacuum tube. I tried turn up the voltage converter and it just stop at 2.6 volt.

The other voltage converter module that goes to the op-amp also only read 8 volt instead of 12 volt.

So at this point I know that using batteries is not possible due to small current from it.

At this point, I hope you are not disappointed especially when this is just an experimental project.

I am really excited to going deeper in this hybrid vacuum tube headphones amplifier.

See you on the next post.

Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.