Audio Preamp Built From VFD Indicator Tubes!

Hello fellow makers!

A few years back a Japanese company named Korg launched the Nutube 6p1, it is meant to replace a dual triode tube in audio applications but unlike a normal tube that requires dangerously high voltages to perform optimally the Nutube is based on VFD technology hence it requires under 1 volt plus low current for the heaters and it's working voltage is around 30 volts instead of the lethal 300+ volts.

I was very excited to try them out....

That's when I found out the retail price, $50 each if you live in the USA. Unfortunately, if I wanted to try one there would be an outrageous markup.

Luckily I came across a video on YouTube where a guy did some experiments using a 7-segment VFD as an audio preamp, and so I got started on this project.

In this Instructable I will be showing you how I reused some DM160 VFD indicator tubes designed initially for computer application and turned them into a great sounding little low voltage audio preamp/buffer.

Join me on my journey to learn how to make your own.

To replicate this Instructable you will need the following:

• DM160 tube Can be replaced with CV5412, 6977 or Soviet IV-15 tubes

Amazon - DM160

• SMD Soldering station Highly recomended

Amazon - 8786D 2 in 1 Hot Air Rework and Soldering Iron Station

• An old USB cable that can be reused for a power cable
• Access to a 3D printer for the enclosure

Amazon - Ender 3

OR

• Hammond 1590U enclosure or similar would also work

Amazon - 1590U Aluminium

• 1206 SMD Resistor DIY Kit

Amazon - 1206 Kit

• RCA Female Jack 8 Pin PCB Mounted

Amazon - RCA PCB mount 4 plug

• Electrolytic Capacitor Kit

Amazon - Capacitor kit

• Zener Diode Kit

Amazon - Zener kit

• 1N4148 Switching Diode

Amazon - 1N4148

• 2N3904 NPN Transistor

Amazon - 2N3904

• 2N7002 N-Channel Mosfet

Amazon - 2N7002

• 50K Dual Potentiometer

Amazon - 50K potentiometer

• Trimpot Kit

Amazon - Single turn potentiometer kit

• DPDT PCB Mini Relay 5V

Amazon - 5V DPDT relay

• H0515LT or similar isolated DC to DC converter, I also tested the boost converter listed below and it works perfectly.

Amazon - MT3608 Boost converter

*As an Amazon Associate I receive a small percentage from sales made through provided links at no cost to you, this helps fund future projects.

The circuit for the preamp is a modified version of the 6P1 Nutube Buffer from the legend P Millet.

At the heart of my version is two DM160 VFD indicator tubes that replaces the Korg Nutube but still features the same 2N3904 input and 2N7002 output stage as they are easy to come by and sound great.

Because I wanted to power the amp from a normal USB port I needed to find a low noise solution to step up the USB's 5v, In this iteration I went with an isolated 2 watt DC to DC converter that takes the 5 volts from the USB and gives us a extremely low noise 15 volts.

I would recommend opting for the 5V to 24V DC to DC converter, unfortunately the 15V version was the only one in stock when I built this amp.

I also added a delayed relay on the signal output to eliminate any turn on "pop", this is completely optional and the circuit works perfectly fine without it.

Start by cutting your PCB to size, this can be done by scoring it with a sharp knife and then breaking it, cutting it with a hacksaw or I always use a Dremel with an abrasive cut off wheel.

Now take some fine steel wool and give the copper a good scrub, be thorough making sure you remove all of the oxidation.

After the steel wool you should be left with a shiny copper board, now we need to take some acetone or alcohol and with a clean cloth wipe the entire board clean making sure not to touch any part with your bare hands.

Next we need to cut a piece of the photoresist film that is just slightly larger than your PCB.

- Always work with the photoresist film in a dark room or you will expose the film -

Now remove the protective plastic film from the back of the photoresist film and lay it on your PCB, we will now need to heat up the board to make the film adhere.

For this I use the heated bed of my 3d printer, simply heat it up to 80-100degrees and leave the board on it for a minute.

When the board is heated take a soft cloth and rub the film onto the board, making sure to remove any air bubbles.

Now it's time to expose our PCB design onto the board.

With my 500mw laser I "engrave" the PCB layout picture at 5% power and with a speed of 500mm/m.

You will need to experiment a bit with your power and feed rate to get the desired results as each laser module will be slightly different.

The key to getting a perfect exposure is low power with a well focused beam.

- Remember to do all of this in a dark as possible environment -

To remove the unexposed film we need a development solution:

Add 1 teaspoon of washing soda to about 200ml of water, you want about a 3% solution. Adjust this according to your specific brand of photoresist film. It's best to first try with a spare piece of film, you want it to take about 3 minutes to dissolve. If your mixture is too strong you'll also disolve the developed film.

Now remove the top protective film and submerge your PCB with the film into the solution.

You will see the undeveloped film starts to get opaque and dissolve into the water.

Agitate your solution until all the undeveloped film has dissolved and you are just left with the dark blue developed resist.

Rinse with clean water.

- WEAR ALL NECESSARY SAFETY EQUIPMENT BEFORE CONTINUING! -

In a well ventilated room or outside mix two parts hydrogen peroxide and one part of hydrochloric acid together in a separate container. Ferric Chloride can also be used instead of this solution if you have some available. (this is your etching solution so handle with care)

Tip: The hydrogen peroxide + hydrochloric acid is a safer alternative to Ferric Chloride and is great to use when making your own PCB's

Carefully submerge the PCB into the etching solution, the copper that needs to be etched has to be completely submerged and facing upwards.

Agitate the mixture regularly.

Leave it in the solution until it has dissolved all of the exposed copper leaving behind only the copper under the resist.

When done rinse thoroughly in clean water.

Now it's time to start soldering the components into place.

Part List:

Tubes:

• DM160 or CV5412, 6977, Soviet IV-15 X2

Resistors:

• 475K X8
• 10K X2
• 33k2 X2
• 1K X2
• 82E X2
• 40E2 X1

Capacitors:

• 470uf >35V X2
• 10uf >35V X10

Potentiometers:

• 1K Trimpot X2
• 50K Dual volume control

Diodes:

• 1N4148 X2
• ~3.6V Zener X1
• 5mm LED X1

Transistors:

• 2N3904 X2
• 2N7002 X3

I recommend starting with the SMD components as it will be easier to place and solder them when there are no through hole components on the other side of the board.

A little tip I have if you are intimadated by SMD components is firstly a fine point soldering tip is a absolute must, then I smear all the places where the components need to go with some of the soldering flux this will hold the components in place making it easier to get one solder point secured.

Next I place a small blob of solder on one of the connection points on the board where the component will go, this way you can solder in the one point of the component to secure it onto the board without having to struggle with the solder wire, then when that connection has hardened you can just solder the rest of the connections using the solder wire as you now have a free hand.

Solder all of the components in place using the attached schematic as your guide.

Time to assemble the tube board.

There are two reasons I designed the PCB holding the tubes separate from the rest of the amplifier. Firstly I wanted to display the tube on the front of the enclosure that is on a different plane than the main PCB.

But most importantly because of how pencil tubes are built all of them suffer from microphonics, this means that if you tap against the tube whilst it is operational it produces audible ringing. To eliminate this we need to isolate it from the enclosure with a dampening material.

Making sure of the pinout of the tubes solder them onto the board as pictured along with an power indicator LED and wires to connect it to the main PCB. I like to always reinforce wire connections with some hot melt glue to prevent them breaking off.

Finally I added a drop of B-6000 adhesive behind each of the tubes, B-6000 dries crystal clear and remains rubbery so it holds the tubes in place and also provides extra dampening.

The enclosure was designed entirely in Fusion 360 and as with my other designs it can be printed with zero supports to eliminate waste.

I decided to print all of the pieces of the enclosure with PETG to eliminate possible deformation as the preamp might be subjected to sunlight.

Unlike regular tube based amps that requires good ventilation to get rid of the excessive heat generated, the VFD tubes generate negligible heat and the interior of the enclosure stays around ambient temperature so it would be possible to print with PLA as well if it will not be placed in direct sunlight.

My print settings are as follows:

Material: PETG

Speed: 40mm/s

Temp: 240 degC Nozzle and 95 degC Bed

Nozzle: 0.5mm (the main body was printed with a 0.8mm nozzle to speed up the print time)

In order for the preamp to fit with the rest of my audio setup and give it a more premium look I need to paint it.

First we need to get rid of the printing defects and layer lines, I start by using a course 80 grit sanding paper to sand away the most obvious imperfections and to give a rough surface to which some Bondo filler can adhere to.

I smeared a thin layer over the entire enclosure, lid and the knob and when it has cured I sanded over with 80 grit moving up to 220 grit to get a smooth flat finish that is mostly free of surface imperfections.

Next I sprayed it with a few layers of high fill primer and wet sanded it with some soapy water and 400 grit sandpaper once the primer has completely dried.

I finished it by applying some satin black spray paint for a clean classic look.

As these little tubes were originally indicator VFD tubes the emit a beautiful neon blue while operating and we definitely want to put that on display!

In order to protect our tubes from dust, dirt and possibly little sticky fingers I decided to cover them with a piece of glass.

Now you can make life easier and just use a piece of clear plastic like some poly-carbonate sheet but unfortunately after cleaning a few times you will be left with some noticeable scratches and haziness.

The glass I'm using for this was originally in a cheap old photo frame and is only 1.5mm thick. Using a diamond tipped glass cutter I scored the glass with a 5X5cm square and broke it loose.

Then using the same B-6000 glue I stuck it into place in the enclosure.

To prevent scratching and give a smooth operation we need a spacer in between the volume knob and enclosure.

For this I threw a piece of black felt onto my laser engraver (a cheap 500mw blue diode laser module that can be fitted to any CNC or even a 3D printer and is super useful) and quickly cut out the spacer.

It is 35mm in diameter with a 12mm hole.

In order to eliminate any microphonics we do not want the tube PCB solidly attached to the enclosure.

An easy solution for mounting was to use black weatherproofing foam with the added bonus of giving a nice border around the tubes.

Make sure the weatherproofing is thick enough so that the tubes won't be pressing against the glass when stuck down.

Unfortunately the weatherproofing only has adhesive on one side so I had to add some thin double sided (usually used for things like repairing phone screens) on the side without adhesive.

Then we can start the final assembly!

Time to assemble.

I started by sticking the tube PCB down behind our glass window, make sure that the board isn't touching the enclosure somewhere directly.

Next we can take the old USB cable snip off the side we won't be using (keeping the USB-A side) and reveal the wires.

Feed the cable through the hole in the enclosure and solder the wires to the board, red goes to the positive voltage input of the step up module, black connects to ground and finally I soldered the white and green wires together as some devices require this to enable a higher current output.

Slide the board into place making sure all of the holes align, insert and tighten the screw of the RCA jacks on the back of the enclosure.

Now all that's left is to push in the lid, make sure the holes align and then screw it into place.

Time to test it out!

• Use the two 1K Trimpots to adjust the bias, with the 15V supply I have I set them to 0.2V.

• I found out that some of my USB power supplies don't switch the relay correctly, I believe this is due to the current spike when the relay engages. Total consumption of the amp with the relay energised is under 300ma.

I will be posting any updates on the design here:

I hope you guys find this Instructable useful and if you have any questions please feel free to leave me a message or comment bellow.

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Happy making!