Introduction: Custom Tube Guitar Amp

Have you ever wanted to have the warm tube sound of a guitar, but could not afford a good tube amp? If so, then this instructable is for you. A while back, a friend wanted me to build and sell him a guitar amp, so I found a few tubes and a cool chassis(CD Drive) and started building. The video below is a demonstration of the tube amp, it is played by my friend, Daniel. This instructable will show you how to build this awesome tube amp. Lets get started!

Step 1: Tools

For this project, you will need a few different tools. You will need:

  • A soldering Iron
  • A Drill
  • A hot glue gun
  • Metal and wood drill bits of varying sizes
  • Large 1/2 inch metal drill bit.

Step 2: Materials:

For this project, you will need quite a few materials:

  • Power transformer that can put out 277-300 volts
  • 6 volt filament transformer
  • Switch
  • 6V6 Beam Power Tetrode tube
  • 12AX7 Tube
  • Wood
  • CD Drive
  • 2x 100k Potentiometers
  • 1/4 inch audio jack.
  • 3x .02uF Capacitor
  • .002uF Capacitor
  • 120uF Electrolytic Capacitor
  • 10uF Electrolytic Capacitor
  • Resistors: 10k, 100k, 32k, 1M
  • Bridge Rectifier
  • Inductive Choke
  • Output Transformer 900:4

Step 3: Preparing the Chassis

When I started this project, I needed to find a metal chassis to house the electronics. After looking around, i came across an old CD drive from a computer. It makes a great chassis, and puts a modern spin on the age old idea of a tube amp. To start, remove the bottom of the drive and empty out all of the plastic components and electronics. After that, press inside the hole in the top to remove the piece of metal held in by the sticker. This should make a large hole in the top of the case, perfect for a power tube. Next, use the 1/2 inch drill bit to make holes for the preamp tubes. Finally, drill holes in the front of the chassis to mount the power switch, potentiometers, and audio jacks. These can then be screwed into place.

Step 4: Manufacturing Tube Sockets

The tube sockets are what electrically and physically connect the tube to the amp. You can buy ready-made tube sockets online, but I decided to manufacture my own out of wood. To do this, I used a pencil to coat the tube pins with graphite, then pressed them on a piece of particle board to make drill marks. I then drill holes on the marks and hot glue wires in place so one bare end is in the hole. The tube sockets are then cut down to the smallest size possible to save space in the chassis. One tube, the 6au6, serves as a pilot light, so it does not need wires. There is a hole in the middle for the LED. The sockets are now done.

Step 5: The Power Supply

To build the power supply for this amp, please refer to the above schematic. The power supply for this amp uses a miniature auto-transformer, so it is a hot chassis, which makes it more dangerous than a conventional amp. To make it safer, either use an isolation transformer or use a traditional power transformer. Also, make sure to use an inductive choke and smoothing transformer to reduce hum. This power supply should deliver a steady 300-350 volt B+ voltage and a 6 volt filament voltage.

Step 6: The Wiring

To wire the amplifier, reference the above schematic. Make sure to use leads that are very short to minimize hum. The pinouts of the tubes are also above. For this step, just be creative and solder the leads and components together in the best way you see fit. Also, make sure that wires that should not touch do not touch.

Step 7: Testing

After your amp is done, its time to test! Hook up the amp to an isolated variac transformer and steadily raise the voltage, just to make sure nothing shorts or starts smoking. If it works, then plug in your guitar, iPod, or banjo, and start playing your music really loud. This instructable does not show how to make a cabinet, but I'll save that instructable for a later day. Make sure to vote for me in the Amp and Circuits contests. Good luck building!

Disclaimer: This project deals with potentially lethal high voltage, build at your own risk.

Edit: I have been getting many complaints in the comments section regarding the safety of this amp section. I fully agree, this amp does pose a safety hazard to people without an adequate knowledge of electronics. These complaints also focus on poor design of the power supply section. The power supply does not contain a power transformer because I did not have one readily on hand. Same with the tube sockets. I built this out of the supplies that I had available. In addition, this amp will eventually be placed in a cabinet. Please stop with all the negative comments, I am fully aware of all the issues this amp contains.

Comments

author

WOW!!!... IT BRINGS ME MEMORIES OF THE SEVENTIES WHEN I EXPERIENCED WITH CATHODIC TUBES AND SEMICONDUCTORS TO CONFIGURE THE PINS.... WONDERFUL JOB MY FRIEND!!!! EXCELLENT!!!!

author
rykonen (author)2017-05-04

This instructable should be changed to reflect the less lethal version suggested by rbd56home (among other users here who all agree this is not a safe design for a tube amp!!). Otherwise it is a death trap and should be taken off instructables. Playing guitar puts the player in contact with strings and this amp can potentially put high voltage/current through the strings/player. There are known instances of amps just like this one harming and killing people.

author
tanner_tech (author)rykonen2017-05-04

This version of the time amp is unsafe, I agree. I have since built a safer version with a better person supply. the only modification to this time amp needs to be replacing the bridge rectifier with a single diode. With this modification, as long as you use a polarized cord where the neutral is grounded to the case, the tube amp should be safe.

author
fstedie (author)2016-11-04

Why post this when you know it is dangerous? Having a disclaimer is no excuse, I can just see some 13-yr old trying to build this thing to save money and killing himself in the process...

author
CurtisC52 (author)fstedie2017-03-15

A little natural selection goes a long way.

georgecarlin.jpg
author
rdb56home (author)2016-10-29

I have been building valve amplifiers for many years and studied the circuit diagram in detail. The critical part is the isolation between the mains input and the rest of the circuit. A fully isolated transformer is absolutely essential as any hazardous voltages can be contained within the circuitry without coming into contact with the input and chassis. The bridge rectifier design that has been used here is grounded to the chassis which means that the chassis is floating with a DC potential far greater than the mains supply. If the chassis is connected to the ground for the input, the input will be negative by at least 300v with respect to the mains input. DC voltages at this level are lethal. To make this design safer, the bridge rectifier can be removed and replaced with a single diode so that the neutral feed from the mains is at the same potential as the chassis. The mains plug must be polarised to ensure this is never connected the wrong way round. The simplest way around this is to replace the mains transformer as suggested in other comments to be absolutely safe. To build this design will be lethal to anyone using it especially if other metal devices come into contact with anything connected to it. Please be safe.

author
GáborL (author)2016-10-28

PLEASE don't use auto-transformer in a guitar amp!

They are OK in old radios where the whole chassis is isolated by the wooden housing and the pot buttons are not conductive but with guitar amps you are connected to the chassis-ground through the strings. If anything goes wrong you are dead. And put a fuse in it at least. Just for example: Several guitarists died because of failing vocal amps that shocked the guitarist through the microphone and the current was going through the man and the guitar amp to the ground. They could have survived if the guitar amp had proper short-circuit protection.

author
Aerowise (author)2016-10-27

Dream differenT

author
UnclTodd (author)2016-10-27

HA! Flashing back to my high school electronics class, fall, 1965... built a Fender Deluxe guitar amp (stolen schematic) from a "raw" chassis as my project, part of final exam was sketching an AM 5-tube superhet schematic with appropriate tubes and approx. component values. I passed, end of my "electronics" education since "semiconductors" came on stage and made me obsolete at age 14!!! LOL The class was never offered again, one heck of a high school class though. Transistors... ugly little suckers. :-(

author
aw1929 (author)2016-10-25

I know the comments many of you have made are very well intentioned and, yes, as a veteran of electronic design (I am 68 and started out when I was about twelve) and vacuum tubes, there ARE a number of dangerous aspects to this design and its execution.

I don't know how long tanner_tech has been fooling with tube electronics, so I'm hesitant to be really critical. On the other hand if this is a device he let his friend Daniel take home in this form, THAT is giving Daniel the means to hurt or possibly kill himself with very little trouble. I thinkg that aspect should be reconsidered.

As for the design, there are some errors in it, though it works to some degree, I'm sure. The really troubling part is in the construction--if it's meant to be a "breadboard" and used only for testing by the builder, while unsafe, it's a viable (if labor intensive) prototyping method. It should never be used as a permanent device!!

I think most of the hazards have been pointed out, so I won't go there. Just remember, things like tube sockets, power transformers and fuses were invented to make the process both easier and safer so, since we're not in some sort of post-appocalyptic world please care both for yourself and your friends, and make use of that stuff. One other point is that the ENTIRE electrical porting needs to be placed inside an enclosure so that it's not possible to stick a finger into the electrical parts. And, yes, if the tube envelope is broken, one could potentially receive and electrical shock, however IF the envelope IS broken, the tube will cease to function and, unless the person fooling with it has no sight, I'd say that one is pretty low on the list.

Anyway, have fun and learn--that's how technology has advanced through the ages and often because the person building the device didn't know any better!!

author
HunterG5 (author)2016-10-25

I applaud the ingenuity, especially with the wood tube sockets. I'm all for DIY tube amp building, but this is a house fire or electrocution waiting to happen and should be taken down immediately. There's a reason they quit building hot-chassis products decades ago -- they were a shock hazard. No fuse in the power supply either.

author
uncle frogy (author)2016-10-25

an OK improvised junkyard amp suitable for the lab or prototype testing but not "road worthy".

I have a fondness for this kind of "post apocalyptic design"

uncle frogy

author
DIY Hacks and How Tos (author)2016-10-23

Cool amp. I love the look of the exposed tubes.

author

Yes you like the look but it is totally unsafe. Inside that thin layer of glass is a lethal voltage that can kill you. Every reputable guitar amp makers on Earth places some kind of protection between you and those tubes. Yes I know "I'll be carful" But what about your 5 year old kid and the dog. Exposed tubes are fin for a lab bench experiment but not a working amp you move around and use.

Then there is that serious ill-design power supply that is not isolated from AC mains.

Fender and marshal placed their chassis in a wooden box so the box offers some degree of safety.

author

The original equipment has their vacuum tubes exposed to release the heat. Only certain equipment needed metal shields over the vacuum tubes to isolate the internal tube elements from stray magnetic fields or radio signals.

author
TheKitMurkit. (author)2016-10-23

Cool! Would be nice if the schematic was explained more detailed for less experienced makers.

author
RaymondR6 (author)TheKitMurkit.2016-10-25

I aghree. The hand drawn schematic is very hard to read. If you can't use a professional drawing program, ask someone who has excellent drawing skills to do it for you. And I ask if you wired the twin heater filaments of the 12AX7 in parallel with the heater filament of the 6V6.

author

Not only is it hard to read, it is just plain WRONG and would never be allowed by any safety standard. If you want one that is drawn right look at the Fender web site and download a user/repair manual for a Fender Champ. They use reasonably designed power supplies and professionally down schematics and have the been making them correctly for 50+ years

author
bonagva (author)TheKitMurkit.2016-10-25

From left to right and top-down:

The power transformer with the low voltage (6.3V) winding for the tubes' heaters
Just below, the high voltage (300-350V) winding which is also the mains winding.
As mentioned by the author, this is an auto-transformer where a single winding is used for input main and output high voltage and, as also mentioned by the author, especially seen the metallic chassis, I strongly suggest you go for an isolated transformer (main winding is separated from the secondary winding.
For safety reasons, you should also connect the metallic chassis to the earth of your mains. Or use an isolated chassis (plastic, wood, acrylic, ...) !

On the bottom, you have a diodes rectifier bridge that converts the AC into DC, this bridge typology is very used as it converts the negative part of the sine wave into a positive one which eases the further filtering.
Then following the bridge you have a LC filtering cell (L=inductance, C=capacitor) to smooth the current and reduce its noise (ripple). The following 12K resistor adds an additional filtering to the current going to the input tube (right of the schematic).

If you go up after the choke (inductance) of the supply, you reach the output transformer and connected to the secondary (few windings) the loudspeaker.
A point to note here: the output transformer must have a gap in the magnetic circuit to avoid saturation by the DC current. This type of transformer is also known for use in single-ended circuits (i.e. only one output tube instead of the two in a push-pull).
Connected to the output transformer, the power tube: B+ is connected to the anode (aka plate) through the transformer that acts as plate load.
Between the cathode of the power tube and ground, you have a RC cell aimed to generate a voltage difference between the cathode and ground making the cathode set to a slight positive voltage and thus the grid becoming negative compared to the cathode.

On tetrodes, you have two grids: one is connected to the ground through a resistor making a fixed voltage and the second is the input grid also connected to ground through a resistor that sets the input impedance of the stage.
To be noted, the power tube is a beam tetrode, the flow of electrons is directed from cathode to plate in a restricted beam. This is done by metallic parts internally connected to the cathode (which have the same potential) and restrict the place where the electrons can flow (charges of same nature repel each others).

Then you have a variable potentiometer connected to ground: volume control.
Another variable potentiometer together with a capacitor connected to ground: (basic) tone control

The 100K resistor between the controls and input tube coming from B+ is the plate resistor of the second stage amplifier.

Then you have the input tube mounted as a two-stage amplifier, in-between the tw stages a decoupling capacitor that stops the DC coming from the first stage, a 1M resistor that sets the load of the first stage and the 100K resistor acting as plate load of the first stage.

Each half tube of stages 1 & 2 have cathode resistors aimed to set the working point of the tube (makes the grid slightly negative vs the cathode) like in the power tube.

Then you have two 1M resistor that sets the input impedance of the amplifier, a capacitor in-between to avoid any DC coming in.

That's it ...

If you're interested by the subject, you can see the basics of tube electronics on "Tubes for Dummies" section of my website:

http://bonavolta.ch/hobby/en/audio/audioel.htm

Don't forget to read the safety advices: tubes involve voltages potentially lethal, please be careful !!!

author
albertson.chris (author)2016-10-25

Who designed the power supply? Please do NOT build this the wiring is just plain wrong and there is no isolation of the AC mains.

If you want a safe and same schematic google for "Fender Champ scematic" and see the real thing.

Technically what is wrong here is that any decent power transformer will have a primary and secondary that are well insulated from each other. Modern ones even wind the cold on separate bobbins. This way even after a fault the AC mains are galvanically isolated from the DC power. This matters a LOT if you are hiding the metal guitar strings in your hands. People have been killed by designed like is posted here.

Please, copy a real schematic, google will find them for you. Using the CD case for a chassis works find, cake pans work too, old ammo boxes or whatever. Just get a real transformer and wire it correctly. The voltage can kill you.

author
SteveJ25 (author)2016-10-25

doesn't look safe. potential to start a fire or kill somebody. 300 volts is nothing to play with

author
BeachsideHank (author)2016-10-23

Great job, especially in making the tube sockets.

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