Introduction: Battery Powered Tube Amplifier
Tube amplifiers are loved by guitar players because of the pleasant distortion that they produce.
The idea behind this instrunctables is to build a low wattage tube amplifier, which can be also carried around to play on the go. On the age of bluetooth speakers, it is time to build some portable, battery powered tube amplifiers.
Step 1: Select the Tubes, Transformers, Batteries and High Voltage Supply
Because power consumption in tube amplifiers is a huge problem, the choosing the right tube can spare a lot of power and increase the playing hours between recharges. Some long time ago there were battery powered tubes, which powered from small radios to airplanes. Their great advantage was the lower filament current required. The picture shows a comparison between three battery powered tubes, the 5672, 1j24b, 1j29b and a miniature tube used in guitar preamps, the EF86
The choosen tubes are:
Preamp and PI: 1J24B (13 mA filament current at 1.2V, 120V max. plate voltage, russian made, inexpensive)
Power: 1J29B (32 mA filament current at 2.4V, 150V max. plate voltage, russian made, inexpensive)
For such lower power settings a cheaper transformer can be used. Some experiments with line transformers showed that they are quite good for smaller amplifiers, where the bottom end is not a priority. Because of the lack of an air gap the transformer works better in push-pull. This also requires more taps.
100V line transformer, 10W with different taps
(0-10W-5W-2.5W-1.25W-0.625W and on the secondary 4,8 and 16 ohms)
.Luckily the transformer I got also had the number of turns per winding specified, otherwise some math would be necessary to identify the adequate taps and the highest impedance available. the transformer had the following number of turns at each tap (starting from the left):
725-1025-1425-2025-2925 on the primary and 48-66-96 turns on the secondary.
Here it is possible to see that the 2.5W tap is almost in the middle, with 1425 turns on one side and 1500 on the other. This small difference could be a problem in some bigger amplifiers, but here it will just add up to the distortion. Now we can use the 0 and 0.625W taps for the anodes to obtain the highest impedance available.
The primary to secondary turns ratio is used to estimate the primary impedance as:
2925/48 = 61, with an 8 ohm speaker this gives 61^2 *8 = 29768 or approx. 29.7k anode-to-anode
2925/66 = 44, with an 8 ohm speaker this gives 44^2 *8 = 15488 or approx. 15.5k anode-to-anode
2925/96 = 30, with an 8 ohm speaker this gives ^2 *8 = 7200 or approx. 7.2k anode-to-anode
Because we intend to run this in class AB the impedance that the tube is actually seen is only 1/4 of the calculated value.
High voltage power supply
Even this small tubes also require higher voltages at the plates. Instead of using several batteries in series, or using those huge old 45V batteries I used a smaller switched mode power supply (SMPS) based around the MAX1771 chip. With this SMPS I am able to multiply the voltage coming from the batteries to values as high as 110V without any problems.
The choosen batteries for this project are Li-Ion batteries, easily obtained in the 186850 package. There are several charger boards available online for these. One important note is to buy only known good batteries, from trusted sellers, to avoid unnecessary accidents.
Now that the parts are roughly defined it's time to start working on the circuit.
Step 2: Working on a Circuit
To power the tubes filaments a series configuration was choosen. There are some difficulties that must be discussed.
- Because the preamp and power tubes have different filament currents resistors were added in series with some filaments to bypass part of the current.
- The battery voltage drops during the use. Each battery has initially 4.2V when fully charged. They quickly discharge to the nominal value of 3.7V, where they slowly decrease to 3V, when it must be recharged.
- The tubes have direct heated cathodes, meaning that the plate current flows through the filament, and the negative side of the filament corresponds to the cathode voltage
The filament scheme with voltages looks like this:
battery(+) (8.4V to 6V) -> 1J29b (6V) -> 1J29b // 300ohms (3.6V)->1J24b // 1J24b // 130 ohms (2.4V)->1J24b // 1J24b // 120 ohms (1.2V) -> 22 ohms -> Battery(-) (GND)
where // represents in parallel configuration and -> in series.
The resistors bypass the extra current of the filaments and the anode current flowing at each stage. To correctly predict the anode current it is necessary to draw the loadline of the stage and choose an operation point.
Estimating an operation point for the power tubes
This tubes come with a basic datasheet, where the curves are plotted for a screen grid voltage of 45V. Since I was interested in the highest output I could get, I decided to run the power tubes at 110V (when fully charged), way above the 45V. To overcome the lack of a useable datasheet I tried to implement a spice model for the tubes using paint_kip and later increase the screen grid voltage and see what happens. Paint_kip is a nice software, but requires some skill to find the correct values. With pentodes the difficulty level also increases. Since I only wanted a rough estimation I did not spend much time looking for the exact cnfiguration. The test rig was built to test the different configurations.
OT Impedance: 29k plate-to-plate or approx. 7k for class AB operation.
High voltage: 110V
After some calculations and testing the grid bias voltage could be defined. To achieve the choosen grid bias the grid leak resistor is connected to a filament node where the difference between the voltage of the node and the negative side of the filament. For example, the first 1J29b is at the B+ voltage of 6V. By connecting the grid leak resistor to the node between the 1J24b stages, at 2.4V the resulting grid voltage is -3.6V in relation to the GND line, which is the same value seen on the negative side of the filament of the second 1J29b. So, the grid leak resistor of the second 1J29b can go to ground, as it normally would in other designs.
The phase inverter
As seen in the schematic, a paraphase phase inverter was implemented. In this case one of the tubes has a unity gain and inverts the signal for one of the output stages. The other stage acts as a normal gain stage. Part of the distortion created in the circuit comes from the phase inverter losing balance and driving one power tube harder than the other. The voltage divider between the stages was choosen so that this only occurs at the last 45 degrees of the master volume. The resistors where tested while the circuit was monitored with an oscilloscope, where both signals could be compared.
The preamp stage
The last two 1J24b tubes consist of the preamplifier circuit. Both have the same operation point since the filaments are in parallel. The 22 ohms resistor between the filament and ground elevates the voltage at the negative side of the filament giving as small negative bias. Instead of choosing a plate resistor and calculating the bias point and necessary cathode voltage and resistor, here the plate resistor was adapted according to the desired gain and bias.
With the circuit calculated and tested it is time to make a PCB for it. For the schematic and PCB I used Eagle Cad. They have a free version where one can use up to 2 layers. Since I was going to etch the board myself it makes no sense using more than 2 layers. To desing the PCB it was first necessary to also create a template for the tubes. After some measurements I could identify the correct spacing between pins and the anode pin at the top of the tube. With the layout ready it is time to start the real build!
Step 3: Soldering and Testing the Circuits
First solder all the components of the Switched mode power supply. For it to work correctly the right components are required.
- Low on resistance, high voltage Mosfet (IRF644Pb, 250V, 0.28 ohms)
- Low ESR, high current inductor (220uH, 3A)
- Low ESR, high voltage reservoir capacitor (10uF to 4.7uF, 350V)
- 0.1 ohm 1W resistor
- Ultrafast high voltage diode (UF4004 for 50ns and 400V, or anything faster for >200V)
Because I am using the MAX1771 chip at a lower voltage (8.4V to 6V) I had to increase the inductor to 220uH. Otherwise the voltage would drop under load. When the SMPS is ready I tested the output voltage with a multimeter and adjusted it to 110V. Under load it will drop a little bit and a readjustement is required.
I started soldering the jumpers and components. Here it is important to check if the jumpers are not touching any component legs. The tubes were soldered on the cooper side after all the other components. With everything soldered I could add the SMPS and test the circuit. For the first time I also checked the voltage at the plates and screens of the tubes, just to be sure that everything was OK.
The charger circuit I bought on ebay. It is based around the TP4056 chip. I Used a DPDT to switch between a series and parallel configuration of the batteries and a connection to the charger or to the circuit board (see figure).
Step 4: Enclosure, Grill and Faceplate and Finish
To box this amplifier I choose to use an older wooden box. Any wooden box would work, but in my case I had a really good one from an ammeter. The ammeter was not working, so I could at least rescue the box and build something nive inside it. The speaker was fixed at the side with the metal grill that allower the ammeter to cool down while in use.
The tube grill
The PCB with the tubes was fixed on the opposite side of the speaker, where I drill a hole so that the tubes are visible from the outside. To protect the tubes I made a small grill with an aluminium sheet. I make some rough marks and drilled smaller holes. All the imperfections were corrected during the sanding phase. To give a good contrast to the faceplate I ended up painting it black.
The Faceplate, sanding, toner transfer, etching and sanding again
The faceplate was done similarly as the PCB. Before I started, I sanded the aluminium sheet to have a rougher surface for the toner. 400 is rough enough in this case. If you want you can go up to 1200 but it is a lot of sanding and after the etch there will be even more, so I skipped that. This also removes any finish that the sheet had previously.
I printed the mirrorred faceplate with a toner printer on a glossy paper. Later I transfered the drawing using a normal iron. Depending on the iron there are different optimal temperature settings. In my case, it is the second setting, just before the max. temperature. I transfer it during 10 min. approx., until the paper starts to get yellowish. I waited for it to cool down and protected the back of the plate with nail polish.
There is the possibility of just spraying over the toner. It also gives good results if you can remove all the paper. I use water and towels to remove the paper. Just be careful not to remove the toner! Because the design here was inverted I had to etch the faceplate. There is a learning curve in etching, and sometimes your solutions is stronger or weaker, but in general when the etch seems deep enough it is time to stop. After etching I sanded it starting with 200 and going up to 1200. Normally I start with 100 if the metal is in bad shape, but this one was need and was already in good shape. I change the sandpaper grain from 200 to 400, 400 to 600 and 600 to 1200. After that I painted it black, waited one day and sanded again with the 1200 grain, just to remove the excessive paint. Now I drilled the holes for the potentiometers. To finish it I used a clear coat.
Batteries and parts were all screwed to the wooden box after the faceplate was positioned, from the speaker side. To find the best SMPS position I turned it on and verified where the audio circuit would be less affected. Since the audio circuit board is much smaller than the box the adequate spacing and correct orientation was enough to make the EMI noise inaudible. The speaker baffle was then screwed in place and the amplifier was ready to play.
Close to the batteries end there is a noticeable volume drop, before I could not hear it, but my multimeter showed that the high voltage decreased from 110V to 85V. The heaters voltage drop also decrease with the battery. Fortunately the 1J29b works without a problem until the filament reaches 1.5V (with the 2.4V 32mA setting). Same goes for the 1J24b, where the voltage drop reduced to 0.9V when the battery was almost drained. If the voltage drop is a problem for you, there is the possibility of using another MAX chip to convert to a stable 3.3V voltage. I did not wanted to use it, because it would be another SMPS in this circuit, which could introduce some extra noise sources.
Considering the battery life, I could play a whole week before I had to recharge it again, but I only play for 1 to 2 hours a day.
Participated in the
Audio Contest 2018
5 months ago
Hey! Nothing not awesome about this article. The instructions, the demo, everything is awesome!! Can’t wait to start on this project. I realize it’s a very old threat but hoping you still active here … I have a question.: any limitation on the speaker ? (Aside from impedance matching, anything else I should be aware of ?) thanks and rock on!
Reply 5 months ago
No limitations. If it has 8 ohms, it should work. Lower efficiency speakers (<80db) might result in a low output. This barely has 0.1W. In my case it was loud enough.
Reply 4 months ago
Thanks mate! If i'm gonna use ready-made SMPS, is it critical it'll be based on MAX1771 ? What else should I be aware of ? And also - Why would you toggle between parallel and series batteries ? Thanks !!
Reply 4 months ago
Hi, you can use a different SMPS (555, ...), it only needs to work with 6v (batteries almost discharged) and multiply to the necessary high voltage. I also tested it with the 555 SMPS, for example.
Series is for use, you need 7.2v down to 6.0v for the heaters and for the SMPS. The SMPS will not work with 3.7v (3.0V discharged). The parallel connection is for charging. I only had a normal 5V USB charger.
5 months ago
HI , amazing project. Just a question . can I use a 6w output transformer instead ?
Primary 6 W/1.7 kOhm;
3 W/3.3 kOhm;
1.5 W/6.7 kOhm
Secondary 4; 8 ohm
Reply 5 months ago
No, the impedance is too low at only 6.7 kOhms and it also has a different number of turns and not enough windings for a push-pull transformer.
That is why you need a transformer with the 0.625 W tap. The number of turns between common and 2.5 W is roughly the same as from 2.5 W to 0.625 W.
For it to work you would have to have a tap with 4x the impedance to the centre tap. For the 6W transformer it would be 0.75W/13.4 kOms with the centre tap at 3 W/3.3 kOhm.
Reply 5 months ago
ok many thanks !. an other question. is this the same transformer you have used ? ( and if not how can I calculate the resistor if I dont know the turns ? ) https://www.reichelt.com/de/en/visaton-transformer-for-100-v-technology-vis-tr-10-16-p26915.html?&nbc=1
Reply 5 months ago
The impedance can be estimated based on ohms law:
V=R*I and W=R*I leads to V^2/W = R
100 V * 100 V / 0.625 W = 16 kOhms
100 V ^2 / 2.5 W = 4 kOms and so on.
16 kOhms : 4 Ohms is the same as 32 kOhms : 8 Ohms if you use the same tap (i.e. the 4 Ohms tap) with a speaker with 8 Ohms.
Reply 5 months ago
thank you !
Question 3 years ago
Very cool design. I would like to build one. I have only a few questions: the diagram does not show which lamps in which place? There are 6 of them and there is no lamp in which place. What voltage should capacitors be used in the amplifier system? And what current efficiency is needed at the 110V output because in my country I will not buy the elements to create this converter and I have to use another alternative.
Answer 3 years ago
Hi, the PDF shows the names of the tubes under one of the layout pictures (top right).
In the schematic the two power tubes (1j29B) are the two stacked on the right, all the others are preamp tubes (1J24B).
Capacitors should be at least for 2x the max. voltage, in this case 250v. At some places, where only lower voltages are present at the plates and screen grid you could use 100v capacitors.
For the filaments, where the max. voltage is 8.4V (2x 4.2v full batteries) you can use 16v. capacitors (100uF and 220uF capacitors).
The current at the 110V output should be around 20 mA on the safe side (that is what my SMPS can supply). The circuit needs much less (around 10 mA). My suggestion is you look for a 555 SMPS. Most of the components are easy to obtain.
Tip 4 years ago
Would be cool if you could use a removable battery, like those found in power tools. That way you could swap them out as needed without opening the case (put it on the outside of the case...)
Question 4 years ago on Step 2
May You please add the Eagle-Files to this very nice Projekt?
Thank You in advance
Answer 4 years ago
You'll find the eagle files at the tube circuit section.
4 years ago
I am glad to see a schematic that is populated with the values of the components. It is easier to use, rather than having to refer to a parts list.
Tubes are not dead! And they are usually out there in abundance.
Here is a pre-amp project that I am presently working on. I also used an old test box. This one is made by Leeds&Northrup. It uses 45 VDC for plate voltage, and 1.5 VDC for filaments. The battery supplies will yield about 50 hours of play time. The tube used is a 3S4 miniature. They were popular in the 50's for portable radios.
Reply 4 years ago
I also thought about using those tubes, but after I read the article over radiomuseum about the 1J24b I choose the russian subminiatures. They have the lowest filament current I ever seen for it's output. And it works at voltages as low as 24V. One thing though is that they barely glow and the heat up time is really short, as one would expect from a solidstate amplifier.
Reply 4 years ago
Just a short reply. I really love the miniature tubes. I have also worked with the Korg Nutube with some good results. Talk about low output though!
Reply 4 years ago
I have seen those, but they are kind of expensive. I generally prefer old stock tubes because they are cheap and widely available in the market.
Reply 4 years ago
This is not a video but really good and easy to understand. He puts much of his material online for free but you can buy books too if you want more. But the online stuff is enough so that you can design small amps or at least read and understand the schematic.
4 years ago
Finally You post in instructable. For long time I follow your Youtube Channel. Good luck for audio contest..