instructables An Ultra Low Wattage, High Gain Tube Amplifier by ThomasH358 For bedroom rockers like me, there is nothing worse than noise complaints. On the other hand, it is a shame to have a 50W amplifier hooked to a load dissipating almost everything in heat. Therefore I tried to build a high gain preamp, based on a famous mesa amplifier using some subminiature tubes for ultra low output. https://www.youtube.com/watch?v=Cj6G5DezAjg An Ultra Low Wattage, High Gain Tube Amplifier: Page 1
An Ultra Low Wattage, High Gain Tube Amplifierby ThomasH358
For bedroom rockers like me, there is nothing worse than noise
complaints. On the other hand, it is a shame to have a 50W
amplifier hooked to a load dissipating almost everything in heat.
Therefore I tried to build a high gain preamp, based on a famous
mesa amplifier using some subminiature tubes for ultra low
output.
https://www.youtube.com/watch?v=Cj6G5DezAjg
An Ultra Low Wattage, High Gain Tube Amplifier: Page 1
Step 1: Overview, Tools and Materials
This instructables will be structures as:
1. Circuit overview: The amplifier 2. Circuit overview: The SMPS 3.
Parts list 4. Thermal transfer 5. Masking 6. Etching 7. Finishing
8. Adding sockets 9. Assembling the boards
10. Adjusting the trimpots 11. Mounting everything inside the
enclosure 12. Final result and Soundcheck
There are some tools required to build this amplifier:
Hand drill, with different drill bits (in case you want to drill
the PCB with a hand drill you need a 0.8-1 mm drill bit, not
normally found in kits). Soldering iron Clothes iron Multimeter
Sanding files Access to a toner printer Plastic box for
etching
And some materials
Sanding paper (200, 400, 600, 1200) Spray paint (black, clear) PCB
Coating spray Ferric Chloride Etching Solution Solder
An Ultra Low Wattage, High Gain Tube Amplifier: Page 2
Step 2: Circuit Overview: the Amplifier
Subminiature tubes for batteries
For this project I used 5678 and 5672 tubes. They were used in
portable battery radios, where filament current was a problem. This
tubes only require 50mA for their filaments, making them way more
efficient than the 12AX7. This keeps the current consumption low,
requiring a smaller power supply. In this case I
wanted to power them with a 9v 1A power supply, as commonly used
with guitar pedals.
The 5678 tube has a mu of roughly 23, which makes it a low gain
tube in comparison with the 12AX7, but maybe with some tweaks even
this could be enough. High gain amplifiers are known to have a lot
of filtering between stages, where almost the majority of
An Ultra Low Wattage, High Gain Tube Amplifier: Page 3
the signal is shorted to ground. There may be some air to play
with.
The 5672, on the other hand, has a mu of 10, but was mostly used as
a power tube in hearing aid devices, and was already used in some
other subminiature amplifiers (Murder one and Vibratone, from
Frequencycentral). It can produce up to 65mW clean...ish. Don't be
scared with the low wattage, it's still pretty loud when distorted!
The datasheet specifies a 20k output transformer for this
tube.
As in previous builds, the 22921 reverb transformer will be
used.
Biasing
One of the difficulties is to bias these tubes without using
different batteries, since they have direct heated cathodes. I did
not want to make this more complicated, so I had to use a fixed
bias configuration. This, on the other hand, allowed the use of the
filaments in series, reducing the total filament consumption. With
6 tubes, each dropping 1.25V, I got pretty close to the 9V of the
power
supply, it just required a small resistor, which also improved the
bias of the first stage. This means the total filament current is
only 50mA!
Pretty good for a pedal power supply.
For it to work, some stages have a trimpot to adjust the desired
bias. The bias is calculated as the difference between the voltage
at the negative side of the filament (f-)and the grid of the tube.
The trimpot adjusts the DC voltage at the grid of the tube,
allowing the different bias configurations and is bypassed by a
large capacitor, working as a short to ground for the signal.
The third stage, for example, is biased close to the cut-off point
of the tube at -1.8V, achieved as the difference between f- (pin 3)
at roughly 3.75V and the grid, at 1.95V. This stage emulates the
cold clipping stage found in high gain amplifiers, such as the
soldano or the dual rectifier. The 12AX7 in a dual rectifier uses a
39k resistor to achieve this. The other stages are almost center
biased, at approximately 1.25V.
Step 3: Circuit Overview: the SMPS
High voltage supply
Regarding the plate voltage, these tubes run ideally with plate
voltages at 67.5V, but also worked with 90V or 45V batteries. Those
batteries were huge! They are also difficult to come by and
expensive. That's why I opted for a switched mode power
supply
(SMPS) instead. With the SMPS I can boost the 9V to 70V and add
some massive filtering before the output transformer.
The circuit used in this instructables is based on the 555 chip,
successfully used in previous builds.
An Ultra Low Wattage, High Gain Tube Amplifier: Page 4
Mainboard
An Ultra Low Wattage, High Gain Tube Amplifier: Page 5
C30 22nF/100V_________R30 1.5k
Special attention to the capacitor voltage rating. The high voltage
circuit requires 100V capacitors, the signal path after the
coupling capacitors can use lower values, in this case I used 50V
or 100V since the film capacitors have the same pin spacing. The
filaments need to be decoupled, but since the highest voltage on
the filaments is 9V a 16V eletrolytic capacitor is on the safe side
and way smaller than a 100V one. Resistors can be of the 1/4W
type.
555 SMPS
C1 330uF/16V__________R1 56k______________IC1 LM555N C2
2.2nF/50V__________ R2 10k______________L1 100uH/3A C3
100pF/50V__________R3 1k_______________Q1 IRF644 C4
4.7uF/250V_________ R4 470R____________ VR1 1k R5
150k_______________D1 UF4004 or ES2G (ultra fast) R9 2.2k
Attention to the switching diode! It must be of the ultra fast
type, otherwise it won't work. For the SMPS low ESR capacitors are
also desired. In case a normal 4.7uF/250V capacitor is used an
additional ceramic capacitor of 100nF in parallel helps to bypass
the high frequency switching.
These are the easier parts to find and can be obtained from any
eletronic parts store. Now, the tricky parts are:
OT 3.5W, 22k:8ohm transformer (022921 or 125A25B) Banzai,
Tubesandmore
L1 100uH/3A inductor Ebay, just don't buy the toroidal shaped. You
also find it at Mouser/Digikey/Farnell.
Don't forget to buy:
A copper clad board, 10x10 mm will do for both boards 2x 40 pin sip
sockets for the tubes A 1590B enclosure Some 3 mm screws and nuts
Rubber feet 5 mm rubber wire grommets Six 10 mm knobs
An Ultra Low Wattage, High Gain Tube Amplifier: Page 6
Step 5: Thermal Transfer
To prepare the PCB and the enclosure I use a process based on toner
transfer. The toner protects the surface from the etchant, and as a
result after the etching bath we have the PCB with the copper
tracks or a beautiful enclosure. The process of transfering the
toner and preparing for etching consists of:
Print the layout/image with a toner printer using glossy paper.
Sand the surface of the enclosure and of the copper board using
sanding paper with grit 200 to 400. Fix the printed image to the
PCB/enclosure using tape. Apply heat and pressure with the clothes
iron for about 10 minutes. Make some extra movement with the tip of
the iron at the edges, those are the tricky places where the toner
won't stick. When the paper is looking yellowish trow it in a
plastic container filled with water to cool it down, and let the
water soak into the paper. Remove the paper carefully. It's better
when it comes off in layers, instead of removing everything in a
single attempt.
The drill template helps to identify the positioning of the
components, you just need to add your own art, and you're good to
go.
An Ultra Low Wattage, High Gain Tube Amplifier: Page 7
https://www.instructables.com/ORIG/FQ2/ATR6/JTCSFEYF/FQ2ATR6JTCSFEYF.pdf…
Download
https://www.instructables.com/ORIG/F75/YB69/JTCSESNZ/F75YB69JTCSESNZ.pdf…
Download
Step 6: Masking
For the enclosure, mask larger areas with nail polish. Since the
reaction with aluminum is much stronger than with copper, there
could be some pitting in larger areas.
Giving an extra protection guarantees that there will be no marks
to ruin the enclosure.
An Ultra Low Wattage, High Gain Tube Amplifier: Page 8
Step 7: Etching
For the etching process I like to use a plastic container with
etchant and one with water to rinse between steps.
First, some safety tips:
use rubber gloves to protect your hands work on a non-metallic
surface Use a well ventilated room and avoid breathing the
resulting fumes Use some paper to protect your workbench from
possible spills
Here I only show the etching of the enclosure, but the PCB was
etched in the same solution. The only difference is that for the
PCB I just waited for about an hour until all the unprotected
copper was gone. With the aluminum there must be some extra care,
since we only want to etch the outside of the box.
For the enclosure I shake the box in the etching mixture for about
30 seconds, until it gets warm due to the reaction an rinse it in
the water. I repeat this step another 20 times, or until the etch
is about 0.5 mm deep.
When the etch is deep enough wash the enclosure with water and soap
to rinse off all the remaining etchant. With the box cleaned sand
the toner and the nail polish off. For the nail polish you can save
some sanding paper by using acetone, but remember to keep the room
well ventilated!
An Ultra Low Wattage, High Gain Tube Amplifier: Page 9
An Ultra Low Wattage, High Gain Tube Amplifier: Page 10
Step 8: Finishing
In this step I used the 400 grit sanding paper to achieve a clean
surface, like in the third picture. This is clean enough for the
drilling step. I drilled all the different sized holes, and used
the files to make the holes for the tubes sockets. The PCB must be
drilled too, I a 0.8 mm drill bit for the components and 1-1.4 mm
for the wire holes. In this build I also used a 1.3 mm drill for
the tube sockets.
With the drilling and filing done I give the box a black coat of
spray paint and let it dry for 24h. It will give a better constrast
between the etch and the enclosure. Obviously, the next step is to
sand it off. This time I
go from 400 to the finest grit. I change the sandng paper when one
grit removed the lines of the previous one. Sanding in different
dirrections makes it easier to identify when all the previous marks
are gone. With the enclosure shining I apply 3 layers of the clear
coat and wait until it dries for another 24h. The PCB can be
protected from corrosion by using a protective coating. As you can
see in the last two figures I like to have a dark green coating.
This coating requires longer times to dry. I waited 5 days to avoid
having finger prints on the board while soldering the
components.
An Ultra Low Wattage, High Gain Tube Amplifier: Page 11
Step 9: Adding Sockets
Soldering the Sockets
According to the layout, the tubes are mounted at the copper side
of the board. This way the board can come closer to the enclosure
and profit from some extra shielding against nasty high frequency
EMI coming from the SMPS. But using the copper side of the board to
solder components has some disadvantages, such as the copper
becoming loose from the board. To avoid this, instead of soldering
the tube sockets, I made larger holes where the sockets could be
pressed in. The pressure of a slighlty smaller hole and some solder
on both sides should solve the problem. For this I used the
machined style pin sockets, without the plastic structure, forced
the metal pin in the hole and soldered on both sides (on
the components side it looks like a blob of solder, but it helps to
keep the pin stuck), as shown in the first 3 pictures. The 4th and
5th pictures show all the sockets and jumpers installed.
Soldering another set of sockets, this time with the plastic
structure, to the tubes improves the connection to the board and
makes it more stable. The original pins of the tubes are very thin,
which can lead to some bad contact or even falling off the sockets.
By soldering them to sockets we solve this problem, since now they
have a tight fit. I think they should have come with proper pins on
the first place, like the larger tubes!
An Ultra Low Wattage, High Gain Tube Amplifier: Page 12
An Ultra Low Wattage, High Gain Tube Amplifier: Page 13
Step 10: Assembling the Boards
To solder the components I started with the resistors, and moved to
the larger parts. The electrolytics are soldered at the end, since
they are the highest components on the board.
With the board ready it's time to add the wires. There are a lot of
external connections here, from the tonestack to the high voltage
and filament cables. For the signal wires I used shielded cable,
shielding the ground mesh at the panel side, closer to the
input.
Critical wires are around the first stage, coming from the input
jack, and going to the gain potentiometer. Before we can build
everything inside the box we need to test it, so that we still have
access to the copper side of the board for some debugging, if it's
necessary.
For the high voltage filtering I added another RC filter in a
smaller board, mounted perpendicularly to the main board, as seen
in the picture. This way the ground, high voltage and transformer
connections are easier to acccess with the board mounted to the
enclosure and can be soldered afterwards.
Building the tonestack
Although I was going to test the board outside the enclosure I
already built the tonestack in the box. This way all the
potentiometers are fixed and properly grounded. Testing the circuit
with ungrounded potentiometers (at least the outside shield) can
result in horrible noises. Again, for longer connections I used a
shielded cable, grounded near to the input jack.
Unfortunately in this build the potentiometers are really close
together, making it difficult to use a board with the components.
In this case I used a point-to- point approach for this part of the
circuit. Another problem was that I only had a PCB style 9 mm 50K
potentiometer, so that I had to anchor it to the neighbouring
potentiometers (panel mount style).
Now is also a good time to install the on/off switch and the LED
with the 2.7k resistor.
As a result of two rows of potentiometers I had to file the inside
wall of the lid, as shown in the picture, so that the box would
close.
An Ultra Low Wattage, High Gain Tube Amplifier: Page 14
An Ultra Low Wattage, High Gain Tube Amplifier: Page 15
Step 11: Adjusting the Trimpots
Adjusting the 555 SMPS
If the SMPS is not working there is no high voltage and the circuit
won't work correctly. To test the SMPS just connect it to the 9V
power jack and check the voltage reading at the output. It should
be around 70V, otherwise it needs to be adjusted with the trimpot.
If the output voltage is 9V there is a problem with the board.
Check for a bad mosfet or 555. If the trimpot does not work verify
the feedback circuit around the smaller transistor. An advantage of
this SMPS is the low count of parts, so it is a little easier to
identify any mistakes or faulty components.
Adjusting the mainboard trimpots
During the testing stage is a good time to adjust the bias with the
trimpots. It can be done later, but if the tone is to dark or to
bright it is easier to make changes now.
The first trimpot controls the bias of the second, third and output
stages and is therefore the most important. I adjusted this trimpot
by measuring the bias of the third stage, the cold clipper. If the
bias is too high the stage will be completely in cut-off, giving a
raw, cold, spongy distortion. If it is biased hotter the output
stage will be too hot, adding some power stage distortion, and
running the tube closer to the max. plate dissipation. In this
case, the lower side of the master volume should be connected to
the negative side of the first stage, so that the bias is still
around 5.9V. In my case it sounded better when the output stage was
running at 5.7V instead of 6.4V.
Just measure the bias at the third stage (middle tube in the back
row) and verify that it is around 1.95V The
second trimpot just needs to be adjusted to taste, or nearly center
biased at 1.2V (measured between pins 3 and 4). Similarly the third
trimpot is also adjusted to approx. 1V.
The voltage readings at the tube's pins 1(plate) to 5 (filament)
are:
V1:
<8.8V><41.6V><1.21V><0.00V><2.50V>
V2:<27.9V><27.9V><2.50V><1.94V><3.71V>
V3:<42.4V><42.4V><3.70V><1.94V><4.90V>
V4:<36.2V><36.2V><4.90V><3.90V><6.10V>
V5:<41.4V><41.4V><6.20V><5.10V><7.60V>
V6:<64.6V><63.7V><8.80V><1.94V><7.60V>
Note that the filaments in the 5672 are backwards than in the 5678,
so that the tubes can't be swapped. Another important aspect to
consider is the tube manufacturer. I found out that the tung-sol
tubes sounded better in the first positions, than the raytheon
tubes. Checking it with an oscilloscope it was visible that the
tung-sol tubes had more gain than the raytheon tubes I had.
Now is also the time to test the circuit and see how it sounds, if
it is too bass heavy I suggest changing the 47nF capacitor between
second and third stage to 10nF, that will filter some bass out from
the initial stages and improve the sound. If it got too thin, just
increase this capacitor to 22nF and so on.
An Ultra Low Wattage, High Gain Tube Amplifier: Page 16
Step 12: Mounting Everything Inside the Enclosure
I started adding the screws for the mainboard. On the inside I
added the rubber wire grommets, to give some clearance between
board and enclosure and also to dampen some vibration. By running
the first stage in pentode mode this could help if the tube gets
microphonic. Then I added the board and screwed it down with the
nuts, connected the tonestack, inserted the input jack and soldered
the remaining wires.
With the mainboard in position I added the output transformer,
adjusted the lenght of the wires and inserted the output jack and
power jack.
At this point I saw that my SMPS board would not fit in the desired
position (at the lateral wall, with the components perpendicular to
this wall) because I added the power jack on the wrong side of the
output jack... To fix this I sawed the SMPS board at the input
side, removing the inductor and capacitor, and soldered the piece
back to the board rotated by 90 degrees, as shown in the picture. I
tested the SMPS again to see if it still was working, and finished
by connecting the high voltage to the main board, through the RC
filter board.
An Ultra Low Wattage, High Gain Tube Amplifier: Page 17
Step 13: Soundcheck
Now just plug the amplifier to your favorite 8 ohms cabinet (in my
case a 1x10" with a celestion greenback) and use your pedal power
supply to play at non-deafening levels!
By the way, if you like the sound of your amp feedbacking when you
stop playing at the end of a sound, wait for the middle part of the
video, it feedbacks quite easily when sitting in front of the
cab.
An Ultra Low Wattage, High Gain Tube Amplifier: Page 18
Beautifully done! Thank you for your post. Where did you learn to
do the etching?
https://youtu.be/Zu_1b5VOxhc
An Ultra Low Wattage, High Gain Tube Amplifier: Page 19
An Ultra Low Wattage, High Gain Tube Amplifier
Step 1: Overview, Tools and Materials
Step 2: Circuit Overview: the Amplifier
Step 3: Circuit Overview: the SMPS
Step 4: Parts List
Step 5: Thermal Transfer
Step 12: Mounting Everything Inside the Enclosure
Step 13: Soundcheck