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Picture of An Ultra Low Wattage, High Gain Tube Amplifier

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.

Step 1: Overview, Tools and Materials

Picture of 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
db9pz5 months ago
That sounds really beautyful! It reminds me a bit of the AX84-project, a similar design. Keep on rocking! And keep good hand made music alive! If I hadn't an AX-84 with a single tube output-stage, this should be 'my living-/bed-room-amp' ! Good luck in the 'pocket-sized contest' !
1BigDuck5 months ago
I would like to buy a kit or the circuit board. Any chance? Price? Buy the one you made? :-)
ThomasH358 (author)  1BigDuck5 months ago
Sorry, it's not for sale. Since I etch my PCBs I'm not confortable in selling them, maybe you know someone nearby that will make the PCB for you.
Thank you for the
reply. J I can appreciate your
situation. I want you to know that your prodject is EXTREEMLY INTERESTING and a
WONDERFUL IDEA! The mount ing of the
tubes is brilliant! ;-) You should consider the PC board option, if there are
enough responses like myself. $30.00 T0 $50.00 WOULD BE

Keep up the great
work!!! KUDOS! J


Ray-N-Tosca6 months ago
Why is a toroidal transformer not suitable?
ThomasH358 (author)  Ray-N-Tosca6 months ago
If you use my layout, size is the first concern.

I used toroidal inductors in some builds and for this particular application they get really hot, and the voltage drops.

Some say that it occurs because the toroidal inductor has no air gap, and saturates more easily with DC current, which would also explain the heat and loss of efficiency I observed.
KushagraK76 months ago
That's a cool project! Great work!
ipetrauskas6 months ago
I can source 2.5W audio transformer, it will also do I believe?
ThomasH358 (author)  ipetrauskas6 months ago
Yes, as I said, it only produces 65 mW. The 3.5 W transformer was easier to find, but any transformer with around 20k:8 ohms will do.
kmpres6 months ago
Wow, that is one tight little package! Looks and sounds great! It's hard to believe you got six tubes and a transformer in a space not much bigger than a pedal. Did you breadboard the circuit before designing the layout or did you design it stage by stage from known good past designs?
ThomasH358 (author)  kmpres6 months ago
I tested it on a perfboard. I started with a given schematic and adapted each stage for the fixed bias.
JonathanL2366 months ago
Beautifully done! Thank you for your post. Where did you learn to do the etching?
ThomasH358 (author)  JonathanL2366 months ago
I followed this tutorial:
I ruined some pedals until I learned the right moment to stop, and that the extra masking is necessary in the larger toner areas.
The paper also plays an important role. You need a good thermal transfer, and using normal paper won't do the trick.
That's why I suggest using glossy paper.