The big project for this autumn has been to build a guitar amplifier. After many hours of late-night-soldering, today was the day when I finally got to switch on the power.
The amp (called Uraltone jr) is built from a kit, designed and sold by Uraltone in Helsinki, Finland. It is a very reasonably priced kit, which contains all the electronics (even including high-quality tubes!) and the metallic chassis for the amp. What You need to buy apart from the kit is materials for the box, and of course the speaker. I had already built a speaker cabinet, so it was just the amp missing. I went for a head/speaker design, but the amp can be built as a combo as well. (I will not go into detail on the wooden box here, the materials and techniques were essentially the same as on the aforementioned speaker cabinet).
The first step was to install all the external parts, transformers, potentiometers, jacks, switches etc. This was in a way the easy part, as the chassis had a hole pre-drilled for every part, so it was really just a matter of getting everything in the right place, and securing everything with the right nuts and bolts.
In the picture above, some of the wires are already cut and soldered to the right place. All the wires on the left are connected to the mains transformer, while those on the right go to the output transformer. The twisted pair of black wire in the middle go to the anode choke, which is essentially just half a transformer (or a very reactive load, with high AC resistance and very low DC dito).
With all the external components in place, it was time to start working on the circuit board. The board itself is a very sturdy PCB/turret hybrid, which holds most of the capacitors and resistors (a few are mounted directly to the pots/tubes).
The board was then installed, and the last wires were soldered, with everything in place, it looked like this on the inside:
It is a mess, there is no denying that, but the instructions were clear, so I actually got everything right on the first try. The outside looked like this:
The front controls are, left to right: On/Off-switch, Master Volume 1&2 (for the different power tubes), Tone (it’s called ‘colour’ by the manufacturer), Volume (gain) and a six-position boost switch.
Back controls: Anode voltage, Feedback and Line out level. These may need some explaining. The first one lowers the voltage on the anodes, which takes down the output power without affecting the sound too much. (this requires som extra parts, which can be bought as an add-on kit). The second knob controls the negative feedback from the output transformer to the preamp. It does not have a huge impact on the sound, but it’s clearly audible, especially on higher output levels, where it does a good job cleaning up the sound a bit. The last one is simply a volume knob for the line out jack. The amp itself has a built in dummy load, which takes care of all the output power when no speaker is connected. I have not tested this feature yet, and I guess it will not be used that much, but it is a nice one to have when recording for example.
Well that’s pretty much it then:
I was a bit concered about messing around with high voltages, but it turned out all right. Common sense applies, as the voltages inside are even higher than mains voltage.
The important part then, the sound. The amp sounds quite fender-ish, with a very nice clean. It can take a lot of different power tubes, at the moment mine runs on the supplied JJ 6V6 and Svetlana 6L6, with a TAD 12AX7 preamp tube, and a JJ 5Y3 rectifier. I guess messing around with different tubes could make a huge difference in sound, but the important thing I think is that it sounds great right away. It can do distortion to some extent, but preamp distortion is not where this amp shines. Instead it does a wonderful job at the level where the power tubes just start to break up. This combined with the ability to take down the output power makes it a really nice blues-machine. With the master volumes just above half-way the sound starts go get a little furry, with just the right amount of harmonics. The tone control is well designed in the sense that it is useful in its entire range, from ‘slightly softer’ to ‘a bit brighter’ (instead of the usual ‘useless muddy mess of low frequencies’ to ‘ear-piercing sqeal’). It also needs to be said, that this amp is REALLY quiet, virtually no hum at all at clean settings, and only the slightest hint of hum when moving into overdrive.
The mandatory ‘glowing-tubes-picture’.
Closing thoughts then. I really enjoyed building this amp, and I feel this might not be the last one I build. This is a well designed kit so it shouldn’t be too hard even for a beginner. I guess a tube amp shouldn’t be your first electronics project, but if You feel that You’re comfortable with soldering and can follow instructions, then I can really recommend this amp.
Please stay tuned for a demo!
What's wrong with this circuit? 
I won’t post schematics this time, as this amp kit is a commercially available product.
well done man, loved the article…im just a bit jealous because i would love to have those “two flowers” in my room or stage,cheers man, i will not forget that you saved my pedal from going to trash (Rocktron Big Crush) i altered the circuit and now it is true by pass,tx
Cool post! I built one myself also in 2015, and that was a sweet little project.
I still have the “adjustable voltage source” parts waiting for a rainy day soldering, but I kind of like having the two master volumes as well. Are both of your master volumes still operational, even with the voltage source mod. installed?
Thanks! Yes they are, the voltage control is on the back of the amp, together with the feedback control and the line out level. I ordered two smaller knobs for these, three of those large knobs supplied with the kit wouldn’t have fit on the (pre-drilled) back panel.
The building manual says you should skip the second master volume if you add the regulator, but obviously you can have both. 😃