Guitar Amplifier Soldano SLO-100

Damian "Ghost" Smółka

Welcome. In this article I would like to present the equipment I recently had the pleasure to commit - Soldano SLO-100. This is my fourth vacuum tube project I have done so far. I think it's the best hardware I build with my soldering iron.

 Technical specifications:

  • Output power – 100W
  • Power tubes – 4 x 6L6WXT+ Sovtek
  • Preamplifier tubes – 5 x ECC83 RFT


  • TS – IGMT-SLO100
  • TG – IGOT-SLO100
  • Choke – IGCH-SLO100


  • Normal – Preamp Volume, Master Volume, Bass, Middle, Treble, Presence, Depth, Switches: Normal/Bright, Clean/Crunch
  • Overdrive – Preamp Volume, Master Volume, Bass, Middle, Treble, Presence, Depth.
  • Equalizer – one for both channels, Presence, Depth
  • Channel switching – front panel switch and Footswitch. LED signaling active channel.


  • FX Loop.
  • BIAS with external measuring points.
  • Outputs – 2 x Jack, Impedance switch 4/8/16 Ohms.

Schematic diagrams and layout which I have used.


The pcb's were purchased from Tube-Town -

In terms of the project realization I divided it on the various stages described below.

Day 1.

With all the necessary elements collected in one place, I started from the adaptation of the chassis to the mechanical parts. I drilled the mounting holes of transformers and PCB's. I used the protective tape which proved to be very helpful. I marked the positions of all important points on this tape and used it as a reference for mechanical engineering.

To make the holes I used a special stepped drill - a variable-diameter drill. In addition, I still had to ream holes in the knobs. The time had come to fill the first elements of the chassis.

As you can see I placed an electrical connector underthe power supply PCB. It's main role is to preserve the relative order of the wiring. It will be seen in subsequent images.

To summarize the first day I installed indicator, power and standby switches, mains socket, mains fuse, tube sockets, BIAS test points, as well as mains transformer. I made some wiring too. Everything you can see in the pictures below.

As you can see the connections on the switches and a power outlet are made on connectors. These connections are additionally secured for safety reasons by applying heat shrink. A similar procedure is performed on fuse sockets. All connections in this area are made with narrow, twisted wires which further simplifies wiring in the chassis.

Day 2.

Having already assembled of the mains transformer I decided to start the Day 2 by assembling the power supply PCB. The first thing I had to do was widening all holes dedicated for external wires coming to the PCB. This was justified by the fact that I decided to give in these holes the "Turret" pins. This procedure allowed me later to solder wires from the top of the PCB.

Having prepared all the elements I proceeded to their fitting to the PCB. After a few minutes I had everything ready and it was possible to start soldering.

Fifteen minutes with a soldering iron in hand and the effect shown below. Unfortunately, I forgot to do some photos of the underside of the PCB. So I present only the pictures from the elements side. Sorry.

Next, I decided to connect the power tubes heaters. Here at once strikes the eye first difference between my version and the original one. I used twisted pair od wire while in the original design we have straight sections of wire. Why so? I did not have adequately thick wire to lead the 9A of heater current and besides I did not want to risk the creation of possible negative side effects - I apply the principle to always use twisted wires in a heater supply circuits.

Wires going to the tube sockets have been combined with the leads from the power transformer and then placed in the "ankle panel".

Then I installed the output (speaker) transformer and choke.

I started by connecting the anode fuse with choke and output transformer. Then I brought the voltage to the lindicator and the constant voltage for tube heaters. I also prepared the connections from the choke to the screen grids of output tubes and to the preamplifier PCB.

Then I installed the trim system (small potentiometer) for biasing.

The next step was to complete the wiring of the connection block. I moved out wires of the local mass and attached one end to the chassis.

Probably wondering is the maneuver executed with a purple line. The reasoning is simple - I cut it too short and it did not reach the point to which it had. Fortunately, there was one not used place left in the connection block, and I could use it.

At this point I decided that power wiring stage was completed. I made an attempt to fit the power supply board.

At this stage, I decided to end the second day.

Day 3.

I came to the conclusion that at this stage the power supply PCB could be safely mounted on the chassis, but before that, I had to do wiring of the tubes heaters.

Next, I soldered heater balancing resistors under the power supply board (resistors are visible under white and purple wires). In this design, a balanced supply is applied just for the preamplifier which has a DC heating. The power stage tubes use AC heating without balancing. This solution was discussed on TRIODA Forum several times so I decided to check how it behaves in practice.

Before attaching power supply PCB I decided to connect supply to output tubes anodes.

Then I put the power supply PCB in place and soldered all the necessary connections. It turned out that all the dimensions are perfect - PCB snapped into place easily and without interfering with other elements. It is possible to remove connectors from switches mains socket without removing the PCB.

Unfortunately, for reasons of lack of time I had to stop the assembly on this day.

Day 4.

I decided that the next thing to do would be to clean up the wire mess of the output transformer.

After prior preparation of mains socket, and then mounting them along with the switch I could cut the output transformer (TG) wires to the desired length and then proceeded to solder them.

In a later stage of the installation, it turned out that by mistake I cut yellow cable too short and I had to exchange it for longer. Fortunately, it was not a big problem. At this I finished another day of amplifier assembling.

Day 5.

The next day I decided to assemble power stage of the amplifier. I installed power tubes sockets just in view of the assembly of the entire system exactly as it looks in the pictures below.

The next step was to assemble the sockets dedicated to external measurement of volatges for biasing of output tubes.

Then I installed a footswitch jack and a switch to change channels on the front of the amplifier.

At this point, I had assembled virtually anything that does not require the need to connect to the preamp PCB. Now is the time to mount it. But I decided to do it until the next day. This day I prepared all the elements, so that I can start the next stage from soldering them.

Day 6

Similarly to the power supply PCB also on preamplifier board I used solder pins in places where wires should be soldered from the top. The next step was to solder all jumpers. Then came the resistors, bridge rectifier of the switching circuit, small capacitors, and at the end of VTL5C1. At this point the PCB looked like the pictures below.

Red wire on top of the plate which connects point B2 and GND is the power supply to the point B2. I am writing this because I have this question on the mail so I decided that I need to explain this issue in order to avoid any misunderstanding.

Then I installed all the other capacitors.

And I also connected the wires that I already cut to appropriate lengths.

At the end of the day I connected the power supply to the LED signaling Overdrive channel activity, I connected Bright / Normal and Clean / Crunch switches, I installed all the potentiometers on the chassis, I assembled the input jack and I made a grounding bus for all potentiometers.

At this point I had to stop assembly for two weeks because I went to college. However, after returning home I finished work the next day.

Day 7.

Before leaving, I had prepared all the elements of the amplifier, leaving only the mounting in place the preamp PCB and making the necessary connections so I was able to start the first tests.

At the beginning I checked the power amplifier and an inverter to verify that I properly connected the output transformer and anodes of output tubes. Then I put the tube in FX back loop circuit, plugged a guitar into the Return input, and I carry out the first test tone. As expected, nice clean sound. Next I checked the equalizer and other adjustments. Everything was OK.

There was nothing left to put the remaining tubes and check the amplifier's possibilities. An hour test with a guitar showed that everything works as it should. The sound is perfect (for me). Clean channel has a nice, classic color of sound. In Crunch mode we have the perfect channel for playing blues and rock ballads. The real power of this monster, however, without a doubt, is in the Overdrive channel.
My preferred level of gain this amplifier reaches around 6 - 7 on the scale so the 11 is still a little uplift for real "geeks throwing debris".

I set the bias current at 42 - 44mA (discrepancy detected in the measurement points). The next day I did the hardware test of endurance. After eight hours of continuous work - playing music from the CD player and guitar - mains transformer heated up but even so quietly that you could hold them with your hand and speaker transformer was only slightly warm. The test was conducted with the amplifier in the housing, in such conditions in which it wiould operate in real conditions.

At this point the project is considered to have ended. To sum up assembly process - no major problems were encountered except inversely connected power to tube anodes and swapping Footswitch wires. These two mistakes were localized and corrected immediately.

If you have any comments or suggestions that may be useful to me for the future don't hesitate and comment this article or ask questions on the TRIODA Forum in this topic -

Just a few pictures at the end.

At the end some sound samples. Files with the "slo" prefix were created using described aplifier. Please, do not comment my professional skills :). I did what I could.

At the beginning of this year the amplifier got a new case - here it is, in all its glory.

Written by Damian "Ghost" Smółka