Soldano Preamp MK2

I’ve been a fan of Soldano amps for many years. A decade ago I built a 5 watt version of the legendary SLO 100 and loved it but eventually sold it because of lack of space in my new apartment. Then I built a single channel version of the preamp, loved it but sold it after few years because it lacked the versatility of the 3 channel version. So now, many years later, I’m itching for SLO sounds again and I’ve set the goal to build the 3 channel version of the preamp in a compact format with simplified switching that doesn’t involve expensive opto-isolators. As a starting point, I used the official SLOCLONE schematic, took the preamp circuit from “Input” jack to “Send” jack and added two Volume controls at the end.

Layout

With this build I wanted to try a different point-to-point layout technique that is challenging and requires a lot of planning, but should yield very compact layout with short leads between the components and following the signal path closely to avoid any risk of excess noise or feedback. It uses point-to-point layout with components arranged in two planes that are parallel with each other and orthogonal to the tube sockets. One plane carries components that go directly from tube sockets to the ground bus (mostly cathode resistors and bypass capacitors, grid reference resistors) and the other plane contains the rest of components that go to tube sockets. The second plane uses a terminal strip that is used to connect the components to the controls that sit on the other side of the terminal strip. Additional components may run from the terminal strip to the ground bus (ground reference resistors, snubber capacitors, etc). I got inspiration for this layout after seeing a similar technique used in some 50s ad 60s NASA tube equipment in the Smithsonian National Air and Space Museum.

Mods

SLO preamp is pretty much perfect as it is, so this time I didn’t bother too much to modify it beyond recognition. As mentioned before, I changed the way channel switching is implemented, mainly to simplify the build and reduce the cost. I applied the same relay switching to Mark IIc+ build and it’s been working for 4 years already with no issues.

The second change I made is a switchable bright cap on the OD channel gain control, a.k.a. switchable Warren Hayes mod. That way you can go between the stock (bright) mode and the Warren Hayes mode (less harsh highs when the gain is rolled down).

One practical change I made is to wire the tube heaters in series and power them with 12.6V instead of the typical 6.3V parallel mode. The reason behind it is that doubling the voltage effectively halves the current, so there’s less EM radiation around heater wires. Another minor reason is that series wiring uses only terminals 4 and 5 on the sockets and allows for a cleaner layout. The downside is that you cannot use Russian tubes anymore. Also, with 12.6V, the current draw of the pair of 100 ohm resistors in the virtual center tap is too high (63mA with 0.8W dissipation!), so larger resistors should be used. A pair of 220 ohm resistors will dissipate around 0.36W, so I went with a pair of 330 ohm 1/2W resistors drawing around 0.25W, just to be safe. If using 12.6V heaters, another alternative is to elevate the heaters by connecting the reference DC voltage to the unused center tap (pin 9) of on of the tubes instead of referencing both sides through a pair of equal resistors.

Finally, I made it possible to choose between having completely isolated channels or leave the normal channel bleed into the overdrive channel like it does on the original amp. Clean signal is out of phase with the overdrive signal and is usually quieter, so it will effectively cancel-out some of the overdrive signal. The difference is subtle and may or may not be intended by the designer of the circuit. I’m leaving it up to you to decide which version to build. The layout below is drawn with channels isolated, but it’s easy enough to change it to have clean channel always on. Just connect the 2M resistor paralleled by a 120pF capacitor to the terminal that joins the 220K grid resistor and 330K ground reference resistor (one terminal to the left from where it is connected on the drawing). That way, normal channel will always go straight to the V3a triode without being affected by the channel switch.

It is worth mentioning that I changed the power supply slightly compared to the schematic. I reduced resistor values from 15K to 4.7K to reduce voltage drop and have voltages similar to the original with the 275VAC secondary and added an extra RC node to improve filtering. Duncan’s PSU Designer app confirmed that these capacitor and resistor values should work well, giving the correct voltages without any noticeable ripple. 4.7K resistor and 22uF capacitor in each node form a low pass filter with cutoff frequency around 1.5Hz which is more than satisfactory for our application.

Switching Options

If footswitch operation is not needed, the simplest way to implement channel switching is using a DPDT switch that switches between different signal paths. The layout is optimized so there are no long leads carrying signal to and from the switch that would pose a risk for noise or feedback. In this case, just use the layout as it’s drawn, ignoring the “Footswitch Circuit” section.

If footswitch operation is needed, the DPDT switch used to switch between the two signal paths can be easily replaced with a miniature DPDT latching relay that is powered from the heaters secondary (or a separate secondary). The relay can be switched either using a footswitch pedal or with a panel-mounted SPDT switch when a footswitch is not plugged in. Note that miniature relays need between 150mW and 500mW of power (DC) for the coil to switch. Heaters secondary must be able to provide extra 0.4-1VA AC (depending on the relay type) or a separate power transformer should be added to power the relay. The AC heater voltage needs to be rectified and then filtered and regulated before we can use it to power the relay. Omron G5V-2, Finder 30.22, Takimisawa RY12W-K or similar relays will work fine here. Choose relay voltage and regulator voltage according to the heater voltage leaving at least 2-3V extra for the regulator to work properly and taking bridge rectifier diode drops into account. 12.6V AC heater winding should be enough to drive a 12V DC relay. 6.3V AC heater winding should be enough to drive a 5V DC relay (probably not enough for 6V, though!). Use a switching jack to disable the panel-mounted channel switch when a footswitch is plugged in.

Footswitch circuit uses a bi-color LED to toggle between the two colors designating the selected channel. A P-channel MOSFET is used to switch between the two LED colors. Alternatively, two separate LEDs can be used the same way with cathodes tied together. For my build I wanted to toggle between red and blue color and have a matching knobs on pots that control each channel. So I got a RGB LED and just used the two diodes ignoring the green one.

Parts
  • The enclosure measures 250 x 180 x 80mm and is made from sheet aluminum, powder coated in black
  • Power transformer is a custom wound 20VA toroidal with 270VAC @ 40mA and 12.6VAC @ 0.6A secondaries
  • Filter capacitors for the power supply are Panasonic NHG 450V electrolytics
  • 1uF capacitors for cathode bypass and output capacitor are Panasonic ECWFE metallized polypropylene film capacitors rated 450V (total overkill for cathode, but it was easier to get all the 1uF capacitors of the same kind)
  • 22nF coupling capacitors are Russian Paper-in-Oil caps rated 400V
  • 1nF and 2.2nF caps are some 630V poly film caps I already had
  • Small capacitors in the picofarad range are NP0 ceramic disk capacitors rated 500V
  • Signal resistors are 1/2W metal-film Xicon, plate resistors are 2W metal-film KOA and power supply uses 2W metal-oxide KOA resistors
  • Pots are 24mm Alpha
  • Jacks are closed-style with isolated threads, so I can control grounding to the chassis
  • Switches are Miyama
  • Tube sockets are Belton Micalex VT9-ST-C with an integrated shield
Construction

I started by mounting the three sockets on the tube bracket.

Power supply is built on a separate terminal strip.

All parallel resistors and capacitors are wired together to simplify wiring to the sockets later.

Heaters are wired together with the 220K resistor on the third tube before the power supply is mounted on the bracket.

Then the second layer is populated with cathode resistors/capacitors.

Finally the top layer is populated together with the ground reference resistors that go between the top layer and the ground bus. I also wired the leads for input and output jacks as well as the lead that comes out of the normal channel’s Gain pot. All shielded leads’ shields are directly soldered to the bus. Teflon coating prevents the heat from melting the insulation and shorting the inner conductor with the shield.

From the side you can clearly see the three separate layers of components. At this stage, the main module is ready to be tested, first without and then with the tubes attached. We can test if heaters light up, check the heater voltage and drop it with a pair of resistors if it’s higher than 12.6V. Then we can test the power supply and make sure that it outputs the expected voltages and we can do the basic testing of the circuit by measuring plate and cathode voltages of all triodes. SLOCLONE schematic lists all important voltages, so we can use those as a reference.

The switching module I put together on a piece of terminal strip with narrower terminal spacing to save some space, as there are no big components to mount here. The three leads on the left that go to the LED(s) are color coded to make the wiring easier.

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    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.