5W Soldano SLO


Soldano SLO needs no special introduction. When it came out in the late 80s, it redefined high gain hot-rodded Marshall sound and served as an inspiration to countless modern amps (looking at you Randall Smith!). It adds a cold biased clipping gain stage that has that refined, smooth, yet punchy distortion character. I always wanted to have one, but didn’t really need all that power and couldn’t afford to buy one, so why not build a scaled down version that has all that awesomeness in a beadroom-friendly package?

The Circuit

I started with the official SLOCLONE schematic and analyzed the circuit, trying to figure out what would need to be changed to accomplish the goal. I figured that output power of around 5W would be plenty and looked around for tubes that could provide that kind of output in push-pull class AB operation with fixed bias, just like the original SLO. In cases like this, many people opt for a simpler solution that uses one 6V6 or EL84 in single ended class A configuration, or use a double triode in push/pull configuration, but I wanted to keep as much of the character of the original SLO, so I really wanted to preserve the topology of the power amplifier and use two power pentodes in push-pull class AB operation..

The smallest current production power pentodes I know of are EL84 but they deliver around 20W which is still too much for me, so I dug deeper into the world of NOS tubes and found some 7-pin small tubes from the 50s and 60s that were designed for much less power. On paper, EL91 sounded like a winner, so I bought a few from eBay and started playing the datasheet. EL91 are small 7-pin power pentodes, probably the smallest ones in the EL family and can deliver 1.7W in single ended and around 5W in push/pull configuration.

Circuit Changes

– The Power Supply

The power amplifier section had to be modified to use EL91 tubes instead of the 6L6 used by the original. Looking at the EL91 tube datasheet, the little El91 can take only 250V on the plates which is half of the voltage used in the original SLO to feed the big tubes. 500V would fry them within seconds for sure. I could reduce the voltage of the whole amplifier to 250V and run the preamp tubes at around 230-240V, but I didn’t want to risk changing the character of the preamp, so I went with a more complicated option that involves cloning the power supply, so we have two independent power supply branches that have separate secondaries on the power transformer and share only the ground reference. One branch delivers ~350V to the preamp and the other branch delivers 250V to the poweramp. Both branches are very similar to the choke-less power supply used in AX84 projects. The negative voltage bias supply is also taken from AX84 projects.

For the power transformers Iā€™m using a custom toroidal transformer, wound by a local company Trafomatic. They can wind even a single custom transformer with any given number of primaries/secondaries. All that at a very good price (much cheaper than EI style Hammond transformer with similar power). For this particular transformer, they wound it according to these specs (voltages are under load): 220VAC primary, 300-0-300V @ 40mA, 200-0-200V @ 100mA and 3.15-0-3.15V @ 3A secondaries.

One more thing I did differently than the original SLO is to use the same secondary for powering the heaters and to power the switching LDRs.

– Output Stage Changes

The small EL91 tubes have less headroom than the big tubes, so I was concerned about overdriving the output stage too heavily. To compensate for smaller tubes, I put voltage dividers instead the two plate resistors (R36: 82K and R37: 100K) of the phase-splitter stage, so I can tap into the output of the phase-splitter and take only a portion of the voltage swing it can produce. Normally, we take the (maximum) output from the plate directly, but we can tap the plate resistor and take potentially less than that. The 82K resistor is replaced with 27K+56K and the 100K resistor is replaced with 33K+68K where the smaller of the two resistor is facing the plate. That way, we are effectively reducing the output of the phase-splitter by about a third.

Another way to achieve very similar results would be to use a lesser gain tube for the phase-splitter, like 12AT7, 12AY7 or even 12AU7, but I was anal about it and wanted to keep the same tube, bias it the same way as the original, just reduce the output.

– Other Mods

  • Replaced 12AX7/ECC83 preamp tubes with Russian military 6N2P-EV tubes. These are about the same as 12AX7 with a tad less gain and slightly different heater wiring that operates only in 6.3V mode. Pin 9 is used as internal shield between triodes that should help reduce noise and it should be grounded.
  • Added Depth control in the negative feedback loop (NFB). It’s possible to get this as an optional feature when buying a SLO from Soldano, so I thought – why not. It gives extra control on the bass making it easier to nail the tone that’s fat, but not boomy. In the SLOCLONE schematic it’s marked as VR9.
  • Reduce fx loop “Send” level to be more pedal friendly. Some say that SLO has a relatively hot FX loop that works well with professional effects that can take line level signal without clipping, but could drive regular stompboxes into clipping. To reduce the level, I added a 1K (R50) resistor in parallel with existing 2.2K resistor on the cathode resistor of the last triode before the “Send” jack. That reduces the output level. To compensate for the lower signal level we send out of the amp, the triode in the recovery gain stage after the “Receive” jack cathode resistor is bypassed with a 1uF capacitor (C13). It boosts all frequencies above ~70Hz for 6db.
  • Switchable treble bleed capacitor on the OD channel gain pot (a.k.a Warren Haynes mod) to change the high end response when gain pot is not turned all the way up. In the SLOCLONE schematic, the switch is S3 and it takes C6 in and out of the circuit.
  • A single 8ohm output jack installed. No impedance switch.
  • Added a switch to choose between 2ohm and 8ohm OT taps for the NFB loop. The 2ohm tap sends less signal to the NFB and it makes amp sound a bit more aggressive.

For the output transformer I got a Hammond 125C which is capable of taking up to 8W of power, so it’s more than capable of handling the output of EL91. The secondary is wired to lugs 2 and 4 which give about 22.5K primary impedance, according to the Hammond 125-series datasheet. Two EL91 in push-pull like to “see” 20K which is close enough.

With only ~5W of power, there’s much less current, so B+ line needs less capacitance to filter out AC ripple. I got JJ 100uF+100uF 500V can capacitor, so that each power supply uses one 100uF section as first filter cap. The rest of the filter caps are mounted on the main board, so there’s no need for a dedicated power supply board.

I should note that photos show ELON and Tung electrolytic capacitors that I bought for the project but they started causing problems pretty much since day one and I finally realized that they are fake Chinese capacitors that have old capacitors pulled from who knows what old device with completely wrong specs. I took them out as soon as I found out and replaced them with F+T caps. It served as a lesson never to buy cheap electrolytic capacitors again. Since then I have switched to using brand name capacitors bought from reliable shops.


For circuit board I used a beautiful blue 1/8″ thick fiberglass board from turretboards.com, sized 14.625″ x 3.125″. The board is drilled according to turret board drill template from SLO Clone forum, with some changes. I have added a section on the board that will have bias power supply, indicator LED supply and screens filter cap. Template is shrinked for few percent to fit everything on the board. Instead of turrets I used eyelets (almost 100 of them). I already used turrets on my Firefly build, so I wanted to try out eyelets. They are nice to work with, easy to install and solder to.

I got a great aluminum chassis powder coated in black really cheap. It’s much smaller than chassis suggested for SLO clone, but without power supply board and choke it’s a nice fit. Aluminum is very nice to work with, compared to steel I used for two previous amps, especially without tools for drilling larger holes. The biggest drilling bit I have is 10mm so all bigger holes (for jacks) need to be either widened from 10mm or (for sockets) use multiple holes of 3-4mm diameter that approximately fit into the circle and then file hole to make it round. Not much fun.

The faceplate is laser engraved in black plexiglass according to my design. After that I used orange marker to highlight engraved area to match tolex color.

At this point heaters have been wired and pots installed. Note that preamp tubes are wired differently than standard 12A*7 tubes. Pins 9 are later grounded to utilize internal shield 6N2P tubes have.

Details of power supply and output stage wiring. There’s a small daughter board on the right that contains the two bridge rectifiers – one for each power supply.

Preamp part of the circuit.

Gut shot with wiring all done.

Outside view to finished chassis. You can see the cool looking toroid here.

I love this photo so I had to put it here, probably one of the best DIY photos I took at the time.

Cabinet is made from 2cm thick plywood and later covered with range vinyl.

And finally finished amp.

And back view…You can see small toggle switch for switching between 2 and 8 ohm taps for NFB loop.

Useful links

SLO Clone Forum

59 Responses to “5W Soldano SLO”
  1. Dylan says:

    Heya Bane. I am thinking of taking this on as my first DIY project. Wondering if you’ve ever chosen to share the actual circuit design file that you used to achieve these changes:

    Sections on the board for bias power supply, indicator LED supply, and screens filter cap.
    Resizing it as you described



    • bancika says:

      Hi Dylan,
      there are no secrets here šŸ˜‰ Schematic for most of the power supply is available in the article – screens supply point is marked with B, bias circuit is also there. I left out switching power supply because it’s the same as in the sloclone schematic. LED indicator is powered off of the bridge rectifier for the switching power supply. You can see on the photo one resistor and one filter capacitor for that led. The resistor is I believe 2.2k, and cap can be anything you want, 4.7uF is a good starting point. You can tweak the resistor to achieve the required current and brightness for the LED. On the board, this whole section is on the right side, see this http://diy-fever.com/wordpress/wp-content/gallery/5w_slo/Picture120.JPG Large tung cap is for the screen supply, below it is the bias supply and you can see where the LED is connected to.
      I do not have layout drawing as this was planned approx 15 years ago and back then I was drawing layouts on paper šŸ™‚
      Also, I would not suggest this as a first project as it’s a fairly complex circuit even without my modifications.
      Hope that helps

  2. Joe says:

    Thanks for these great DIY project logs, I’m really looking forward to building a 5w SLO as well as a hot-rodded AFD style, amp using the same 5w power section design! Do you recall the chassis dimensions you used on this build? I can estimate ~17″W x 8″L x 3″T but I’m not certain? Also assuming I use 12ax7’s in the preamp instead of the 6N2P-EV tubes you used, would it be worthwhile to do DC heaters to cut down on noise or did you find that to be overkill with your first SLO preamp build? Is simply elevating the 6.3v AC heaters enough to make the noise manageable? Thanks again!

    • bancika says:

      Yes, I think your guess re: chassis size is correct. Heater elevation together with the virtual center tap (two 100R resistors) will get rid of most of the noise. On my latest build I used 12.6VAC heaters elevated with center tap (330R this time) and it’s pretty quiet. With 12.6V the current draw is halved, so less EM radiation around the wires. DC will be even quieter, but whether it’s worth bothering, I’m not sure… šŸ™‚

      Whatever you decide to do, proper lead dress is equally or more important to achieve quiet and stable build.

  3. Serj says:

    Hi Bane,
    Do you have a board layout pic? I’m pretty bad at drawing optimal component locations and wiring. Usually end up with some spider-web of wires ))

    • bancika says:

      Hi. I don’t have one, but I used one from sloclone forum. Try searching for sloclone turret board layout.

  4. Maciek says:

    Good job on this one, bro. I listened to the clips you put up on YouTube and it does sound impressive!
    I would like to ask you about the grounding of the power supplies. You wrote that they share a ground reference. How exactly did you connect the grounds? I’m asking because I am looking for verified ways of implementing two power supplies in a tube amp I’m working on right now and I’m not that skilled yet.
    Thanks in advance.

    • bancika says:

      Thank you for the comment, appreciate the kind words. In this amp I used star grounding scheme. You can see the common grounding point with all the lugs going to it on one of the photos. Both power supplies go to one side of the 100+100uF can capacitor which has a common ground that is connected to the star point. All other ground connections go there too.

  5. dhimas says:

    Hi bancika i have request for you, can you build mesa boogie dual rectifier clone?

  6. Nick says:

    Hi! I’ve been planning to build a SLO 5W for a while and after I saw your blog I’m determined to complete it! I love the board design you used but I wanted to know which metal film resistors did you use? brand and specs?
    Thanks a bunch for having a blog that’s a gold mine to us diyers šŸ˜‰


    • bancika says:

      I use various different brands, depending on the situation. For amps where space is not an issue, I use Xicon 1/2W metal film which are larger, easier to work with (also they should have a bit less noise due to the size) and cheap. You can get those on Mouser. In most other cases I use generic 1/2w or 0.6w metal film resistors I get in my local store. They are approx half the size of Xicon but they work ok and they are cheap. When I want to get fancy, I use Dale CMF 1/2W resistors that are more expensive, but at least they look cool šŸ™‚
      It’s hard to go wrong with metal film resistors these days, as most of them will work just fine.


  7. Matteo says:

    really great job !
    I am building a low watt amp using two EL91 in push-pull and OT Hammond 125B for 8 ohm output secondary impedance.
    The ampli sounds great but i cannot get 4 W of power from OT, but only 2 W.
    As for tubes configuration i followed datasheet, they are philips EL91 as you reported on your article, and i built a circuit with 250 V on plates, 560 ohm share cathode resistor bypasser with 22uF cap for a quiescent grid-to-cathode bias of 13.4 V.
    When i check output power, injecting 100mV,1 kHz signal into ampli, the undistorted sinewave i get on my 8 ohm dummy load is only 4.3 VAC rms for a total power of 2.3 watt RMS.
    Have you noticed this behavior on your ampli ? can you please tell me any suggestion you may have ?

    • bancika says:

      I haven’t measured power output on mine, so I can’t tell exactly how much output it was producing.
      With the values you provided, at the cathode you are drawing ~24mA. That’s combined plates and screens. In the datasheet it says 14.5mA per plate and 2mA per screen which should be 31mA in total. That could explain the difference you see at the output.
      Looking at the datasheet, in cathode bias configuration it needs 820ohm resitor per valve, or 410ohm if you are using one resistor for both. Can you try that and see what you get? Also, I’d try a much larger bypass resistor. It should tighten up the sound and make it closer to fixed bias. Did you use 15k primary or close to it?
      Either way, there’s no much difference in loudness between 2.3w and 5w…

  8. Jonathan says:

    Thanks Bane, that’s a relief, as I really couldn’t figure out any way of getting a primary Inductance that high.

    I’ll wind for 5.6H.

    I was also thinking of maybe stacking DC sources from a 200V power amp winding and 100v winding to get the preamp voltage as it gives better window fill in the power transformer and means I can use lower voltage capacitors.

    I’ll let you know how it goes.



  9. Jonathan says:

    Hi Bane,

    Sorry another quick question.

    I want to try this project, and decided to wind my own transformers. However it looks like the EL91 High anode impedance ~ 20K would require a primary winding inductance of ~ 159 Henrys to match the frequency response of the of the SLO100 OPT with 6L6’s.

    The Hammond transformer you used in this project only has a primary inductance of 5.6 Henrys so paired with EL91 in push pull assuming 20K : 16,8,4,2 ohm Primary to Secondary impedance ratios, it looks like you might start to get a noticeable loss from notes and harmonics below the 19th fret on the 4th string.

    Did you notice any lack of clarity in the Bass response?



    • bancika says:

      Hi Jonathan,

      I don’t recall any loss of bass or treble. That transformer is widely used on some low power amps, like AX84 Firefly…someone would have noticed.


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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.