5W Soldano SLO

Introduction

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.
Components

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.

Video Clips

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Click here to list all 9 related video clips.

Pictorial

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Useful links

SLO Clone Forum

Comments
32 Responses to “5W Soldano SLO”
  1. Tim says:

    Is this amp for sale? šŸ™‚

  2. robodile says:

    do you have a layout and BOM?
    -=-rob-=-

    • Bancika says:

      not for my version. I used layout and bom from slocloneforums.com and just changed a few things to accommodate the 5W output.

      • BP says:

        I have been trying to join the sloclone forum but I never get the activation email. Do you know if there are any issues there or who to talk to so I can get the issue resolved?

  3. robodile says:

    What Size chassis?
    -=-rob-=-

  4. robodile says:

    excellent site and work. also the SLO amp is very cool. What did you use as a chassis?
    thank you
    -=-rob-=-

  5. Yvo says:

    Hi Bane,

    I stumbled upon your awesome site! Great work!
    Like many others, I am also interested in your schematic. Especially in the power amp section.
    I read your comment about adding two resistors at the PI plate voltage. This will lower the plate voltage for the PI tube right? Correct me if I’m wrong. In which range is the voltage approximately? Or what were the values of these resistors?
    Also, if the output of the PI is lower, the overall output of the amp will be lower right? Because I’m planning on using 6L6 tubes like the original, but with a lower overall output (lower than 15W hopefully, is this possible by the way?).

    I hope to hear from you and keep the good work comming!

    • Bancika says:

      My output stage is pretty much the same as stock with the addition of voltage divider on PI anodes. I think I used resistors with ratio 1:2 to drop 1/3 of signal. But that’s only to accommodate smaller output tubes that can take less signal before being overdriven. I would not do it to lower the power of the amp, it’s pretty close to having a post PI master volume set to 2/3. If you want to use 6L6 I’d just build a normal 50w version and add VVR (variable voltage regulator) to be able to drop B+ and effectively drop output power to anywhere from 1 to 50w. It should cost only couple of bucks in parts and will give you ultimate flexibility. It’s shame to waste larger iron and 6L6 to do 15w only. You’ll want a louder amp some day šŸ™‚

      • Yvo says:

        Thank you for your kind reply! And fast too šŸ˜‰
        I could also build an SE power amp instead of a PP with only one 6L6. Less efficient, but easier to build.
        Or are there any good low power 6L6 equivalent tubes? Like your used EL91?
        For now, I don’t need the power and it doesn’t seem like I will in the near future. I already built an attenuator for my 15W Marshall Haze because of the noise complaints xD

        • Bancika says:

          Check out my 3w power amp. It uses 6ak6 in push pull. I’d stick with push pull, it goes better for this type of sound…or do 6v6 for 20w. If you use jj brand you can get away eith single power supplt because they should be able to take 400v on plates

          • Yvo says:

            I checked out your 3W amp and it looks awesome! However, after listening to your sound clips of the SLO, I think I will stick with your design that uses EL91’s. The power supply is the biggest problem in that case. Wouldn’t it be possible to make a voltage divider with high wattage resistors and some caps for filtering after the powering stages for the pre amp tubes? Or would it be better to find a PT with one tab for the pre amp tubes and one tab for the power tubes? 12AX7’s have the same specs except the wiring is different right?
            By the way, is your power supply the same as the SLO, except for the bias circuit? Or did you build a rectifier and power stages twice because of your power supply?
            I really appreciate your help šŸ™‚

        • Bancika says:

          I just made two same supplies, but one with 360v, the other 250v IIRC. None of them are the same as SLO, I just used supply from AX84 projects or something like that. No choke. Recto -> 50uF filter -> resistor -> 50uF filter, or along those lines.
          You can’t do it with one supply easily because dropping resistor will cause a huge sag, output stage is class AB, not A. You could try zener diodes, but you’ll need a lot of them to drop over 100v and they burn very easily, so there’s a lot to worry about re: heat dissipation.

          • Yvo says:

            Two supplies would be the only option then. I am pretty busy at the moment with my final tests of this year. Could I maybe sent you a schematic if I find the time to draw one up? Thanks for all the help!

  6. Dan says:

    Could you help me build one of these?

  7. Jaime says:

    Hey when you move on to something else can I buy it from you :0)

  8. Jo says:

    Hi there,

    This may not be at all anything that you can help with but I work for a climate change charity and we’re looking to do something around making/listening/recording music using only 5 watts. We’re only in the research stage at the moment but I just stumbled across your site and you sound like you’re a man that knows a thing or two about music and electricity!!

    Is there any chance you can give us some advice? Does this even sound like something that is possible?!

    Anything you can tell us would be massively appreciated.

    Many thanks, Jo

  9. Cris says:

    Hi,
    5 watt slo. That great. The 100 watt and 50 watt are bleeding my ears. I never mod slo before.
    So, you basically use the stock preamp, then just power down the output stage?
    In this case, you get 5 watt from AX84.com ‘s power supply style?
    AX84.com share the schem and layout. Whict power supply that you use?
    Is this one, “5W_SE: AX84 5W Single-Ended Poweramp Files”?
    I’m not sure, so I ask you about this, sorry. ^^^

    So if basically use the stock preamp, then just power down the output stage. The 5 watt PS design can be done to other amp design?

    Thank’s
    Cris

    • Bancika says:

      Yes, preamp is 100% stock. I changed the PI a bit to reduce the output. Also, I made a separate power supply for the poweramp because it can’t take more than 250V. I was stupid not to draw schematic back then because I can’t remember component value, but I’ll check.

  10. David says:

    Bancika, I’m planning to build an amp with el91 in push pull with 250V on the plates. can you share your schematic for the PA. Thanks

    • Bancika says:

      Hi mate. I don’t have the schematic but there’s not much to it: I used bias circuit from AX84.com push-pull amps (they are all the same). PI is long tail but I wanted to reduce the output to suit smaller power tubes. Plate resistors are split into two in roughly 2:1 ratio (68K+33K and 56K+28K) with smaller resistor facing the plate and coupling cap is connected between them. That should reduce the output by 1/3. Similar could be achieved by swapping the tube to 12AY7, 12AT7 or even 12AU7.
      I believe that grid reference resistors on EL91 are 220K but not 100% sure. I’ll need to draw the schem once, there were quite a few questions like these šŸ™‚

      Let me know if you need anything else.

      Cheers,
      Bane

  11. Rev. D. says:

    Just was going through the old post on Amp Garage and saw your link. Wow man what can I say even though SLO’s are not my thing (more a old blues guy) I just had to give my regards for a fantastic sounding amp. As I say not the style I play but I know what that Genre requires and to think your getting all that girth with 5 watts is incredible. I’ve built quite a few amps but I’m nowhere near as skilled as you are. I did recently pickup some ef184 tubes which I’ll do something with probably high gain, even though I don’t play metal, something along the lines of a EVH type amp would be cool just to stretch my chops into something different. Anyway, just wanted to say job well done.

    Regards,

    Don

  12. Angelo says:

    Nice amp – sounds great.

    Has anyone tried to register on the SLOclone forum – if you click on register and agree to their terms it doesnt do anything. Anyone know how to get a hold of the administrator? I cant find an email address or contact info.

    Thanks

  13. Tim says:

    Super AWESOME !

  14. Phil says:

    Where did you get the info — the schematics and such. I tried looking at the SLO Clone forums, but the site seems to be dead unless one is a member. Thanks.

  15. admin says:

    Hi Alf,
    I don’t have a schematic but there aren’t many changes in comparison to the original SLO. The biggest difference is that I have two power supplies, so you’d need to find a transformer similar to mine (specs are in the text) or use two transformers. The only circuit change is that I used two resistors in series for PI plates that reduces output level for ~1/3. You could also use 12at7 or 12ay7 for PI to reduce output level…
    Cheers,
    Bane

  16. Alf says:

    Bane , I like your new site a lot and am very interested in the schematic of the 5 watt SLO , do you have it on your site ?

    The 6n2p-ev tube I have too and would like to try them .

    How do you like their sound ?

    I also use the EL84 equivalent (6p14p) and am very positive about them.

    Great that you let other people benefit from your experience ! I personally appreciate it enormously.

    regards Alf

    • JBoothe says:

      Hey Bancika!

      Great site you have here. I’m just getting into amp building and it wasn’t until I actually came across your site and eventually made it to the sloclone forums that I decided on my first build! I also want to do the separate preamp and poweramp builds so I can stack them in a modular way and interchange them. I may even build separate tone stacks and have those inserted in a modular way as well. I have been building effects pedals and designing various FET preamps over the past couple of years so thanks for having such an awesome resource for fellow DIYers! It is often hard to find good quality resources online but this is definitely one of those rare gems.

      Thanks Again!
      Jeremiah

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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.