Soldano Preamp

Build name: Black box

Background

This preamp is part of a bigger picture idea I had in mind. To build a couple of bare-bone preamps, no switching, no EQ and other nonsense, to get a decent graphic EQ pedal and to build digital logic for switching them. Concept of having digital circuits only control all-analog signal path was always interesting to me and this preamp is the first step in that direction.

I wanted to start with a high gain preamp that will cover Dream Theater grounds, so I started off with Soldano SLO 100 design. I removed the tone stack, FX loop, clean channel and the power amp. Later I found that it's actually closer to the x88r preamp. Removing the clean channel left me with a spare triode in the first tube. I decided to wire it in parallel with the first gain stage. To compensate for the change, the first plate resistor is lowered to 120K so that each triode sees 240K (close enough to stock 220K value) and I put two 1.8K||1uF pairs on the cathodes as I didn't have 900ohm resistors, nor 2uF caps. With those changes, parallel stage should be biased about the same as in the stock circuit. Benefits of the parallel stage should be lower noise, a bit less treble due to doubling of Miller effect and more complex tone, since two triodes are never the same. I also included cathode follower mods suggested by Merlin that's supposed to reduce the stress on tube during startup without affecting the tone. Finally, I decided to use 12.6DC heaters, regulated and heavily filtered. Heaters are also elevated to 75V to make cathode follower tubes' lives easier. Without the elevation cathode followers are operating at higher cathode-grid voltage than they should, according to data sheet - Vh-k(max)=180V.

Schematics
Black Box Pre schematic Preamp - click to open full size
Black Box Pre power supply schematic Power supply - click to open full size
Black Box Layout Chassis layout - click to download PDF file
Construction

I found a nice sheet metal pre-painted box in the local store that seemed OK for the job. What's nice about it is that every panel may be removed individually which makes it easier to install boards, and mess with the circuit later. Front and back panels are aluminum and the rest of it is steel.

For circuit boards I decided to use some pre-made generic turret board and tag board I had lying around. Making circuit boards is not my favorite part of making amps, especially drilling boards, so these boards saved me some times and nerves.

As usual, I got a custom wound toroid transformer that has everything I need:
  • 300-0-300VAC@40mA high voltage secondary
  • 14-0-14VAC@2A heater secondary
I got both secondaries center-tapped because it's easier to layout full wave rectifier with only two diodes and it doesn't require virtual center tap to elevate the heaters. 14VAC are there to provide few extra volts before the regulator.

Both volume and gain pots' wipers are connected to the board using shielded teflon wire. Shield is grounded on the board side and connected to the pin 3 on the pot. That way pot is grounded together with the stage it controls.

Pictorial

Starting off with the power supply boards. The board on the left is for high voltage power supply and heater elevation circuit. The heater supply board is shown on the right. 7812 regulator used for heaters is not shown here because it's mounted on the chassis with thermal paste and mica insulator to prevent it from shorting with the chassis. Since the regulator body is connected to the center pin, it is also elevated to 75V so it has to be isolated from the grounded chassis.

Tube rail is made from sheet aluminum and has both sides bent downwards at 90 degrees for added structural support.

Here's the main board wired to the tube rail. You can't see the filter caps because they are mounted on the other side of the board.

While testing I used cheapo Chinese tubes but later they got replaced with JJ tubes. JJs sound warmer and are less noisy. Also two of the Chinese tubes turned to be microphonic. I'm not buying them any time soon.

Gut shot from the other angle, with tube shields installed

Wiring for the first tube. I made terminal with a single eyelet mounted on a piece of fiber board screwed to the rail using a L shaped aluminum profile. It works pretty well as strain relief. I don't like soldering grid resistors directly to the lead. It will eventually break while moving the lead around.

The second tube uses the same way of mounting the grid resistor. Since I'm running the heaters in series pin 9 is not connected so it's heat-shrinked on all tubes.

Here you can see the protection diode in series with 10K resistor going from grid to cathode. It's heat-shrinked to prevent it from shorting with something. Grid resistor doesn't need strain relief because it's not connected to a "flying" lead, but directly to the board.

The last tube has the same diode protection on the cathode follower. Plate resistor is mounted directly on the socket.

Result

This preamp turned out exactly as I wanted at that time. Tons of gain! It gives awesome saturated, oozing lead tone that I can shape later with my graphic EQ pedal. Leaving the 4th preamp tube from the SLO circuit makes the preamp more versatile. With gain pot I can control the amount of distortion that comes mostly from the cold clipper stage, but with volume pot over half the last stage gets overdriven. It gives different overdrive characteristics, since it's warmer biased and followed by the cathode follower. Backing down on gain and cranking volume sounds warmer and more compressed than doing it the other way round. By playing with two knobs I can dial anything between warmer crunch tones and heavier distortion which is very cool.

However, it must be noted that this is really a one-trick-pony for metalheads that play only high gain sounds. And it's great at that. After few years I was beginning to miss the 3-channel SLO I built before with it's lush cleans and nice breakup from the crunch channel, so I eventually sold this preamp and started thinking about multi-channel builds with less gain.

Mods
As any other build I just couldn't let it be, so I fiddled around and changed a couple of values. I noticed that bass is a bit flubby when doing that lower-gain palm mute rhythm, so I did three things:
  • Lowered first coupling cap from 22nF to 10nF to tighten the low end just slightly.
  • Lowered one of the two bypass caps on the input stages from 1uF to 0.47uF, thus making each of the parallel triodes "see" ~0.73uF which is awfully close to Marshall input stage bypass cap. This mod made the most noticeable effect on reducing the "flabbyness".
  • Added 470K resistor after the first coupling cap to ground. When I took the lead channel out of SLO/x88r I neglected the load of other channel(s) to the first stage. This compensates for it and reduces gain a bit (which is a good thing in this case!).
Video Clips
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Comments
22 Responses to “Soldano Preamp”
  1. Shaun says:

    Hi Bancika,

    Just looking at your SLO preamp, very nice ! I was wondering if I could ask you if you can remember the approximate voltages for the B+1 B+2 and B+3 supplies (within 10%). I have a 400 V 50 ma supply which I can drop using resistors and cap filtering, and would also allow me to elevate the elevate the heaters (as you have done). Don’t worry if you cannot remember i’ll just start out at 380 V on B+1 and see where it takes me 🙂

    • Bancika says:

      I don’t have exact numbers, but I was aiming for something close to SLO. 360-380V sounds about right.

      • Shaun says:

        Many thanks, based on our conversation I’ll go for 350V 360V and 380V and let you know how it turns out.
        Many thanks for taking time to reply 🙂
        Regards
        Shaun

  2. abstrus says:

    Hi all!
    Looks like a very interesting project!

    Which tubes do you use here, i guess its not written here…
    JJ EC803S, JJ ECC83S ?
    Any recomendations?

    thanks..
    klaus

  3. Stephen says:

    Very nice. The layout, the dress– the.. heck, you’ve approached this as though designing a case for a PC – which is very smart really.

    One question: where do you go to get your custom toroidal tranny built? Would appreciate knowing this, and if you don’t mind – how much you paid. It’s a fantastic way to go… if ya get around the one or two issues of Toroid, they’re just perfect for amps like this.

    Well done – brilliant job man.

    Stephen

    • Bancika says:

      Thanks for the comment, appreciate it. I buy my toroids from two Serbian companies specialized in toroids. Trafco.rs and trafomatic.com. Custom preamp toroids about 20va with several secondaries used to cost about 20$ each, for one off builds. Larger orders would cost less. I also get larger toroids from them for whole amps and they are affordable as well.
      Cheers

  4. flametubes says:

    hey man, have u tried the schematic with the minitubes? 6021?

  5. Carl says:

    Sounds great! Is there a detailed parts list available?

  6. Marius says:

    Hello,

    thanks for posting the schematics! I have a little problem and hope you could help. I built the preamp according to your schematics but there is a horrible feedback inside the amp. Lots of hum and screaming. If I connect the guitar the pitch of the scream changes hehe. Connecting the first 4 tubes by themselves with the guitar works nicely, and connecting the last 4 tubes by themselves with a signal generator before the 5th works too. But all of them together is a disaster! My only explanation would be that there is a feedback through the power supply section. Do you have any ideas or did something like that happen to you at some point?

    Thanks a lot
    Marius

  7. hi,

    ok,i figured out what the tubes are all about,my next question is what resistors are plate resistors?

    thanks !!!! grtz Tom.

  8. hi again,

    a few more questions,on the shematic tube V1,is this one tube or are these two seperate tubes that are called V1 and what about tubes V3 and V4 are these each time 2 separate tubes with the same name.
    i supose with tube V2a and V2b you maen one tube from witch you use both sides of the twin triode.

    thanks !!!
    grtz Tom.

  9. hello,

    thank’s for the fast reply,it all comes clear now,i will try to build this preamp.

    greetz Tom.

  10. hello,
    i am not verry familiar with tube stuff,but what i was wonderig what type resistors and capacitors you use,film,mica
    ceramic,etc…..voltage?1:4 watt,0,5 watt,1watt?

    your preamp sounds amazing,nice job,i’am also a fan of dream theatre.

    thans Tom.

    • Bancika says:

      Resistors are 1/2 except for plate resistors which are 2W. Coupling caps are mostly papar in oil but the original uses film caps (orange drops or mallory, don’t remember). You can use any film or paper caps as long as they are rated at least 400V. Mica caps are used for values less than 1nF.
      Cheers

  11. oneleg says:

    DO YOU HAVE Soldano Preamp FULL SCHEMATICS?PLEASE SEND ME,THANK YOU!!!!!!

  12. Mykas says:

    Very nice..impressed with the build
    quality and the tone.

    ~Mike

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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.