Build name: Black box
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.
Power supply - click to open full size
Chassis layout - click to download PDF file
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.
- 300-0-300VAC@40mA high voltage secondary
- 14-0-14VAC@2A heater secondary
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.
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.
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.
- 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!).