Dual channel bass/guitar blackface-style 1U preamp

with transformer balanced low impedance outputs

In 2004, I built my own adaptation of an Alembic F-2B dual channel blackface-style bass preamp. In 2007, I sold it to a bass player who saw it on my web page. I also received numerous emails from bass players all over the world who were interested in building their own version. In response to this interest, in early 2010 I started designing an updated version of a 1U dual channel preamp. The two main objectives for improvement were to increase the amount of clean headroom (input and output), and to be able to drive +4 dBu balanced outputs. I finished building the turret boards by May 2010, but the arrival of good weather, followed by the arrival of true love, left the project stalled at 75% complete until Fall 2011. I finished putting it together in October 2011, and spent the following month writing this page and updating the layout and assembly drawings to reflect as-built component values and voltage measurements. Forgive the lack of a true schematic; the layout contains the component values and wiring connections. The current layout diagrams do not show how the AC mains connections are wired. It's completely standard, but you should understand exactly what this means if you want to build your own. The assembly drawings are scaled properly, but the placement of the parts in the enclosure is not what I'd call blueprint accurate. If you attempt to build something like this project from my documentation, you are unlikely to use exactly the same sized switches, pots, jacks, etc. that I did. A 1U rack enclosure doesn't leave much margin for placement error, so it would be prudent to test fit everything by hand before drilling holes in your enclosure.

Dual channel blackface-style guitar/bass preamplifier
	Version 2 (2010/11)	Version 1 (2004)
Chassis form factor	1U rack mount, 10 inches (255 mm) deep	1U rack mount, 6 inches (150 mm) deep
Internal layout	Power supply and each preamp channel on separate turret board	Power supply and each preamp channel on single turret board
High voltage power supply (B+)	>= 300VDC @ 40 mA. Enough filament and B+ current for several preamp tubes per channel	240 VDC @ 3 mA. Only enough filament and B+ current for one 12AX7 per channel
Output	high current, low-impedance transformer balanced output on XLR and 1/4" TRS phone jacks, >16V rms at clipping into 600 ohm load	capacitor-coupled high impedance unbalanced output on 1/4" phone jacks, ~2V rms at clipping into 10k ohm load

Chassis form factor

As in my first dual preamp project, I wanted to retain a 1U rack mount form factor. With the additional features and larger power requirement of this project, though, I needed a deeper rack box. I selected a Par-Metal 10-19112B steel and aluminum chassis, with a net internal depth of 10" (255 mm), and an internal height of 1.42" (36 mm). The chassis is very strong, with a 0.125" (3.2 mm) anodized aluminum front panel, 0.06" aluminum rear panel (U channel), and 0.05" painted steel top and bottom plates, and painted steel channel sides. It includes aluminum rack handles.

Layout

I wanted to be able to experiment with preamp circuit variations without having to build a new platform. For example, I could build this project with a Fender blackface channel and an Ampeg B15 channel. Or I could have an Ampeg B15 channel and an SVT channel. Or I could assemble any combination of mix-and-match preamps I might fancy. I didn't need plug-in modularity, but I wanted to be able to swap a module in a day or so without tearing the whole rig apart, so I chose to build each preamp channel on a separate turret board. The power supply filter capacitors are located on their own turret board. The vacuum tubes are mounted horizontally to the chassis behind the turret boards on 1/16" aluminum angle bracket.

Power supply

My first dual preamp was powered by a small 6 VA power transformer with dual primary and dual secondary coils. Using one of the primary windings as a secondary and feeding it to a full wave voltage doubler gave me about 240 VDC at 3 or 4 mA. This is only enough for a single 12AX7 tube per channel. After a couple of filtering stages, the B+ was only about 170 VDC, which is kind of low for a blackface style preamp. A Princeton Reverb has a filtered preamp B+ of 240V, a Vibrolux Reverb gets 300V, and the Super Reverb, Twin Reverb, and Showman get 400+V. I wanted more clean headroom, so I needed a higher B+. Since I also wanted to be able to use two or three tubes per channel, I also needed more filament and B+ current.

To obtain a filtered B+ of at least 300V, I chose an Antek AN-05T280 toroidal power transformer. It has dual primary windings, allowing operation on 120 VAC or 240 VAC mains current. high voltage taps at 260 or 280 VAC at 250 mA. It provides 2 x 6.4 VAC @ 2A filament windings, which I connected in series to obtain 12.6 VAC for the filaments. The only issue with this transformer is its height - 1.6" inches, which I think includes the mounting hardware. Without the mounting hardware, it just fits between the chassis top and bottom plates, making a perfect sandwich. To secure the toroid in the chassis, I cut a cylindrical plug from round 1.125" delrin stock (plastic), fastened the plug to the chassis with a #6 machine screw, and placed the center hole of the toroid over the plug. I measured 335 VDC at the input of the second stage filter capacitor.

Given the relatively low gain of the preamp circuit, I decided it was suitable to run the filaments on AC.

Another small chunk of learning I gained from the project concerns driving the power-on LED from the12.6 VAC filament line. It's easy to calculate the current limiting resistor to run this LED from a 12 V DC source (220 ohms), but it's not a good idea to subject an LED more than 5 or 6 volts of reverse bias. As described in this wikipedia article, I added a diode in inverse parallel to the LED to protect it from reverse voltage. The diode and current limiting resistor are encased in heatshrink tubing as shown in the below right image.

Antek 05T280 50VA toroidal power transformer Antek 05T280
toroidal power transformer

power supply turret board with full wave bridge rectifier diodes, 2 R-C filter stages, and filament voltage reference divider power supply turret board

Preamp output

The output of my first bass preamp project coupled the plate of a 12AX7's second triode to a 1/4" phone jack with a 0.1 uF film capacitor. This high impedance unbalanced output was adequate for a reasonably sensitive power amp with a 10kohm or greater input impedance and a short cable. I wanted to be able to drive balanced line inputs of a mixing board or power amp over a long cable with minimal loss of signal fidelity and minimal noise pickup. Alembic's single channel F-1X provides a transformer balanced output, driven by some silicon. I have nothing against silicon, but in the interest of keeping my project all tube, I decided to drive the output transformer for each channel with a White Cathode Follower, optimized for maximum voltage swing. The output transformers are Edcor WSM15k/600 ohm 0.5 W plate to line units, the same model I used in my dual Pultec MB-1 microphone preamp clone. The output of each transformer connects to a male XLR and a 1/4" TRS phone jack on the rear panel. Both channels connect in parallel through 220 kohm series isolating resistors to an additional 1/4" TRS phone jack for a mono output.

Both individual XLR and 1/4" TRS outputs drive about 16V peak-to-peak into a 600 ohm load. This will upset the sound engineer who expects a microphone-level input from your "direct out". These outputs were designed for driving +4 dBu balanced input power amps, like this Bryston , or your favorite Crown, QSC, etc. It would also work well into any pro signal processing equipment you might have. The mono out (1/4" TRS) is attenuated, giving 6V P-P into a 600 ohm load. In retrospect, it is desirable to be able to drive lower-level inputs without overload. Therefore, I plan to incorporate a switchable pad for each channel's output to provide 2 levels of attenuation to accommodate the input signal levels for a wider range of power amps, mixing consoles, and outboard signal processing gear.
preamp
output transformers and jacks

Preamp modules

For my initial build, I went for two Fender blackface-style preamp boards, each containing a 12AX7 and a 12AU7. The first triode of the 12AX7 is a gain stage, followed by the standard passive treble-middle-bass tone stack, with the second triode providing makeup gain. Both triodes of the 12AU7 implement the output stage.

White Cathode Follower

Here we depart from the standard blackface circuit. The output of the second gain stage is direct coupled to the cathode follower, which in turn is coupled to the output transformer by a 2.2 uF polycarbonate film capacitor (mil-spec things that apparently aren't made anymore). A consequence of direct coupling the second gain stage to the WCF (12AU7) is the grid of the WCF is at the DC potential of the prior stage's plate, roughly 150V. In order to not exceed the maximum heater-cathode voltage difference rating of the 12AU7, the filament must be referenced to a positive voltage so the difference between the heater and the WCF cathode (DC + signal) is less than 200V. I chose a reference voltage of 90V, created by a voltage divider on the power supply board (with a 1uF filter cap) whose center point is tied to the filament windings. All the filaments in the unit are referenced to this elevated DC voltage.

The design equations for the so-called optimal white cathode follower are well-documented on the internet. I first learned about it in this article by Alex Cavalli on the Headwize site, devoted to the care and feeding of headphones. The inimitable John Broskie has analyzed it at length in his Tubecad Journal . This companion article on Cavalli's own site presents the equations in very good detail..

My initial plan was to use an SRPP output stage, but the additional gain from this topology would have been completely excess. The parallel 12AU7 cathode follower in the Pultec would no doubt have been a fine alternative, but I thought it would be interesting to try a well-developed topology that would be new for me. Thus, the optimal White Cathode Follower.

Irregularities

If you look at the pictures, the two preamp turret boards are slightly different, but the circuit is the same. I changed my output stage design from SRPP to WCF after I had drilled the first board and set the turrets. The second board was made with my slightly more compact layout for the cathode follower, allowing me to move the second filter stage resistor from the power supply board to the preamp module. I made one module with carbon composition resistors, and the other with metal film resistors. The film capacitors are Mallory 150's.

assembled preamp turret
board and tube sockets

Constructing the preamp turret board and tube sockets

As is the case with most of my projects, I laid out the turret board in the style I adapted from Doug Hoffman of Hoffman Amps. The top row of turrets faces the tube sockets, the bottom row of turrets is the B+ (high voltage) buss, and the row of turrets above that is the ground buss. The boards are 2.25" x 5.75" and made from 0.125" thick G10-FR4 0.125" from McMaster Carr. I use the blue color in 24" x 24" sheets. Many DIY vacuum tube parts vendors sell it in strips by the inch so you don't have to ruin your table saw blades slicing it up. I draw the layouts in an ancient version of Microsoft Visio Professional, print them out full size, align and tape the layout to the top of the board, and punch the location of each hole to be drilled with a spring loaded center punch.
The punch layout pattern is removed, and I drill all the marked locations on my drill press. The drill size is #33 (0.113"), which is just 0.001" larger than the turret body for a very snug fit. You don't want to go oversize on the holes. If you use a different size turret, choose the proper size drill. The lateral space between components and the vertical position of the turret rows depends a bit on the size of components you use, and how tight your layout needs to be to fit into the available space. In this case, I'm using a 0.375" lateral grid, with turret rows at 0.1875", 1.125", 1.625", and 1.9375" from the top of the board. If you're using physically larger capacitors, you may need to make your board longer and/or wider. There is some space for both in this size box. It's also practical to change the lateral or vertical spacing in areas where you need to.
The Keystone turret lugs are Mouser Electronics part 534-1509-4. These two-level terminals are some of the most expensive ones, but I prefer them because they have a through-hole large enough to accommodate several component leads. I now buy them in 1000 quantity, and sell them to my DIY friends to get the price down to a merely painful $0.25 per turret. I set the turrets on my drill press (not plugged in) using the Keystone staking tool 534-TL-8, also available from Mouser. I formerly used a home-made tool, but it was sensitive to the amount of applied force. I tended to crack the swage (flare) on at least a few turrets per board. Since NASA doesn't accept cracked swages in spaceflight-qualified guitar amps, I had to drill them out and replace them. The pointed end of the Keystone tool has a complex profile that flares every turret perfectly. It was worth an $18 tool to not ruin any more turrets.
The appropriate turrets are laced together with #22 soft tinned buss wire, using the Doug Hoffman technique. Doug describes his board manufacturing process on a series of pages beginning here. Boards laid out the Hoffman way (and my way) tend to be a little longer and narrower than those laid out Marshall or Fender style, but I simply prefer the appearance of the Hoffman buss-row style, and because it minimizes the need for jumper wires and crossovers.
Here's the blackface preamp turret board with the components inserted. At this stage, I only solder the leads of components to turrets that won't be getting flying leads to pots, jacks, tube sockets, etc. It is my practice to orient the component markings so they can easily be read from above, orienting everything in the same direction if possible. Of course, polarized electrolytic capacitors must be oriented with proper polarity! If you are taking this much care to make your projects this way, you might as well make it pretty.
Turning the board over, I solder the single jumper this layout requires to get the ground from the negative lead of the filter cap to the ground buss turret row.
The tube sockets are fastened to the 1" x 1/16" aluminum angle stock, and the flying leads are soldered to the socket pins. It's so much easier to solder when you have free access to the pins, than when it's all screwed down inside the chassis. Note that I use the leads of the grid stopper resistors to wire from the socket to the board. It's considered good practice to place the body of the resistor close to the tube pins. I insulate the resistor leads with leftover bits of insulation stripped from the connecting wires. I use #22 silver plated teflon insulated solid wire, most of which I picked up years ago at various hamfests. I used #22 stranded on the filaments, which is more than adequate for the low-current filaments of most small-signal tubes. I'd use #20 if I were wiring the higher-current filaments of power tubes. Next I wire the flying leads from the tube sockets to the appropriate turrets on the board. I made an assembly / testing jig from a piece of perforated steel to hold the board and tube socket bracket in the same relative positions they will have in the rack box, so the leads connecting the board and sockets can be trimmed to the correct length. After a few of the connections were complete to fix the distance between board and sockets, I unscrewed them from the jig to make the connections with several of the turrets from the underside. Debugging and future maintenance would be easier if all connections were made from the top, but I prefer to make most wire connections underneath for better appearance (or should I say, non-appearance). I do make the grid resistor connections from above.

Constructing the power supply turret board

Below is the construction sequence for the power supply board. Note that I changed the layout of this board in my document after I made this prototype. I made it slightly more compact, and changed a few of the resistor values.

Testing the power supply and preamp modules

I assembled a completed preamp module and the power supply module in the test fixture. An input jack and the potentiometers are assembled on another piece of scrap aluminum drilled in the same pattern as they will be on the rack box's front panel.

Despite the liberal use of clip leads and quite a few unsoldered connections, everything works. Of course you've been told a thousand times, solder is not glue, and your wiring connections are supposed to be mechanically solid before you solder them. My hack for pinning down component leads and wires that are loose in unsoldered turret holes is to wedge them in with a short bit of previously clipped component lead
preamp and power supply module in test
fixture

preamp and power supply module in test
fixture

Front panel

Front panel, drilled and ready to mount jacks, pots, switches, and LED. There is almost no clearance above or below the controls when the front panel is fit into the rack box. I had to file away a chunk of the lip of the box's bottom panel beneath the power toggle with a round file allow the switch body to fit.
Front panel with everything mounted. These Alpha 24 mm potentiometers are about as large as will fit between the top and bottom panels of the rack box.
Front panel with everything mounted, as seen from the rear. There is also little clearance between the solder lugs on the pots and the top cover of the rack box. I put a strip of cloth electrical tape along the front of the top cover above the pots.

I use Doug Hoffman's old AB763 board layout diagram as a reference for wiring the volume control and treble - middle - bass tone controls. I use the Hoffman page of common hookups as my reference for wiring the input jacks. I complete most of the front panel wiring before attaching it to the enclosure. The board and potentiometer grounds are connected to the chassis at the input jacks.
front panel
controls - wired

Assembly and testing

I placed the turret boards, transformers, power entry, fuse holder, etc. in the rack enclosure and shifted things around by hand until I was satisfied with the clearances and spacings. Then I measured where things were, and adjusted the positions to lie on 1/8" increments. Everything went together mechanically without surprises. Working in my usual slow way, I made the circuit connections among the modules and the front and rear panels. Finally I wired in the power supply components, and made sure the B+ lines all showed high resistance to ground. Correspondingly, I made sure all the points that were supposed to be at ground potential had low resistance to ground. Since I had tested the power supply and one preamp module in my test fixture, I felt confident that I was ready to power up. I connected both channel outputs to my mixer's balanced line inputs with the gains turned all the way down, bridged both channel inputs together with a jumper cable, and plugged in my guitar. I powered up slowly on my variable autotransformer (a.k.a variac), and watched the power supply voltages. Still no surprises. When I brought up the channel volumes, one channel was perfect, but the other channel showed about 10x as much gain, and the tone controls did nothing. A quick check of the tone control wiring for that channel turned up a missing ground from one leg of the mid pot. After correcting that, both channels worked and sounded excellent, with no extraneous hums, buzzes, or other noises. Feeding the preamp with my HP 200CD oscillator at 1 KHz, I measured the output clipping point at >16 V peak-to-peak into the mixer input. The frequency response extends down to 17 Hz and beyond 20 KHz with the tone controls set as flat as can be achieved with a Fender TMB tone stack. Clipping point for the inputs is about 1.4 V peak-to-peak. Some players with a high-output instrument or a percussive style prefer to substitute a lower gain tube for the 12AX7, such as a 5751, 12AY7, or even a 12AU7.

I still need to electrochemically etch the front and rear panels. I will be experimenting with processes for this before I try it on this project. Since it won't work on anodized aluminum, I will probably make a new front panel blank from non-anodized aluminum. Fortunately, it is otherwise uncomplicated to fabricate.
I simulated the power supply and preamp circuits in LTSpice IV. I may post the schematic files created for this.
Update the Visio documentation to show the wiring from the AC mains to the power transformer and the power supply board
Upload Excel spreadsheet with bill of materials
Try some experiments with output pads to add signal level flexibility.
Begin studying other bass preamp circuits for ideas to try in a new preamp module.

Send me an email if you have questions or comments about this project.