Sortaflex: DIY Tube Bass Preamp (2020)


Way back in 2013, I built a clone of an Ampeg Portaflex “Fliptop” head, using information I harvested from several schematics I was able to find. The post for that project is here:

Last year (2019), I decided I wanted to build the preamp section of the Portaflex. I wasn’t super-strict about sticking to the original, but I did keep the EQ section more or less the same. Hence the name “Sortaflex”. What I came up with worked, but it had a lot of issues, so I set it aside for a while.

Usually I’m a hermit, but now that everyone else is too, I decided to take another shot at building a tube bass preamp that runs on AC power.

The Sortaflex has three controls: Volume, Bass, and Treble. Input is 1/4″. Outputs are unbalanced 1/4″ and balanced XLR with a ground-lift switch.

History of the Ampeg B-15 Portaflex

And now, an action-packed video:


Sortaflex main board schematic
Sortaflex main board schematic
Sortaflex power section schematic
Sortaflex power section schematic


Images for etching: If you would like to etch your own Sortaflex PCBs, download the two PDFs below. The first one is for the main board and the second one is for the power (RC-filter) section.

Layouts: The two PDFs below contain layouts for the two Sortaflex boards. The first one is for the main board and the second one is for the power (RC-filter) section. Each component on the layout diagrams corresponds to the components in the schematic diagrams above.

Layout diagram: I seem to have misplaced it but I’ll post it here as soon as I find it, I promise.

Update 8/30/2023: Never mind about the layout diagram. I don’t know what I did with it.

The main differences between this gizmo and the venerable Ampeg Portaflex are:

  1. 12AU7/12AX7 nine-pin tube instead of a 6SN7/6SL7 octal tube. Most of the fliptop schematics I’ve seen specify 6SL7 for the preamp tubes; one showed 6SL7s. I used a nine-pin tube here to make it easier to stand the PCB up and stuff everything into a Hammond 1590D enclosure.
  2. Different plate resistor values (R3 and R14 in the schematic) and a different biasing voltage. (See the last paragraph in the ‘About the XLR Output’ section below for more of me complaining about my own failures). R3 and R14 could be tweaked for different gain and different impedance, but since the output signal is pretty hot to begin with, I’ve tried to keep the overall gain down.
  3. Addition of capacitor on cathode of V1A. Can be omitted. See schematic.
  4. No phase inverter or power stage (because it’s a preamp). More about this below.
  5. 68k grid-stop resistor at the grid of V1B. I’m not sure why I thought that should be there, but it’s part of the 2013 project as well. Maybe it’s a holdover from a previous guitar-amp project.
  6. The volume control is after the Baxandall EQ and before the V1B input. In a typical fliptop amp, the volume control would be just before the phase inverter (from what I’ve been able to learn from various schematics).
  7. Different values for the coupling capacitors. I used mostly big WIMAs and small WIMAs. Most of mine are 1uF, but “canonical” values (22nF) could be used instead.

So, does the Sortaflex sound like a Portaflex? Sort of. Maybe some… or not. Using a different tube probably makes it apples to oranges by default.

And it’s just the preamp section. Although I can’t prove it, I’ve always thought that the phase inverter stage of the Portaflex contributes to a quality in its sound that I have trouble describing: strech-y? Rubber-band-y? Slightly compressed? Something in the midrange that I can’t quantify.

Update 8/30/2023: I’ve concluded that it doesn’t sound like a Portaflex. The EQ is close but the unique sound of the B-15 just isn’t there. Maybe I should rename it the Notaflex.

Another update 8/30/2023: Tube Depot doesn’t sell the power transformer I used for this project and that’s quite unfortunate for me because the 1) that PT was perfect because it sourced only about 5mA from the primary and something like 300 or 400 mA from the heater taps, which is perfect for a single-tube project. If I ever build any more doodads like this, I’ll have to go on a hunting expedition for a suitable replacement.

dimensions of that PT were perfect for a Hammond 1590D.

What I like most about the Sortaflex:

  • With a 12AU7, it’s got plenty of clean headroom.
  • The low-end response is respectable and the EQ is wide and responsive.
  • I’m not hearing any AC hum – it’s quiet as a church mouse.
  • The balanced (XLR) output sounds pretty good from what I can tell.
  • It looks cool in the dark.

What I like least about the Sortaflex:

  • It’s big and heavy, and seems like overkill for a preamp.
  • It’s f***ing expensive to make.

About the Circuit Boards

The PCBs are one-sided so that they can be etching to copper clad board, but I had some made because I’m insane in the membrane. They are 0.062″ with a 1-oz. copper thickness, not the thicker kind (0.094 or 0.125 with 2-oz. copper thickness) that one often sees in conjunction with tube amps. I made the traces nice and wide though – most of them are 0.04″, and 0.07″ for the ground traces on the RC-filter board. If power tubes had been involved, I probably wouldn’t have used PCBs at all. But so far I haven’t had anything go wrong with either of them.

The small holes in the boards are for the screws that hold the standoff posts in place. On the back sides, you’ll see clearance around the groundplane so that the standoffs are isolated from any traces. On the main board, the big hole toward the bottom allows the pair of filament wires from the power transformer (the thick green twisted ones in the photos above) to pass through and solder to the heater connections from the back.

About the XLR Output

The balanced XLR output is achieved by a 12:1 Crimson Audio CT-121 di transformer (, the cost of which (about $50 US) is somewhat out of proportion with a typical D.I.Y. project’s budget. I had an extra one because I bought two for another project. I’m quite pleased with the result.

The CT-121 di has an 11.5:1 (primary to secondary) ratio. I don’t know what its input impedance is but it seems to be similar to (but less expensive than) a Jensen DT-DB-E, which is 12:1 with a 140 kOhm input impedance. For a version with no balanced output, the PCB could be populated without the transformer section, by omitting C11, R18-R23, C12, C13, the transformer, the XLR jack, and the ground-lift switch (and C14 and R24).

Incidentally, I don’t remember why I thought the value of R18 should be so high (220 kOhms – see schematic).

I should have thought about what would happen when the tube and the Mosfet buffer are tied to the same B+. If I ever build another version of this, I’ll try separating them with an RC stage. Without the Mosfet buffer, the B+ sits at around 200 Vdc and the voltage at the plates of the tube is around 125-130 Vdc, which is nice. But with the tube and the Mosfet buffer tied together, the B+ is only about 100 Vdc and the tube plate voltage is around 70 Vdc. In the future, I might try changing the resistor values in the buffer.

About the Ground Wiring

Since this is a high-voltage project, I tried to use star grounding even though it’s just a glorified pedal. That’s why you see so many thick black wires connected to the RC-filter PCB (see photos above).

About star grounding:

About amp grounding in general:

For the RC filter board, the diode rectifier, the filtering capacitor that follows it (C1), the next RC filter stage (R1 and C2), and the voltage divider for the LED all have their own own ground wires (see schematic and photos above). They run to the terminal strip along with the ground wires for the power transformer’s filament supply and the tube circuit board. The two articles referenced above will explain why such a seemingly fussy approach can be desirable. It’s worked well for me when I build tube amps. Or what I should say is: I always use this approach for tube amp projects –> I usually get very little hum noise in said projects –> I’m superstitious.

By the way, referring to the two photos above, the purple and orange wires extending from the PCB are for the bright blue LED. A voltage divider prevents the LED from being fried (see schematic above).

About the Components

The tube in the videos is a JJ ECC82 / 12AU7. I’ve also tried an Electro-Harmonix 12AU7 and a Tung-Sol 12AU7, with similar results, and 12AX7s from Electro-Harmonix and Tung-Sol, which yielded much higher gain. That could be nice if you want tube bass fuzz.

The power transformer SKU is TR-PW-13. I get them from TubeDepot. Their website has a data sheet for it. It can drive a single tube, with ~= 200VDC/5mA at the secondaries (not center-tapped) and 6.3V/300mA for the filaments (center-tapped).

The RC-filter board uses Nichicon electrolytic capacitors rated at 450 Vdc for smoothing the oscillating AC into a useable DC voltage. They’re pricey but I like them. I get them from TubeDepot or Amplified Parts.

The AC switch assembly came from someone who sells on Amazon. There are many similar listings but not all are appropriate for this project. The fuse is worked into the unit. Separate components for the plug, the switch, and the fuse would also suffice. This sort of mousetrap should always be fused. And there should always be an Earth ground, distinct from any other ground points (see schematic and photos).

Most of the resistors are Vishay 2-Watt metal-film. I like them better than 1/2-Watt. I get them from Mouser.

Most of the nonpolar capacitors are WIMA, also from Mouser. The larger ones are in positions where a higher DC tolerance (> 100V) is required.

The tube socket is from TubeDepot, SKU SK-9PINPC2.

The enclosure is a Hammond 1590D.

For wiring, I prefer the stranded, tin plated, aerospace-grade tefzel wire I get from, just in case someone wants to take one of my homemade preamps to Mars.

Most of my PCB standoff posts come from Angela Instruments. The RC-filter PCB has 1/4-inch standoffs and the ones for the main board are 1-1/2 inches, to make room for the pots and the 1/4″ jacks.

3 thoughts on “Sortaflex: DIY Tube Bass Preamp (2020)

  1. Larry

    I notice some Ampeg tone stacks have the 270k from the bass pot pick to ground (like yours does) and some do not. What is the purpose of this resistor? Similarly, you have resistors and a capacitor in series after the vol pot on the signal path to the second grid. Most Ampeg schematics I’ve seen do not have these, so I wonder about their purpose and also about their effect on the tone.

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