Ampeg B-15-N Portaflex tone stack
Tone stacks and tone controls from amps and pedals, from their schematics, with the stage that drives each one and what it feeds. Turn the knobs and watch the curve. Pin a setting to compare, change any part, and listen through a speaker. Pickups and guitar knobs are on the Guitars page.
The Bass 1 and Treble 1 controls of the Ampeg B-15-N Portaflex (1968), with every part as on the bench.
Swipe sideways for the whole drawing. Tap a part to change it. The source drives the end marked in, and the load hangs on the end marked out.
At this setting
0 dB is the source’s voltage with nothing hooked to it, so the curve shows what the stack cuts. A tube source changes the curve only through its output resistance, not its gain.
Pin this curve keeps what the graph shows, up to three pins in their own colors. Pins stay when you switch stacks, so you can lay a Marshall treble sweep over a Vox one. In a sweep, the lightest line is the knob at 0 and the darkest is 10.
Listen
You hear a picked string through a guitar on full, with 15 feet of cable. Play without it leaves the stack out.
Source and load
Driven by
Driving
The knobs
- Bass 1
- Bass, 1 MΩ, with a cap across each half. Its wiper feeds the output through 120 kΩ. Toward the top the lows from the 220 kΩ get through, toward the bottom they’re pulled to the 22 kΩ. The caps let the mids and treble past whatever it’s set to. The taper isn’t marked, so linear is assumed.
- Treble 1
- Treble, 1 MΩ. Its top gets the treble through the 470 pF, its bottom goes to ground through .0047 µF, and its wiper is the network’s output. Up, more treble from the top. Down, the .0047 µF pulls treble off the output. The taper isn’t marked, so linear is assumed.
Parts
- 100 nF
- Coupling cap into the tone network.
- 220 kΩ
- Feeds the Bass pot’s top. With the 22 kΩ under the pot it sets how far the network drops the signal, about 20 dB with the knobs centered.
- Bass 1
- 1 MΩ, Bass, 1 MΩ, with a cap across each half. Its wiper feeds the output through 120 kΩ. Toward the top the lows from the 220 kΩ get through, toward the bottom they’re pulled to the 22 kΩ. The caps let the mids and treble past whatever it’s set to. The taper isn’t marked, so linear is assumed.
- 1 nF
- From the Bass pot’s top to its wiper. It passes the mids and treble around the upper half of the pot.
- 10 nF
- From the Bass pot’s wiper to its bottom. It passes the mids and treble around the lower half.
- 22 kΩ
- From the Bass pot’s bottom to ground, the bottom of the network’s divider.
- 270 kΩ
- From the Bass pot’s wiper to ground. It drops the level slightly more and keeps the Bass knob from moving the mids as much.
- 470 pF
- Into the Treble pot’s top, .00047 µF. Only the treble gets through it.
- Treble 1
- 1 MΩ, Treble, 1 MΩ. Its top gets the treble through the 470 pF, its bottom goes to ground through .0047 µF, and its wiper is the network’s output. Up, more treble from the top. Down, the .0047 µF pulls treble off the output. The taper isn’t marked, so linear is assumed.
- 4.7 nF
- Under the Treble pot to ground. With the knob down it pulls the treble off the output.
- 120 kΩ
- From the Bass wiper to the Treble wiper. The lows and mids reach the output this way.
- 270 kΩ
- Mixing resistor, 270 kΩ, from the channel one’s tone network to V3’s grid, where the other channel’s 270 kΩ meets it. Each channel loads the other through the pair, about 6 dB.
Source and load
Source: 110 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 310 kΩ with 60 pF across it, the input of what follows it on the bench, from its impedance at 1 and 10 kHz.
Values, wiring and drawing are taken from the Ampeg B-15-N Portaflex (1968) on the bench, so the two always match.
Warnings. The summary above the graph turns red if the stack can’t be solved as set, usually from a part at zero or a shorted pot track.
How this tool works. We solve each stack with the circuit bench’s engine: the source stage’s operating point first, then every part at 161 frequencies from 20 Hz to 20 kHz. A stack that’s also in a bench circuit is built from that circuit file, so the two always match.
The curves are small-signal. They don’t show clipping or grid current in the next stage. For the stage before or after a stack, try the gain stage tool.
Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.
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