Fender 6G7 Bandmaster 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 brownface two-knob network: 250 pF to the treble pot, 100 kΩ to the pot’s bottom, and a bass pot below with 10 nF from its top to its wiper and 10 kΩ from the wiper to ground.
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
- Treble
- A 250 kΩ pot between 250 pF from the plate and the 100 kΩ low path.
- Bass
- A 250 kΩ pot from the low path to ground. 10 nF bypasses it from its top to its wiper, and 10 kΩ runs from the wiper to ground. There’s no middle control. The middle sits where the treble and bass paths cross.
Parts
- 50 nF
- Coupling cap from V1’s plate.
- 250 pF
- Treble cap, 250 pF, to the top of the treble pot.
- 100 kΩ
- The low path, 100 kΩ, to the bottom of the treble pot and the top of the bass network. With the bass network it sets how much bass and middle reach the output.
- Treble 250k
- 250 kΩ, Treble, 250 kΩ linear, as marked. Its wiper is the output, between the treble cap at its top and the low path at its bottom.
- Bass 250k
- 250 kΩ, Bass, 250 kΩ audio, from the low path to ground, its wiper tied to 10 nF and 10 kΩ. Turned up, the wiper nears ground and shorts the 10 kΩ, and the whole track, bypassed by 10 nF, holds the low path up at low frequencies. Turned down, the 10 nF shorts the track, and the low path sees about 10 kΩ to ground at every frequency.
- 10 nF
- Bass cap, 10 nF, from the top of the bass pot to its wiper. It bypasses the track above the wiper for everything but the lows.
- 10 kΩ
- From the bass pot’s wiper to ground. It sets how far the low path gets pulled down with the bass turned down, and so how deep the cut goes.
Source and load
Driven by: triode plate, 100 kΩ plate resistor. Driving: volume pot, 500 kΩ. From Fender’s 6G7 schematic, which draws both channels with the same network. Source: one triode of the 7025 (a low-noise 12AX7), with a 100 kΩ plate resistor and about 255 V at its top, worked out from the drawing’s +170 V on the plates and +1.4 V on the cathodes. The shared 820 Ω cathode resistor carries both triodes’ current, so it’s entered as 1.64 kΩ. Load: the 500 kΩ volume at full, marked linear, then V2’s grid. The vibrato channel’s 47 pF across the volume does nothing at full. The schematic doesn’t mark which way the pots turn, so up is assumed to be more treble and more bass.
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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