Peavey 5150 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.
A Marshall-type stack fed through 470 kΩ, with 33 kΩ to ground at its input and a 10 nF and 47 kΩ treble shunt. So the stack gets a heavily divided, darkened signal from the stage before.
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
- Marked High on the panel. A 250 kΩ linear pot, with 570 pF (470 pF and 100 pF in parallel) and a 47 kΩ slope resistor.
- Bass
- Marked Low on the panel. A 1 MΩ audio pot used as a variable resistor into the bass path, through 22 nF.
- Middle
- Marked Mid on the panel. A 50 kΩ linear divider with the mid cap on its wiper, as in the Marshall stacks.
Parts
- R89
- 470 kΩ, Series resistor into the tone stack.
- R24
- 33 kΩ, Tone stack input to ground. With R89 it’s a divider of about 1 in 15.
- C59
- 10 nF, With R94, a treble shunt at the stack input.
- R94
- 47 kΩ, In series with C59.
- C14, C60
- 570 pF, Treble cap, 470 pF and 100 pF drawn in parallel on the schematic.
- R28
- 47 kΩ, Slope resistor.
- C18
- 22 nF, Bass cap.
- C21
- 22 nF, Mid cap, to the Mid pot’s wiper.
- High
- 250 kΩ, Treble, 250 kΩ linear. Its wiper is the stack’s output.
- Low
- 1 MΩ, Bass, 1 MΩ audio, wired as a variable resistor from the High pot’s bottom to the Mid pot.
- Mid
- 50 kΩ, Mid, 50 kΩ linear.
Source and load
Driven by: ideal. Driving: volume pot, 1 MΩ. Source: ideal. The stage before has 1 MΩ of shunt feedback from its output to its grid, pulling its output resistance down to a few kΩ, small next to the 470 kΩ in series. Load: the lead post gain pot (1 MΩ) at full. The cathode follower after it has a bootstrapped grid resistor and hardly loads it.
History of this network
People often call this the FMV stack, for Fender, Marshall and Vox, or the TMB stack, for treble, middle and bass. Nobody knows who designed it first. The earliest use we know of is Gibson’s GA-77 amp, whose schematic is dated 1953 to 1954. Its first channel has bass and treble controls.
Fender added a middle control to the Bassman’s stack in 1957. Marshall’s JTM45 of 1962 to 1963 followed the 5F6-A Bassman of 1958 closely, and that’s how the network got to Marshall. The Marshall versions here show how its values changed after that.
Vox’s Top Boost for the AC30 is credited to Dick Denney and was first drawn up formally at JMI in December 1961. It uses the same network with a fixed 10 kΩ where the middle pot would be. The bass pot’s wiper goes to that 10 kΩ and its far end to ground, so at full it shorts the 10 kΩ. Accounts differ on where it came from. One Vox history names the Gibson GA-77, another says Denney worked from a Fender Twin, and others call it the Bassman’s stack without the middle control.
Passive bass and treble networks were common in radio and hi-fi gear by the early 1950s, so the idea may be older than the GA-77. We don’t know of an earlier drawing of it.
Values and wiring are the same as in the six-stage high-gain lead amp circuit 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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