High-power amp with rotary high and low filters, early 1970s style

Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.

How to use the bench

Every part is simulated. Nothing you hear is a recording.

  1. 1Pick a circuit

    Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.

  2. 2Set the knobs and the input

    Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.

  3. 3Press Play

    Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.

  4. 4Tap a part

    Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.

  5. 5Tap a wire

    Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.

  6. 6Try a change

    Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.

Good to know

Modeled after the Dumbleland 150 W (Sleepwalk redraw of #009).

Two triodes with a Fender-type tone stack between them, then a filter of two seven-way switches into a third triode and a master, a 12AU7 long-tail pair, a 12BH7 driver and four 6550s.

Input

Add

Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.

How it’s played
Tap a part to change its value, or a wire to probe it. To add or take out parts, open Modify this circuit, below. Red numbers are DC voltages to ground, not warnings. Scroll or drag sideways to see the whole circuit.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.

Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.

Add a part

Tap where each leg goes. Tap both ends of a part to put the new one across that part.

Change a part

With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.

Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.

Change a wire

With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.

With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.

Keep and share

Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.

Test tone for the graphs
The graphs use a steady tone. Play sounds a plucked string that dies away.

Input

pickup, unloadedat the pedal input

Output

at the amp input

Probe

Spectrum

Frequency response

What the curves mean

The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.

Output

Bias

every voltage and current at rest

What to listen for. The front end is the Overdrive Special’s tone stack with Rock/Jazz, Deep and Bright. After V1b comes the same kind of filter as the Steel String Singer’s: a Trimmer, then a red path through a coil into a tapped resistor string (Low) and a wht path through a choice of caps (High), both into V2a’s grid. The power amp has a 12BH7 gain stage between the inverter and the 6550s, with an Accent switch on the feedback.

Download.

Parts and what they do

Stock values. Tap a part on the schematic to change it.

10k, 10 kΩ
Dropping resistor from node 3 to node 4. Its value isn’t marked on the drawing, so a typical 10 kΩ is assumed.
20µ, 20 µF electrolytic
Filter cap on node 4, 20 µF, part of a can.
1M, 1 MΩ
Input resistor to ground at the jack.
68k, 68 kΩ
Input 1’s 68 kΩ into V1a’s grid.
68k, 68 kΩ
Input 2’s 68 kΩ. With nothing in input 2 its tip is tied to input 1’s, so the two sit in parallel, 34 kΩ.
V1a, 12AX7 / ECC83 / 7025
First stage, half a 7025. Its plate drives the tone stack directly.
100k, 100 kΩ
V1a plate resistor. It’s also part of the tone stack, since the stack hangs right on the plate.
1.5k, 1.5 kΩ
V1a cathode resistor.
5µ, 5 µF electrolytic
V1a bypass cap, 5 µF.
330p, 330 pF
Treble cap, 330 pF, from V1a’s plate to the top of Treble.
100k, 100 kΩ
Slope resistor, from V1a’s plate to the bass and middle caps.
.1µ, 100 nF
Bass cap, to the top of Bass.
.047µ, 47 nF
Middle cap, .047 µF, to Middle’s wiper.
Middle, 100 kΩ pot, linear taper
Middle, 100 kΩ linear (“100kB”), a variable resistor to ground. Turned up, its resistance rises and less of the mids go to ground.
Treble, 250 kΩ pot, linear taper
Treble, 250 kΩ linear (“250kB”). Its wiper is the stack’s output, the top of Volume.
Bass, 250 kΩ pot, audio taper
Bass, 250 kΩ audio. On Rock its wiper is tied to its top, so it’s a variable resistor. On Jazz the wiper feeds Volume through 100 kΩ.
1.8k, 1.8 kΩ
Under Bass, to ground.
100k, 100 kΩ
On Jazz, from Bass’s wiper to Volume. On Rock its switch end hangs open.
Volume, 1 MΩ pot, audio taper
Volume, 1 MΩ audio.
270k, 270 kΩ
Deep network, from Bass’s top. With Deep off its far end hangs open.
270k, 270 kΩ
Deep network, from Volume’s wiper. With Deep off its far end hangs open.
.01µ, 10 nF
Deep network cap, with 1 MΩ across it.
1M, 1 MΩ
Across the Deep cap.
10k, 10 kΩ
Bottom of the Deep network, to ground.
220k, 220 kΩ
From Volume’s wiper into V1b, 220 kΩ with 500 pF across it. With the plate-to-grid 40 MΩ it sets how much of that feedback reaches the grid.
500p, 500 pF
500 pF across the 220 kΩ, so the treble gets to V1b’s grid at full level.
V1b, 12AX7 / ECC83 / 7025
Second stage. Its plate feeds the Trimmer and the filter.
100k, 100 kΩ
V1b plate resistor.
1.5k, 1.5 kΩ
V1b cathode resistor.
5µ, 5 µF electrolytic
V1b bypass cap, 5 µF.
20M, 20 MΩ
Two 10 MΩ in series from V1b’s grid toward its plate. With the .05 µF and two more 10 MΩ it feeds back a tiny share of the plate’s signal.
.05µ, 50 nF
In the plate-to-grid path, so the plate’s DC stays off the grid.
20M, 20 MΩ
Two more 10 MΩ, from the .05 µF to V1b’s plate.
.01µ, 10 nF
Coupling cap from V1b’s plate to the Trimmer. It isn’t on the drawing, but without it the plate’s DC would reach the filter and V2a’s grid, so .01 µF is assumed, as on the Steel String Singer’s filter.
Trimmer, 1 MΩ pot, linear taper
Trimmer, 1 MΩ, a variable resistor (its third lug unused) from V1b’s plate to the filter. Turned up, its resistance falls and more signal reaches the filter. Its taper isn’t marked, so linear is assumed.
100k, 100 kΩ
From the wht node toward the coil, 100 kΩ.
5H, 5 H
The filter’s coil. The drawing shows no value. 5 H is inferred, as on the Steel String Singer. The Coil jumped switch removes it.
coil R, 500 Ω
The coil’s own resistance, 500 Ω, typical, not measured.
470k, 470 kΩ
Between the red and wht nodes, across the 100 kΩ and the coil.
.01µ, 10 nF
High switch position 1, .01 µF.
.005µ, 5 nF
High switch position 2, .005 µF.
.0022µ, 2.2 nF
High switch position 3, .0022 µF.
.001µ, 1 nF
High switch position 4, .001 µF.
500p, 500 pF
High switch position 5, 500 pF.
250p, 250 pF
High switch position 6, 250 pF.
100p, 100 pF
High switch position 7, 100 pF.
820k, 820 kΩ
From the wht node to V2a’s grid, 820 kΩ, in every position of the High switch.
39k, 39 kΩ
Low switch string, 39 kΩ from position 1 to 2.
68k, 68 kΩ
Low switch string, 68 kΩ from position 2 to 3.
92k, 92 kΩ
Low switch string, 92 kΩ from position 3 to 4.
180k, 180 kΩ
Low switch string, 180 kΩ from position 4 to 5.
270k, 270 kΩ
Low switch string, 270 kΩ from position 5 to 6.
390k, 390 kΩ
Low switch string, 390 kΩ from position 6 to 7.
12k, 12 kΩ
Bottom of the Low switch’s string, 12 kΩ to ground.
.01µ, 10 nF
Across the whole string, from the red node to position 7.
270k, 270 kΩ
From the Low switch’s wiper to V2a’s grid, 270 kΩ.
100k, 100 kΩ
V2a’s grid resistor, 100 kΩ to ground, the bottom of both filter paths.
V2a, 12AX7 / ECC83 / 7025
Third stage, after the filter, half a 7025.
100k, 100 kΩ
V2a plate resistor, from node 3.
1.5k, 1.5 kΩ
V2a cathode resistor.
4.7µ, 4.7 µF electrolytic
V2a bypass cap, 4.7 µF.
.02µ, 20 nF
Coupling cap to the Pre Out jack and the Master.
Master, 1 MΩ pot, audio taper
Master, 1 MΩ audio. Its wiper drives the inverter.
.02µ, 20 nF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 5 Hz.
V3a, 12AU7 / ECC82
12AU7 (ECC82), the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A.
V3b, 12AU7 / ECC82
The other half of the inverter. Its grid sits at the feedback point for signal, so it’s driven through the shared cathodes, and its plate swings with the input. It drives tube B.
1M, 1 MΩ
Grid resistor for the driven half, returned to the bias point above the tail, not to ground.
1M, 1 MΩ
Grid resistor for the other half.
1.5k, 1.5 kΩ
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
22k, 22 kΩ
The tail resistor. It’s large, so the pair shares a nearly fixed current. When one half conducts more, the other conducts less, which splits the phase.
.1µ, 100 nF
Ties the second grid to the feedback point for signal, so the feedback acts across the pair.
17k, 17 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
10k, 10 kΩ
Plate resistor for the other half.
3.3k, 3.3 kΩ
The feedback resistor from the speaker. With the 100 Ω below it, it sends about 3 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
100, 100 Ω
The bottom of the feedback loop, 100 Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.1µ, 100 nF
Coupling cap from the inverter to side A’s driver.
.1µ, 100 nF
Coupling cap from the inverter to side B’s driver.
1.2k, 1.2 kΩ
The drivers’ cathode resistor, 1.2k Ω to ground for both halves, with no bypass cap. The two cathodes share it, so the pair works somewhat like a long-tail pair: what one half gains in current the other loses, which evens out the two sides’ swings.
470k, 470 kΩ
Grid leak for side A’s driver, to ground.
V8a, 12BH7 (GE, JJ)
12BH7, side A’s driver: a gain stage that raises the inverter’s signal about 4 times on its way to tube A’s grids. The other half drives side B, and the two share one cathode resistor.
12k, 12 kΩ
The driver’s plate resistor, 12k Ω from the node for the driver.
.1µ, 100 nF
Coupling cap from the driver’s plate to tube A’s grids.
470k, 470 kΩ
Grid leak for side B’s driver, to ground.
V8b, 12BH7 (GE, JJ)
The other half of the 12BH7, side B’s driver.
12k, 12 kΩ
The driver’s plate resistor, 12k Ω from the node for the driver.
.1µ, 100 nF
Coupling cap from the driver’s plate to tube B’s grids.
500, 500 Ω
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 1k. The resistors are drawn as one part, the two in parallel.
500, 500 Ω
Grid stopper for tube B. Each of the two tubes has its own 1k. The resistors are drawn as one part, the two in parallel.
V4, V6, 6550
n6550, tube A of the push-pull pair, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V5, V7, 6550
n6550, tube B of the pair. Two tubes in parallel, drawn as one.
25, 25 Ω
Plate resistor for tube A, 50 Ω between the plate and the output transformer. It damps oscillation at radio frequencies. The drop across it is the plate current times 50 Ω, a few volts. Each of the two tubes has its own 50. The resistors are drawn as one part, the two in parallel.
25, 25 Ω
Plate resistor for tube B, the same. Each of the two tubes has its own 50. The resistors are drawn as one part, the two in parallel.
6.5, 6.5 Ω
Cathode resistor for tube A, 13 Ω to ground. The bias is fixed, so it’s there to set the bias by: the voltage across it is the cathode current times 13 Ω. It isn’t bypassed, so it also lowers the tube’s gain slightly. Each of the two tubes has its own 13. The resistors are drawn as one part, the two in parallel.
6.5, 6.5 Ω
Cathode resistor for tube B, the same. Each of the two tubes has its own 13. The resistors are drawn as one part, the two in parallel.
100k, 100 kΩ
Grid leak for tube A, returned to the bias supply. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
100k, 100 kΩ
Grid leak for tube B.
23.5, 23.5 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard. Each of the two tubes has its own 47. The resistors are drawn as one part, the two in parallel.
23.5, 23.5 Ω
Screen resistor for tube B. Each of the two tubes has its own 47. The resistors are drawn as one part, the two in parallel.
5 H, 5 H
The choke from B+ to the node for the driver, 5 H, typical of the part, not measured. It passes DC with little drop. With the filter cap after it, it removes most of the 120 Hz ripple from that node.
100, 100 Ω
The choke’s winding resistance, 100 Ω, typical, not measured. It’s all the DC drop from B+ to the node for the driver.
20µ, 20 µF electrolytic
Filter cap at the node for the driver. With the choke it filters the supply and keeps the stages on this node from coupling to each other through it.
4.7k, 4.7 kΩ
Dropping resistor from the node for the driver to the node for the inverter, as on the Dumbleland 150 W drawing. The current drawn past it drops the voltage, so each stage down the chain runs lower than the one before, and a hard note that pulls B+ down pulls these down too.
30µ, 30 µF electrolytic
Filter cap at the node for the inverter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
10k, 10 kΩ
Dropping resistor from the node for the driver to the node for the preamp, as on the Dumbleland 150 W drawing. The current drawn past it drops the voltage, so each stage down the chain runs lower than the one before, and a hard note that pulls B+ down pulls these down too. The preamp runs off this node instead of its own HT supply.
20µ, 20 µF electrolytic
Filter cap at the node for the preamp. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 1.9 kΩ to 8 Ω, 20 H primary, 5 mH leakage
The output transformer, 1.9 kΩ plate to plate into 8 Ω, center tap on B+. Tap any piece of it to change its figures. The core saturates as the flux nears its knee. Every figure is typical of the part, not measured.
PT, 400 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 400 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part, not measured.
PT, 400 V ac, 60 Hz
The other half of the winding, from the grounded center tap. It runs opposite the first half, so the rectifier charges the reservoir on every half cycle.
PT R, 30 Ω
Resistance of one half of the winding, plus the primary’s share. With the rectifier’s own drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 30 Ω
Resistance of the other half. Typical of the part, not measured.
D1, 1N4001
One of the silicon rectifier diodes. It drops under a volt, so B+ holds up better under load than with a tube rectifier.
D2, 1N4001
The other rectifier diode, on the other half cycle.
50µ, 50 µF electrolytic
The reservoir caps’ first. It feeds the output transformer’s center tap. It charges in short pulses at each peak of the mains and drains between them, so B+ carries ripple, and on a hard note it sags.
50µ, 50 µF electrolytic
Another reservoir cap, in parallel with the first.
PT, 400 V ac, 60 Hz
The end of the high-voltage winding the bias comes off, 400 V ac, the same end as the rectifier’s first plate. It’s drawn as its own source so the bias supply’s average can be found. Keep it equal to the winding beside the rectifier.
47k, 47 kΩ
Series resistor into the bias rectifier, 47k Ω. There’s no bias tap, so the bias comes straight off the high-voltage winding, and this resistor drops most of it.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
100µ, 100 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
Bias 10k, 10 kΩ pot, audio taper
Bias adjust, 10k Ω linear. Its wiper feeds both output grids through their grid leaks. Toward the diode end the grids go more negative and the tubes idle cooler.
22k, 22 kΩ
The bottom of the bias divider, 22k Ω to ground.
Coil R, 8.69 Ω
Voice coil resistance, the floor of the speaker’s impedance. The shape of a 1x12 open-back cab with an alnico 12 in, measured (a 93 Ω peak near 74 Hz on 8 Ω), scaled to the 8 Ω tap. One real cab, not every speaker. The Speaker menu by the play buttons is separate. It plays the note through a speaker and cab impulse response, with either load.
Coil L, 256 µH
Voice coil inductance. It makes the impedance rise through the treble. The feedback holds the output steadier against that than it would be without feedback.
Loss L, 464 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 14.3 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 2.13 mH
A second step in the treble rise, with Loss R2 across it: a real voice coil’s losses keep climbing from a few hundred hertz.
Loss R2, 2.19 Ω
Limits what Loss L2 adds.
Tank R, 83.7 Ω
Height of the bass resonance, about 92 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 21.1 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 220 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.
16 Ω tap, 1 V
The 16 Ω tap the feedback comes from, assumed to be an ideal tap on the same winding: 1.414 times the speaker voltage. The load the feedback network puts on the winding is left out. It’s tens of kilohms on a winding of a few ohms.

About this version

Sleepwalk redraw
The 7025s use the 12AX7 model. The tube models are typical of each type, not measured.

Sources

  • Aaron and Talbany, Sleepwalk Amplifiers redraw, based on Dumbleland #009

How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type, not measured parts. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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