Clean amp with rotary high and low filters, reverb and four 6L6s

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 Dumble Steel String Singer #002 (draft drawing).

From a drawing marked draft. Two triodes with a Fender-type tone stack between them, then a filter of two seven-way switches into a gain stage and cathode follower, mixed with the reverb return and sent through Level to four 6L6s.

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. From a drawing marked draft, so some values may be wrong. The filter’s coil has no value on the drawing, so 5 H is inferred. V1a drives a Fender-type tone stack from its plate, with 1 MΩ of feedback from V1b’s plate to V1a’s cathode. After Volume, V1b feeds the filter: a .01 µF path into a tapped resistor string (the Low switch) and a .001 µF path through a choice of caps (the High switch), both into V2a’s grid. 470 kΩ from V2b’s cathode back to V1b’s cathode puts the filter and V2a inside a feedback loop, which lowers the gain and evens out the filter’s steps. V2b and the reverb return’s V3b are cathode followers, mixed through 220 kΩ each into Level. V4a and V5 drive the tank. The springs are a measured tank, played from its impulse response. The FET input buffers the guitar before V1a. A 12AX7 cathode follower drives each pair of 6L6 grids directly, with no coupling cap between them, so the bias holds when the output grids pull current.

Download.

Parts and what they do

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

1k, 1 kΩ
Dropping resistor, 1 kΩ 3 W from B to A, in the box marked FET on the drawing.
20µ, 20 µF electrolytic
Filter cap on A, 20 µF 500 V.
1k, 1 kΩ
Dropping resistor, 1 kΩ 3 W from B to C, the reverb stages’ supply.
20µ, 20 µF electrolytic
Filter cap on C, 20 µF 500 V.
1M, 1 MΩ
Input resistor to ground at the jack.
34k, 34 kΩ
Grid stopper into V1a, 34 kΩ on the drawing. With the tube’s input capacitance it keeps radio and fizz out.
V1a, 12AX7 / ECC83 / 7025
First stage. 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, above the 100 Ω.
5µ, 5 µF electrolytic
Bypass cap across the 1.5 kΩ only, so the 100 Ω under it stays unbypassed.
100, 100 Ω
Under V1a’s cathode, 100 Ω to ground. The feedback from V1b’s plate connects at the top of it.
1M, 1 MΩ
Feedback from V1b’s plate to V1a’s cathode, 1 MΩ. Against the 100 Ω it feeds back a small share of the signal, which lowers the gain slightly and flattens the response over both stages and the tone stack.
.1µ, 100 nF
In the feedback path, so V1b’s plate voltage stays off V1a’s cathode.
360p, 360 pF
Treble cap, 360 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, audio taper
Middle, 100 kΩ as a variable resistor to ground (its top lug isn’t connected). Turned up, its resistance rises and less of the mids go to ground. Audio taper is typical, not on the drawing.
Treble, 250 kΩ pot, audio taper
Treble, 250 kΩ. Its wiper is the stack’s output, the top of Volume. Audio taper is typical, not on the drawing.
Bass, 250 kΩ pot, audio taper
Bass, 250 kΩ. On Rock its wiper is tied to its top, so it’s a variable resistor. On Jazz the wiper feeds Volume through 100 kΩ. Audio taper is typical.
1.8k, 1.8 kΩ
Under Bass, to ground. With Bass at 0 the stack still has 1.8 kΩ at the bottom.
100k, 100 kΩ
On Jazz, from Bass’s wiper to Volume, so bass reaches the volume around Treble. On Rock its switch end hangs open.
Volume, 100 kΩ pot, audio taper
Volume, 100 kΩ. It sets how hard V1b and everything after it get driven. Audio taper is typical.
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. Shorted by the switch with Deep off.
1M, 1 MΩ
Across the Deep cap.
10k, 10 kΩ
Bottom of the Deep network, to ground.
68k, 68 kΩ
Grid stopper from Volume’s wiper into V1b.
V1b, 12AX7 / ECC83 / 7025
Second stage. Its plate feeds both halves of the filter.
100k, 100 kΩ
V1b plate resistor.
1.5k, 1.5 kΩ
V1b cathode resistor, above the 1 kΩ.
5µ, 5 µF electrolytic
Bypass cap across the 1.5 kΩ only.
1k, 1 kΩ
Under V1b’s cathode, 1 kΩ to ground, unbypassed. It lowers V1b’s gain, and the LNFB from V2b’s cathode connects on top of it.
470k, 470 kΩ
LNFB, 470 kΩ from V2b’s cathode back to V1b’s cathode. Against the 1 kΩ it feeds back a small share, so the filter and V2a are inside a feedback loop with V1b.
.22µ, 220 nF
In the LNFB path, so V2b’s cathode voltage stays off V1b’s cathode.
5H, 5 H
The filter’s coil, in series with the .01 µF from V1b’s plate to the red node. The drawing shows no value. 5 H is inferred, not from the drawing: with the .01 µF and the .01 µF across the string it makes a broad peak near 1 kHz on the Low switch’s lower positions. The Coil jumped switch removes it.
coil R, 500 Ω
The coil’s own resistance, 500 Ω, typical of a coil this size, not measured.
.01µ, 10 nF
In series with the coil, from V1b’s plate to the red node, the top of the Low switch’s string. On the drawing the cap comes first, then the coil. In series, the order makes no difference.
.001µ, 1 nF
From V1b’s plate to the wht node, the High switch’s caps. Only the upper mids and treble get through it.
470k, 470 kΩ
Between the red and wht nodes.
.01µ, 10 nF
High switch position 1, .01 µF.
.0051µ, 5.1 nF
High switch position 2, .0051 µF.
.0024µ, 2.4 nF
High switch position 3, .0024 µF.
.001µ, 1 nF
High switch position 4, .001 µF.
330p, 330 pF
High switch position 5, 330 pF.
150p, 150 pF
High switch position 6, 150 pF.
.003µ, 3 nF
From the wht node to ground, .003 µF. With the .001 µF from V1b’s plate it divides the treble down before the High switch.
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.
100k, 100 kΩ
Low switch string, 100 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. The treble skips down the string, so the lower positions lose mostly bass and mids.
270k, 270 kΩ
From the Low switch’s wiper to V2a’s grid, 270 kΩ. With the High switch’s path it sets the mix of the two.
100k, 100 kΩ
V2a’s grid resistor, 100 kΩ to ground. It’s the bottom of both filter paths.
V2a, 12AX7 / ECC83 / 7025
Third stage, after the filter. Its plate drives V2b’s grid directly.
100k, 100 kΩ
V2a plate resistor, from B.
1k, 1 kΩ
V2a cathode resistor.
1µ, 1 µF electrolytic
V2a bypass cap, 1 µF. It bypasses only above about 160 Hz, so the bass gets less gain.
V2b, 12AX7 / ECC83 / 7025
Cathode follower, its grid on V2a’s plate. Its low output drives the mix, the LNFB and Level.
100k, 100 kΩ
V2b’s cathode resistor, 100 kΩ to ground.
220k, 220 kΩ
From V2b’s cathode to the mix node. With the 220 kΩ from V3b’s cathode, the dry signal is about halved here.
220k, 220 kΩ
From the reverb return’s cathode follower to the mix node.
.01µ, 10 nF
Coupling cap from the mix node to Level, by shielded lead.
Level, 1 MΩ pot, audio taper
Level, 1 MΩ audio. Its wiper is the effects send, and with nothing in the loop it feeds the phase inverter, so it works as the master.
100k, 100 kΩ
V4b’s grid resistor, across the tank’s output coil.
V4b, 12AX7 / ECC83 / 7025
Reverb recovery stage. The tank drives its grid.
100k, 100 kΩ
V4b plate resistor, from C.
1.5k, 1.5 kΩ
V4b cathode resistor, above the 100 Ω.
5µ, 5 µF electrolytic
Bypass cap across the 1.5 kΩ only.
100, 100 Ω
Under V4b’s cathode, 100 Ω to ground. The feedback from V3b’s cathode connects at the top of it.
.003µ, 3 nF
From V4b’s plate to the reverb pot, .003 µF, by shielded lead.
Reverb, 100 kΩ pot, audio taper
Reverb, the return level, 100 kΩ audio. Its wiper is V3a’s grid. Turn it up for more of the springs.
V3a, 12AX7 / ECC83 / 7025
Reverb return gain stage. Its plate drives V3b’s grid directly.
100k, 100 kΩ
V3a plate resistor, from B.
1k, 1 kΩ
V3a cathode resistor.
1µ, 1 µF electrolytic
V3a bypass cap, 1 µF.
V3b, 12AX7 / ECC83 / 7025
Reverb return cathode follower. Its low output sits on the mix node through 220 kΩ, so it also sets how much the dry signal is cut there.
100k, 100 kΩ
V3b’s cathode resistor, 100 kΩ to ground.
270k, 270 kΩ
Feedback from V3b’s cathode back to V4b’s cathode, 270 kΩ, around the reverb return stages.
.1µ, 100 nF
In the reverb return’s feedback path, so DC stays off V4b’s cathode.
22k, 22 kΩ
Dropping resistor, 22 kΩ 2 W from E to G, the reverb driver’s supply.
20µ, 20 µF electrolytic
Filter cap on G, 20 µF 500 V.
68k, 68 kΩ
From Volume’s wiper into V4a’s grid, 68 kΩ. The grid has no resistor of its own, so it rests on Volume through this.
V4a, 12AX7 / ECC83 / 7025
Reverb send stage. It feeds the signal from Volume’s wiper to the reverb driver.
100k, 100 kΩ
V4a plate resistor, from C.
1.5k, 1.5 kΩ
V4a cathode resistor, above the 100 Ω.
5µ, 5 µF electrolytic
Bypass cap across the 1.5 kΩ only.
100, 100 Ω
Under V4a’s cathode, 100 Ω to ground, unbypassed.
.001µ, 1 nF
From V4a’s plate to the send pot, .001 µF. Only the upper mids and treble go on to the tank.
Reverb send, 250 kΩ pot, audio taper
Reverb send, 250 kΩ audio. It sets how hard V5 drives the tank. The drawing shows no setting.
10k, 10 kΩ
Grid stopper into V5, 10 kΩ.
2.2M, 2.2 MΩ
Feedback from V5’s plate to the send pot’s wiper, 2.2 MΩ. Against the pot’s resistance it lowers V5’s gain somewhat.
.05µ, 50 nF
In V5’s feedback path, so the plate’s DC stays off the grid.
V5, 12AT7 / ECC81
The reverb driver, both halves of a dual triode in parallel, drawn as one tube. The drawing doesn’t mark its type, so a 12AT7 is assumed, as in most tank drivers. It drives the tank’s low-impedance input coil through a transformer.
2.2k, 2.2 kΩ
Cathode resistor for the reverb driver.
25µ, 25 µF electrolytic
Bypass for the driver’s cathode.
Reverb T, 22.5 kΩ to 8 Ω, 20 H primary, 50 mH leakage
The reverb transformer, primary from G to V5’s plate, secondary to the tank’s input coil. It loads the driver and the supply. Every figure is typical of the part, not measured.
Tank in R, 800 mΩ
Resistance of the tank’s input coil, 0.8 Ω, measured on a long tank. The current through it drives the springs, which play from a measured tank’s impulse response.
Tank in L, 1.1 mH
Inductance of the tank’s input coil, 1.1 mH, about 7.6 Ω at 1 kHz with its resistance. Measured on a long tank.
Tank out L, 350 mH
The tank’s output coil, 0.35 H, about 2270 Ω at 1 kHz, from V4b’s grid to ground. Measured on a long tank.
Tank out R, 181 Ω
Resistance of the tank’s output coil, 181 Ω, measured on a long tank.
220k, 220 kΩ
Feeds the FET input from E, the screens’ node through 220 kΩ 2 W, which drops most of the voltage. With the 8.2 kΩ below it, the FET’s supply sits around 10 to 15 V.
8.2k, 8.2 kΩ
The bottom of the FET’s supply divider, 8.2 kΩ to ground. It holds the supply low enough for the transistor.
200µ, 200 µF electrolytic
Filter cap on the FET’s supply, 200 µF.
10k, 10 kΩ
The FET’s drain resistor, 10 kΩ.
Q1, NTE452
The FET input’s transistor. The drawing marks it only “F1175”, so a 2N4416-type part is assumed, typical, not measured. With its drain on 10 kΩ it has a gain of a few times, and its gate pulls no current, so the guitar sees about 3.3 MΩ.
2.7k, 2.7 kΩ
Source resistor, 2.7 kΩ. It sets the FET’s current. The drawing reads 3.77 V on the drain.
5µ, 5 µF electrolytic
Bypass cap on the FET’s source, so the source resistor sets the current without lowering the gain.
3.3M, 3.3 MΩ
From the FET jack’s tip to ground, 3.3 MΩ. It sets the gate’s DC at ground and most of what the guitar sees, about 3.3 MΩ against the Normal jack’s 1 MΩ.
1M, 1 MΩ
1 MΩ from the jack into the gate, with 100 pF across it. The gate pulls no current, so it costs nothing at DC.
100p, 100 pF
100 pF across the 1 MΩ, so the treble gets past it to the gate.
5µ, 5 µF electrolytic
Coupling cap from the drain to the Trim.
Trim, 10 kΩ pot, linear taper
Trim, 10 kΩ linear, inside the amp. Its wiper goes to the Normal jack’s switch, so it sets the FET input’s level into V1a. The drawing shows no setting; 0.5 of its turn is a guess.
.02µ, 20 nF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 5 Hz.
V6a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A.
V6b, 12AX7 / ECC83 / 7025
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.
820, 820 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
18k, 18 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.
113k, 113 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
113k, 113 kΩ
Plate resistor for the other half.
68p, 68 pF
A small cap from plate to plate. It rolls off the far top end the inverter passes on, which keeps the feedback loop steady.
2.7k, 2.7 kΩ
The feedback resistor from the speaker. With the 270 Ω below it, it sends about 9 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
270, 270 Ω
The bottom of the feedback loop, 270 Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.1µ, 100 nF
Coupling cap from the inverter to the first cathode follower’s grid.
.1µ, 100 nF
Coupling cap from the inverter to the second cathode follower’s grid.
1M, 1 MΩ
Grid resistor for the first cathode follower, 1M Ω to the divider that sets both followers’ grids.
1M, 1 MΩ
Grid resistor for the second cathode follower, the same.
820k, 820 kΩ
Top of the followers’ grid divider, 820k Ω from the bias supply. With the 130k Ω below it, it puts the followers’ grids at about 14 % of the bias supply. Their cathodes follow a volt or two above, and the output grids sit on the cathodes, so this divider sets the output tubes’ bias.
130k, 130 kΩ
Bottom of the followers’ grid divider, 130k Ω to ground. Bigger runs the output tubes colder. The drawing shows no bias figure. The drawn 130 kΩ puts each 6L6 at about 33 mA, where crossover distortion on a quiet note is just gone, not measured on an amp.
V7a, 12AX7 / ECC83 / 7025
Cathode follower driving tube A, one half of a 12AX7, its plate on the screen supply. Its cathode connects straight to the output grids, so they’re driven from a low impedance and can be pushed into grid current without the coupling cap charging up. That keeps the bias from shifting on a hard note.
V7b, 12AX7 / ECC83 / 7025
The other half, driving tube B.
220k, 220 kΩ
The first follower’s cathode resistor, 220k Ω to the negative bias supply, so the cathode can sit below ground at the output grids’ bias.
220k, 220 kΩ
The second follower’s cathode resistor, the same.
750, 750 Ω
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
750, 750 Ω
Grid stopper for tube B. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
V8, V10, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V9, V11, 6L6GC
6L6GC, tube B of the pair. Two tubes in parallel, drawn as one.
470k, 470 kΩ
Grid resistor for tube A, 470k Ω to ground. The cathode follower drives the grid through the stopper, so this only loads it slightly. Each of the two tubes has its own 470k. The resistors are drawn as one part, the two in parallel.
470k, 470 kΩ
Grid resistor for tube B, to ground. Each of the two tubes has its own 470k. The resistors are drawn as one part, the two in parallel.
235, 235 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 470. The resistors are drawn as one part, the two in parallel.
235, 235 Ω
Screen resistor for tube B. Each of the two tubes has its own 470. The resistors are drawn as one part, the two in parallel.
4 H, 4 H
A choke from B+ to the screen supply, as in the amp this style is based on. It passes DC with little drop, so the screens sit close to B+. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
100, 100 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
1.8k, 1.8 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Dumble Steel String Singer #002 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.
20µ, 20 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.
4.7k, 4.7 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Dumble Steel String Singer #002 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, 2 kΩ to 4 Ω, 20 H primary, 5 mH leakage
The output transformer, 2 kΩ plate to plate into 4 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, 340 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 340 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, 340 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, 35 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, 35 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.
25µ, 25 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, 340 V ac, 60 Hz
The end of the high-voltage winding the bias comes off, 340 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.
3.3k, 3.3 kΩ
Series resistor into the bias rectifier, 3.3k 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.
10µ, 10 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
Coil R, 4.34 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 4 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, 128 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, 232 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 7.15 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 1.06 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, 1.09 Ω
Limits what Loss L2 adds.
Tank R, 41.9 Ω
Height of the bass resonance, about 46 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 10.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 440 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

Draft drawing
From the revision 7 draft drawing of #002, which says it may contain errors. The tubes are 12AX7s, typical of the type, not measured. The drawing names only V6 in the power amp as a 12AX7.

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.

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.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

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