Shared-plate input tube amp, late 1950s

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 Watkins Dominator 20/T.

Two inputs, each into its own triode, the two triodes sharing one plate resistor and one cathode resistor. A tone and volume network follows, then a long-tail pair and two EL84s with no feedback.

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. Each jack goes straight to its own triode, and the two triodes share one plate resistor and one cathode resistor, so the two inputs mix at the plate with no mixing resistors. The triode not in use still conducts, and its plate resistance, about 75 kΩ, sits across the 100 kΩ plate resistor. That lowers the stage’s gain from about 105 to about 38 before the tone network loads it further, to about 24 at 1 kHz. Then a passive tone and volume network, and the long-tail pair inverter. Two EL84s share one 220 Ω cathode resistor and run with no negative feedback. The amp’s tremolo channel meets this one at the inverter’s second grid. It isn’t modeled here, so that grid goes to ground through its .002 µF.

Download.

Parts and what they do

Stock values for the Watkins 20/T version. Tap a part on the schematic to change it.

1M, 1 MΩ
Grid resistor for V1a, 1 MΩ from input 1’s tip to ground. There’s no grid stopper: the tip goes straight to the grid.
1M, 1 MΩ
Grid resistor for V1b, 1 MΩ from input 2’s tip to ground.
V1a, 12AX7 / ECC83 / 7025
Input 1’s triode. It shares its plate and cathode resistors with V1b, so the two inputs mix at the plate.
V1b, 12AX7 / ECC83 / 7025
Input 2’s triode. With nothing in input 2 its grid is grounded, and it loads the shared plate like a resistor of its own plate resistance, about 75 kΩ.
R4, 100 kΩ
The shared plate resistor, 100 kΩ (WEM: 56 kΩ).
RK1, 820 Ω
The shared cathode resistor, 820 Ω (WEM: 1.5 kΩ). Both triodes’ current runs through it.
CK1, 25 µF electrolytic
Bypass across the shared cathode resistor, 25 µF 12 V (WEM: 50 µF).
CT1, 10 nF
From the plate to the top of the tone pot, .01 µF.
Tone, 500 kΩ pot, audio taper
Tone, .5 MΩ log, its wiper on ground. Toward the .01 µF end it shunts the plate to ground through the cap and cuts treble. Toward the other end it puts only 100 kΩ across the volume, and with the .005 µF ahead of it that thins the bass. Up is assumed to be brighter.
CA, 5 nF
Coupling cap from the plate to the volume, .005 µF. With the volume and the tone network below it, it sets how much bass gets through.
RT, 100 kΩ
From the volume’s top to the tone pot’s bottom end, 100 kΩ.
Volume, 500 kΩ pot, audio taper
Volume, .5 MΩ log (WEM: 2 MΩ).
CO, 10 nF
Coupling cap from the volume’s wiper to the inverter’s grid, .01 µF.
V3a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A. Its grid is fed through the preamp’s coupling cap, so there’s no second one here.
V3b, 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.
47k, 47 kΩ
The tail resistor, to ground. It’s large, so the pair shares a nearly fixed current. When one half conducts more, the other conducts less, which splits the phase.
.002µ, 2 nF
Ties the second grid to ground for signal, so that half is driven only through the shared cathodes. There’s no feedback loop in this style.
100k, 100 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
100k, 100 kΩ
Plate resistor for the other half.
.01µ, 10 nF
Coupling cap to tube A’s grid.
.01µ, 10 nF
Coupling cap to tube B’s grid.
10k, 10 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
10k, 10 kΩ
Grid stopper for tube B.
V4, EL84 / 6BQ5
EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, EL84 / 6BQ5
EL84, tube B of the pair.
500k, 500 kΩ
Grid leak for tube A, to ground. The bias comes from the cathode resistor. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
500k, 500 kΩ
Grid leak for tube B.
100, 100 Ω
One screen resistor shared by both tubes, with no cap after it. It carries both screens’ current. The two screens’ signal currents are opposite and cancel in it, so its drop follows the even harmonics and grows as the stage is driven harder.
220, 220 Ω
The shared cathode resistor. Every output tube’s current runs through it and lifts the cathodes above the grids, and that’s the bias. Driven hard, the pair draws more current on average, so the cathodes climb and the tubes run colder, which softens the clipping.
50µ, 50 µF electrolytic
Bypass cap across the cathode resistor, so the cathodes hold still at the signal’s frequencies. It charges and discharges slowly as the average current changes on a hard note.
1.5k, 1.5 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
40µ, 40 µF electrolytic
Filter cap on the screen supply.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the preamp and inverter, as on the Watkins Dominator 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 and inverter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 7.68 kΩ to 7.5 Ω, 40 H primary, 10 mH leakage
The output transformer, 7.68 kΩ plate to plate into 7.5 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, 275 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 275 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, 275 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, 180 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, 180 Ω
Resistance of the other half. Typical of the part, not measured.
V6, EZ81
One plate of the EZ81 rectifier. It conducts near the peak of its half of the winding, and its own drop rises with the current, which adds to the sag.
V6, EZ81
The rectifier’s other plate, on the other half cycle.
40µ, 40 µF electrolytic
The reservoir cap. 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.
Coil R, 8.15 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 7.5 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, 240 µH
Voice coil inductance. It makes the impedance rise through the treble. With no feedback, the voltage at the speaker rises and falls with its impedance.
Loss L, 435 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 13.4 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 2 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, 2.05 Ω
Limits what Loss L2 adds.
Tank R, 78.5 Ω
Height of the bass resonance, about 87 Ω in all at its peak. With no feedback, the voltage at the speaker rises and falls with its impedance.
Tank L, 19.8 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 235 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

Versions

Watkins 20/T
From a hand drawing of serial 1809 with its measured voltages. ECC83 (12AX7) tubes. The tube models are typical of the type, not measured.
WEM Custom 15, 1965, modeled after the WEM Dominator (Custom 15, 1965)
WEM’s 1965 drawing. 56 kΩ on the shared plates and 1.5 kΩ with 50 µF on the cathodes, and a different tone network: 680 pF from the plate into a 2 MΩ Tone pot with 100 pF to ground at its far end, its wiper on a 2 MΩ Volume. It opens on the Watkins power amp. WEM’s own power amp differs: the inverter’s grids sit on a 100 kΩ and 33 kΩ divider off the supply with 47 kΩ under the cathodes, each EL84 has its own 270 Ω and 50 µF, and the supply runs 320, 300 and 250 V.

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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