Two-channel EL84 reverb amp with transistor tremolo, late 1960s

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 Gibson Falcon (EL84 version).

Two 12AX7 channels with treble and bass, mixed into a 12AX7 mixer with the reverb, then a driver and cathodyne into two cathode-biased EL84s. A triode oscillator drives a transistor that shunts channel 1’s grid.

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 channel is one 12AX7 stage into a passive treble and bass network and a volume pot. Both volumes feed a mixing node through 1 MΩ each. From there one path runs through a 220 kΩ / 220 kΩ filter with a .0047 µF to ground and 500 pF across it, a notch near 470 Hz, to the mixer stage, and another drives the reverb. The mixer’s .0022 µF into the driver’s 220 kΩ cuts the deep bass. The driver has 15 kΩ of feedback from the speaker on its cathode, then a cathodyne splits the signal for two EL84s on a shared 100 Ω. The tremolo works on channel 1 only: the oscillator’s cathode current drives a germanium transistor, which shunts that channel’s grid through 15 kΩ and the Depth control. The springs are a measured tank, played from its impulse response. The tube models are typical of the types, the transformers typical figures, not measured.

Download.

Parts and what they do

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

R1B, 47 kΩ
Channel 2’s input 1 resistor, 47 kΩ into the grid. Input 2 has no switch, so a guitar in input 1 sees only this one into the grid.
R3B, 47 kΩ
Channel 2’s input 2 resistor, 47 kΩ into the grid. With nothing in input 1 that jack grounds its own tip, so the two 47 kΩ make a divider and input 2 is about 6 dB quieter.
R1A, 47 kΩ
Channel 1’s input 1 resistor, 47 kΩ into the grid. Input 2 has no switch, so a guitar in input 1 sees only this one into the grid.
R3A, 47 kΩ
Channel 1’s input 2 resistor, 47 kΩ into the grid. With nothing in input 1 that jack grounds its own tip, so the two 47 kΩ make a divider and input 2 is about 6 dB quieter.
V1a, 12AX7 / ECC83 / 7025
One half of the first 12AX7, channel 2’s only gain stage.
100k, 100 kΩ
Plate resistor for V1a.
820, 820 Ω
Cathode resistor for V1a, not bypassed, so the stage has less gain.
.01µ, 10 nF
Coupling cap to channel 2’s tone node.
.0022µ, 2.2 nF
Channel 2’s treble cap. With the Treble control under it, it bleeds treble from the tone node to ground.
Treble 2, 1 MΩ pot, audio taper
Channel 2’s Treble, a 1 MΩ pot as a variable resistor under the .0022 µF. Turned up, its resistance rises, less treble goes to ground and the channel gets brighter. Taper typical, not on the drawing.
Bass 2, 5 MΩ pot, reverse audio taper
Channel 2’s Bass, a 5 MΩ pot as a variable resistor in series with the signal, with 330 pF across it. Turned up, its resistance drops and more of the low end gets past. Reverse audio taper, typical (the drawing marks none, and Gibson marks its controls wired this way reverse audio on the GA-19RVT).
330p, 330 pF
Across channel 2’s Bass control. It always lets the treble past, whatever the Bass is set to.
Vol 2, 1 MΩ pot, audio taper
Channel 2’s Volume, 1 MΩ audio. Its wiper feeds the mixing node through 1 MΩ.
1M, 1 MΩ
Mixing resistor from channel 2’s volume.
V1b, 12AX7 / ECC83 / 7025
The other half of the first 12AX7, channel 1’s gain stage. The tremolo works on its grid.
100k, 100 kΩ
Plate resistor for V1b.
820, 820 Ω
Cathode resistor for V1b, not bypassed.
.01µ, 10 nF
Coupling cap to channel 1’s tone node.
.0022µ, 2.2 nF
Channel 1’s treble cap, to ground through the Treble control.
Treble 1, 1 MΩ pot, audio taper
Channel 1’s Treble, a 1 MΩ variable resistor under the .0022 µF. Turned up, less treble goes to ground. Taper typical, not on the drawing.
Bass 1, 5 MΩ pot, reverse audio taper
Channel 1’s Bass, a 5 MΩ variable resistor in series with the signal, 330 pF across it. Turned up, its resistance drops and more bass gets past. Reverse audio taper, typical (the drawing marks none, and Gibson marks its controls wired this way reverse audio on the GA-19RVT).
330p, 330 pF
Across channel 1’s Bass control. It lets the treble past.
Vol 1, 1 MΩ pot, audio taper
Channel 1’s Volume, 1 MΩ audio. Its wiper feeds the mixing node through 1 MΩ.
1M, 1 MΩ
Mixing resistor from channel 1’s volume.
220k, 220 kΩ
First half of the filter from the mixing node to the mixer’s grid.
220k, 220 kΩ
Second half of the filter into the mixer’s grid.
.0047µ, 4.7 nF
From the middle of the two 220 kΩ to ground. With the 500 pF across both 220 kΩ it makes a bridged-T notch near 470 Hz, a deep dip in the low mids.
500p, 500 pF
Across both 220 kΩ. It carries the treble past the filter.
470k, 470 kΩ
Grid resistor for the mixer.
V2a, 12AX7 / ECC83 / 7025
One half of the second 12AX7, the mixer, where the dry signal and the reverb meet.
100k, 100 kΩ
Plate resistor for the mixer.
1k, 1 kΩ
Cathode resistor for the mixer, not bypassed. The drawing reads +1.5 V on it.
.0022µ, 2.2 nF
Only .0022 µF into the driver’s 220 kΩ, so everything below about 330 Hz is cut slightly more each octave down. It keeps the bass from muddying the small output stage.
15k, 15 kΩ
From channel 1’s grid to the Depth control. With Depth at full and the transistor on, it and the 47 kΩ ahead of the grid make the divider that sets the deepest swing.
Depth, 1 MΩ pot, reverse audio taper
Depth, a 1 MΩ pot as a variable resistor between the 15 kΩ and Q1. Turned up, its resistance drops, so the transistor pulls the grid down harder. Reverse audio taper, typical (the drawing marks none, and Gibson marks its controls wired this way reverse audio on the GA-19RVT).
Q1, 2N388
The tremolo driver, a germanium NPN. Its base sits on the oscillator’s cathode, so the tube’s cathode current runs into it. As that current rises and falls, the transistor turns on and off and shunts channel 1’s grid to ground through Depth and the 15 kΩ. Its collector has no supply, so it works as a switch, not an amplifier.
150k, 150 kΩ
Plate resistor for the oscillator. The drawing reads +157 V on the plate while it runs.
V3b, 12AX7 / ECC83 / 7025
One half of the third 12AX7, the tremolo oscillator. The other half is the cathodyne in the power amp.
330, 330 Ω
Cathode resistor for the oscillator. Q1’s base sits across it, so the base-emitter junction clamps the cathode to a few tenths of a volt.
470k, 470 kΩ
From the supply to the oscillator’s cathode and Q1’s base. It feeds the base a steady half milliamp or so.
.033µ, 33 nF
First cap of the oscillator’s phase-shift ladder.
68k, 68 kΩ
In series with the Speed control, the ladder’s first leg. It sets the top speed.
Speed, 500 kΩ pot, reverse audio taper
Speed, 500 kΩ as a variable resistor in the ladder’s first leg. Turned up, its resistance drops and the oscillator runs faster. Reverse audio taper, typical (the drawing marks none, and Gibson marks its controls wired this way reverse audio on the GA-19RVT).
.022µ, 22 nF
Second ladder cap.
510k, 510 kΩ
Second ladder leg to ground, 5 % on the drawing.
.022µ, 22 nF
Third ladder cap, into the grid. The three caps and legs turn the plate’s swing round by half a cycle at one frequency, so it oscillates there.
1.5M, 1.5 MΩ
Grid leak for the oscillator, and the ladder’s last leg.
470k, 470 kΩ
Grid resistor for the reverb driver, from the mixing node to ground.
V4a, 12AX7 / ECC83 / 7025
One half of the fourth 12AX7, the reverb driver. Its grid is the mixing node itself. The drawing reads +2.75 V on its cathode, about 1.25 mA.
2.2k, 2.2 kΩ
Cathode resistor for the reverb driver.
1µ, 1 µF electrolytic
Bypass for the reverb driver’s cathode, full gain above about 70 Hz.
T2, 25 kΩ to 8 Ω, 20 H primary, 100 mH leakage
The reverb transformer, from the driver’s plate to the spring unit. It loads the driver and the supply. Typical of a small 12AX7-driven reverb transformer, not on the drawing.
Unit in R, 800 mΩ
Resistance of the reverb unit’s input coil. The current through it drives the springs, which play from a measured tank’s impulse response. Typical of a long spring tank, not measured.
Unit in L, 1.3 mH
Inductance of the reverb unit’s input coil. Typical of a long spring tank, not measured.
Unit out L, 360 mH
The reverb unit’s output coil. The springs come back in series with it. Typical of a long spring tank, not measured.
Unit out R, 200 Ω
Resistance of the unit’s output coil. Typical, not measured.
.15µ, 150 nF
From the unit’s output to the recovery grid.
5.6k, 5.6 kΩ
Grid resistor for the recovery stage, only 5.6 kΩ, loading the unit’s output coil.
V4b, 12AX7 / ECC83 / 7025
The other half of the fourth 12AX7, the reverb recovery stage. It brings the springs back up.
100k, 100 kΩ
Plate resistor for the recovery stage.
470, 470 Ω
Cathode resistor for the recovery stage, not bypassed.
.01µ, 10 nF
Coupling cap to the Reverb control.
.0047µ, 4.7 nF
From the Reverb control’s top to ground. It trims the springs’ top end.
Reverb, 500 kΩ pot, audio taper
Reverb, 500 kΩ. Its wiper feeds the mixer’s grid through 470 kΩ. Turn it up for more of the springs. Taper typical, not on the drawing.
470k, 470 kΩ
From the Reverb control to the mixer’s grid.
V2b, 12AX7 / ECC83 / 7025
12AX7, the driver. A cathodyne has no gain of its own, so this stage raises the preamp’s signal about 30 times, enough to drive the output tubes. Its grid is fed through the preamp’s coupling cap, so there’s no second one here.
220k, 220 kΩ
Grid resistor for the driver.
100k, 100 kΩ
Plate resistor for the driver.
4.7k, 4.7 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or so. It sits on the feedback leg, so the fed-back signal moves the driver’s cathode.
.01µ, 10 nF
Coupling cap from the driver to the cathodyne.
V3a, 12AX7 / ECC83 / 7025
The cathodyne, or split-load inverter. The same current runs through its plate resistor and its cathode resistors, which are about equal, so the plate swings one way and the cathode the other by about the same amount. The cathode drives tube A, the plate tube B. Its gain is slightly under 1 on each side.
150k, 150 kΩ
The cathodyne’s plate resistor, the output to tube B.
3.3k, 3.3 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
150k, 150 kΩ
The cathodyne’s cathode load, the same value as its plate resistor. The cathode above the bias resistor is the output to tube A.
470k, 470 kΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage.
15k, 15 kΩ
The feedback resistor from the speaker. With the 470 Ω below it, it sends about 3 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
470, 470 Ω
The bottom of the feedback loop, 470 Ω to ground under the driver’s cathode resistor. It isn’t bypassed, so the fed-back speaker voltage moves the cathode. The driver’s current runs through it too, so it’s kept small.
.01µ, 10 nF
Coupling cap to tube A’s grid.
.01µ, 10 nF
Coupling cap to tube B’s grid.
V5, EL84 / 6BQ5 (Philips curves)
EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V6, EL84 / 6BQ5 (Philips curves)
EL84, tube B of the pair.
220k, 220 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.
220k, 220 kΩ
Grid leak for tube B.
100, 100 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.
100µ, 100 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.
1k, 1 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the inverter and preamp, as on the Gibson Falcon (EL84) 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 inverter and preamp. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 8 kΩ to 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 8 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, 243 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 243 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, 243 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, 60 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, 60 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.
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, 8.69 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.

About this version

As drawn
All three 12AX7s use the 12AX7 model. Q1 is a 2N388 model, a germanium NPN typical of the part. The reverb transformer and unit are typical, not on the drawing.

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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