Two-channel EL84 amp with grid tremolo, mid 1960s 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 Harmony H415.

Two channels, each a 12AX7 triode with its own Volume and Tone, mixed through 470 kΩ into a second gain stage, then a cathodyne with no driver into two cathode-biased EL84s. A pentode oscillator swings both output grids for the tremolo.

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 two channels differ. Channel 1 has 1.5 kΩ on its cathode, unbypassed, and 390 kΩ from the grid to ground, so jack 1 or 2 is a 68 kΩ / 68 kΩ divider ahead of a stage with about half its full gain. Channel 2’s cathode is grounded, biased by grid current through 2.7 MΩ. Each volume has its own tone pot from a 680 pF cap around the volume to .01 µF to ground. The channels mix through 470 kΩ into V2a, which drives a cathodyne straight into two EL84s with no feedback. The tremolo is a pentode phase-shift oscillator. Its swing goes through the intensity pot to where the EL84s’ grid leaks meet, so both grids move together and the output stage’s gain rises and falls. The footswitch jack grounds that swing with nothing plugged in.

Download.

Parts and what they do

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

68k, 68 kΩ
Jack 1’s resistor into channel 1’s grid (R1).
68k, 68 kΩ
Jack 2’s resistor (R2). With nothing in jack 2 its switch grounds the tip, so this and R1 make a divider: about 6 dB down.
390k, 390 kΩ
Channel 1’s grid resistor to ground (R5).
100p, 100 pF
From channel 1’s grid to ground (C1). With the 34 kΩ ahead of it, it cuts above about 50 kHz, against radio pickup.
V1a, 12AX7 / ECC83 / 7025
Half of V1, channel 1’s gain stage.
1.5k, 1.5 kΩ
Channel 1’s cathode resistor (R6), unbypassed, +0.7 V on the drawing.
100k, 100 kΩ
Channel 1’s plate resistor (R8), from the +150 V node. The drawing reads +100 V on the plate.
68k, 68 kΩ
Jack 3’s resistor into channel 2’s grid (R3). With nothing in jack 3, its switch ties its tip to jack 4’s.
68k, 68 kΩ
Jack 4’s resistor (R4). With nothing in jack 4 its switch grounds the tip, so jack 3 is a divider. A guitar in jack 4 sees R3 and R4 in parallel.
2.7M, 2.7 MΩ
Channel 2’s grid leak (R7). The cathode is grounded, so grid current through it sets the bias.
100p, 100 pF
From channel 2’s grid to ground (C2), against radio pickup. Marked with a + on the drawing, read as 100 pF like C1.
V1b, 12AX7 / ECC83 / 7025
The other half of V1, channel 2’s gain stage, its cathode grounded.
100k, 100 kΩ
Channel 2’s plate resistor (R9). The drawing reads +68 V on the plate: the stage runs hotter than channel 1’s.
100k, 100 kΩ
Dropping resistor from +280 V to V1’s +150 V node (R27).
20µ, 20 µF electrolytic
Filter cap on the +150 V node (C13).
.022µ, 22 nF
Coupling cap to channel 1’s volume (C3).
Vol 1 500k, 500 kΩ pot, audio taper
Channel 1’s volume, 500 kΩ (R10). Audio taper is assumed.
680p, 680 pF
Tone 1’s bright cap (C22), from the volume’s top to the tone pot.
Tone 1 500k, 500 kΩ pot, audio taper
Channel 1’s tone, 500 kΩ (R12), its wiper on the volume’s wiper. One end goes through 680 pF to the volume’s top, the other through .01 µF to ground. Up moves the wiper toward the 680 pF, so treble gets around the volume. Up is assumed to be brighter.
.01µ, 10 nF
Tone 1’s treble cut (C4), to ground.
470k, 470 kΩ
Mixing resistor from channel 1 to V2a’s grid (R14).
.022µ, 22 nF
Coupling cap to channel 2’s volume (C5).
Vol 2 500k, 500 kΩ pot, audio taper
Channel 2’s volume, 500 kΩ (R11). Audio taper is assumed.
680p, 680 pF
Tone 2’s bright cap (C23).
Tone 2 500k, 500 kΩ pot, audio taper
Channel 2’s tone, 500 kΩ (R13), wired as channel 1’s. Up is assumed to be brighter.
.01µ, 10 nF
Tone 2’s treble cut (C6), to ground.
470k, 470 kΩ
Mixing resistor from channel 2 to V2a’s grid (R15).
V2a, 12AX7 / ECC83 / 7025
Half of V2, the gain stage after the mix. The other half is the power amp’s cathodyne.
1.5k, 1.5 kΩ
V2a’s cathode resistor (R16), unbypassed, +1.3 V on the drawing.
100k, 100 kΩ
V2a’s plate resistor (R17), from +280 V. The drawing reads +190 V on the plate.
.022µ, 22 nF
Coupling cap to the cathodyne’s grid (C7).
V5, 6AU6 / EF94
The tremolo oscillator, a small pentode (a 6AU6 by its pins), wired as a phase-shift oscillator.
100k, 100 kΩ
The oscillator’s plate resistor, first part (R29). The tremolo comes from its top end. The drawing reads +160 V on the plate.
150k, 150 kΩ
The oscillator’s plate resistor, second part (R28), from +280 V.
390k, 390 kΩ
Screen resistor for the oscillator (R30). With R31 it holds the screen near the drawing’s +54 V.
150k, 150 kΩ
From the oscillator’s screen to ground (R31).
.25µ, 250 nF
Screen bypass for the oscillator (C17).
.01µ, 10 nF
First cap of the phase-shift ladder (C14).
68k, 68 kΩ
In series with the speed pot, the ladder’s first leg (R34). It sets the top speed.
Speed 1M, 1 MΩ pot, linear taper
Speed, 1 MΩ (R35), as a variable resistor in the ladder’s first leg to ground. Turned up, its resistance drops and the oscillator runs faster. Linear taper is assumed.
.022µ, 22 nF
Second ladder cap (C15).
390k, 390 kΩ
Second ladder leg to ground (R33).
.022µ, 22 nF
Third ladder cap, into the grid (C16). The three caps and legs turn the plate’s swing round by half a cycle at one frequency, so it oscillates there.
2.7M, 2.7 MΩ
Grid leak for the oscillator, and the ladder’s last leg (R32).
.03µ, 30 nF
Couples the oscillator’s swing from the plate resistor’s tap to the intensity pot (C18).
Intensity 500k, 500 kΩ pot, linear taper
Intensity, 500 kΩ (R36), from the cap to ground. Its wiper is where the EL84s’ grid leaks meet, so it sets how much of the swing reaches both grids. Linear taper is assumed. On Preamp only, the wiper drives nothing.
.1µ, 100 nF
From the intensity wiper to ground (C19). It rounds off the oscillator’s swing and keeps audio off the grids’ return.
V2b, 12AX7 / ECC83 / 7025
12AX7, a cathodyne, or split-load inverter, with no driver ahead of it. 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, so the preamp has to bring the whole swing the output tubes need.
47k, 47 kΩ
The cathodyne’s plate resistor, the output to tube B.
5.6k, 5.6 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
47k, 47 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.
2.7M, 2.7 MΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage. It’s the grid’s only path to ground, and the cathode follows the grid, so the preamp sees much more than 2.7M Ω.
C8, 22 nF
Coupling cap to tube A’s grid.
C9, 22 nF
Coupling cap to tube B’s grid.
V3, EL84 / 6BQ5 (Philips curves)
EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V4, EL84 / 6BQ5 (Philips curves)
EL84, tube B of the pair.
R22, 220 kΩ
Grid leak for tube A, returned with tube B’s to the tremolo’s Depth pot, so the oscillator swings both grids together. 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.
R23, 220 kΩ
Grid leak for tube B.
R24, 180 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.
C10, 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.
R25, 560 Ω
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
.001µ, 1 nF electrolytic
Filter cap on the screen supply.
R26, 10 kΩ
Dropping resistor from B+ to the node for the inverter, the preamp and the tremolo, as on the Harmony H415 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.
C12, 20 µF electrolytic
Filter cap at the node for the inverter, the preamp and the tremolo. 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 4 Ω, 10 H primary, 10 mH leakage
The output transformer, 8 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, 265 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 265 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, 265 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, 100 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, 100 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.
C11, 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, 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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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

As drawn
Harmony’s drawing. The tube types aren’t printed: 12AX7s for V1 and V2 and a 6AU6 for the oscillator, by their pins and readings. The transformers 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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