Two-channel EL34 amp with relay and photocoupler switching, 1987 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 Kitty Hawk M1.

A shared input stage, then a clean channel (stack, volume, two more triodes, master) and a lead channel (four more triodes, overdrive, its own stack, master), switched by a relay and two photocouplers, mixed into a long-tail pair and four EL34s.

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. One input stage feeds both channels. The clean side has a treble, bass and middle stack right after it, then the volume with a 220 pF bright cap, and two more gain stages. The lead side has the first photocoupler and a 220 kΩ / 2.2 MΩ network ahead of V2A, the overdrive pot after it, then V3B, its own stack, and V3A with the second photocoupler ahead of it. A relay grounds whichever channel is off. Both masters mix through 100 kΩ into a long-tail pair that drives four EL34s at about 500 V, with feedback and presence. The effects loop and the reverb’s op-amps are left out.

Download.

Parts and what they do

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

1M, 1 MΩ
Input resistor to ground, 1 MΩ. The jack goes straight to the grid.
V1B, 12AX7 / ECC83 / 7025
The input stage, shared by both channels. Its cathode is bypassed for full gain.
1.5k, 1.5 kΩ
V1B’s cathode resistor, +1.4 V on the drawing.
22µ, 22 µF electrolytic
V1B’s cathode bypass.
100k, 100 kΩ
V1B’s plate resistor. The drawing reads +195 V on the plate.
100n, 100 nF
Coupling cap from V1B to the clean channel’s stack.
220p, 220 pF
Treble cap of the clean stack.
100k, 100 kΩ
Slope resistor of the clean stack, from the input to the treble pot’s bottom.
Treble 250k, 250 kΩ pot, audio taper
Clean treble, 250 kΩ. Its wiper feeds the volume. Up is assumed to be brighter. Audio taper is assumed.
Bass 250k, 250 kΩ pot, audio taper
Clean bass, 250 kΩ as a variable resistor from the treble pot’s bottom to the middle pot’s top, with 47 nF across it. Up adds resistance, so more bass. Audio taper is assumed.
47n, 47 nF
Across the clean bass pot.
Middle 25k, 25 kΩ pot, linear taper
Clean middle, 25 kΩ as a variable resistor to ground, with 4.7 nF across it. Up adds resistance, so more mids. Linear taper is assumed.
4.7n, 4.7 nF
Across the clean middle pot.
Volume 1M, 1 MΩ pot, audio taper
Clean volume, 1 MΩ. Its wiper is V1A’s grid. Audio taper is assumed.
220p, 220 pF
Bright cap across the clean volume, from its top to its wiper. With the volume down, it passes treble around the track.
V1A, 12AX7 / ECC83 / 7025
Second half of V1, the clean channel’s first stage after the stack.
1.5k, 1.5 kΩ
V1A’s cathode resistor, +1.4 V on the drawing.
22µ, 22 µF electrolytic
V1A’s cathode bypass.
100k, 100 kΩ
V1A’s plate resistor, +190 V on the plate.
10n, 10 nF
Coupling cap from V1A.
2.2M, 2.2 MΩ
In series into V2B’s grid. With the 220 kΩ to ground it drops the signal about 20 dB.
22p, 22 pF
Across the 2.2 MΩ, so the treble gets past it.
220k, 220 kΩ
V2B’s grid resistor to ground. In lead mode the relay grounds this grid.
V2B, 12AX7 / ECC83 / 7025
The clean channel’s last stage.
1.5k, 1.5 kΩ
V2B’s cathode resistor, +1.4 V on the drawing.
22µ, 22 µF electrolytic
V2B’s cathode bypass.
100k, 100 kΩ
V2B’s plate resistor, +193 V on the plate.
100n, 100 nF
Coupling cap to the clean master.
Master 1M, 1 MΩ pot, audio taper
Clean master, 1 MΩ. Its wiper goes to the mix. Audio taper is assumed.
470p, 470 pF
Coupling cap from V1B to the lead channel. Only 470 pF, so the lead side starts thin in the bass.
2.2M, 2.2 MΩ
The path into V2A’s grid that’s always there. Alone, with the 220 kΩ to ground, it passes about a tenth of the signal.
Cell 1, 5 kΩ
Photocoupler 1’s photoresistor, lit: about 5 kΩ, typical. Dark it’s megohms and isn’t modeled.
22p, 22 pF
Across photocoupler 1’s cell.
220k, 220 kΩ
In series with photocoupler 1’s cell into V2A’s grid. Lit, this is the main path: about half the signal gets through.
220k, 220 kΩ
V2A’s grid resistor to ground.
V2A, 12AX7 / ECC83 / 7025
The lead channel’s first stage.
1.5k, 1.5 kΩ
V2A’s cathode resistor.
22µ, 22 µF electrolytic
V2A’s cathode bypass.
100k, 100 kΩ
V2A’s plate resistor, +192 V on the plate.
10n, 10 nF
Coupling cap from V2A. In clean mode the relay grounds its far side.
470k, 470 kΩ
In series to the overdrive pot. With the pot’s 250 kΩ it drops the signal about 10 dB.
470p, 470 pF
Across the 470 kΩ, so the treble gets past it: a treble boost into the overdrive.
Overdrive 250k, 250 kΩ pot, audio taper
Overdrive, 250 kΩ. Its wiper is V3B’s grid. Audio taper is assumed.
V3B, 12AX7 / ECC83 / 7025
The lead channel’s second stage, ahead of its stack.
1.5k, 1.5 kΩ
V3B’s cathode resistor, +1.5 V on the drawing.
22µ, 22 µF electrolytic
V3B’s cathode bypass.
100k, 100 kΩ
V3B’s plate resistor, +181 V on the plate.
1n, 1 nF
Treble cap of the lead stack.
33k, 33 kΩ
Slope resistor of the lead stack.
Treble 250k, 250 kΩ pot, audio taper
Lead treble, 250 kΩ. Up is assumed to be brighter. Audio taper is assumed.
22n, 22 nF
From the slope node to the lead bass pot’s top.
2.2n, 2.2 nF
From the slope node to the lead middle pot’s top.
Bass 250k, 250 kΩ pot, audio taper
Lead bass, 250 kΩ as a variable resistor. Up adds resistance, so more bass. Audio taper is assumed.
Middle 25k, 25 kΩ pot, linear taper
Lead middle, 25 kΩ as a variable resistor to ground. Up adds resistance, so more mids. Linear taper is assumed.
2.2M, 2.2 MΩ
The path into V3A’s grid that’s always there.
470k, 470 kΩ
In series with photocoupler 2’s cell into V3A’s grid.
Cell 2, 5 kΩ
Photocoupler 2’s photoresistor, lit: about 5 kΩ, typical. Dark it’s megohms and isn’t modeled.
47p, 47 pF
Across photocoupler 2’s cell.
220k, 220 kΩ
V3A’s grid resistor to ground.
V3A, 12AX7 / ECC83 / 7025
The lead channel’s last stage.
10p, 10 pF
From V3A’s plate to its grid. It cuts the stage’s treble, above a few kilohertz with the 470 kΩ ahead of it.
1.5k, 1.5 kΩ
V3A’s cathode resistor, +1.5 V on the drawing.
22µ, 22 µF electrolytic
V3A’s cathode bypass.
100k, 100 kΩ
V3A’s plate resistor, +183 V on the plate.
100n, 100 nF
Coupling cap to the lead master.
Master 250k, 250 kΩ pot, audio taper
Lead master, 250 kΩ. Its wiper goes to the mix. Audio taper is assumed.
100k, 100 kΩ
From the clean master’s wiper to the mix node.
100k, 100 kΩ
From the lead master’s wiper to the mix node.
100k, 100 kΩ
From the mix node to ground.
220k, 220 kΩ
One of two 220 kΩ in series from the clean master’s wiper to the lead master’s.
220k, 220 kΩ
The other 220 kΩ, to the lead master’s wiper.
22n, 22 nF
Coupling cap into the long-tail pair’s grid, with the effects loop switched out.
V4A, 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.
V4B, 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.
470, 470 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
10k, 10 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.
.022µ, 22 nF
Ties the second grid to the feedback point for signal, so the feedback acts across the pair.
82k, 82 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.
47k, 47 kΩ
The feedback resistor, 47 kΩ from the speaker tap. With the presence pot it sends about 9 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
Presence, 5 kΩ pot, linear taper
Presence, 5 kΩ linear, the bottom of the feedback loop. Its wiper goes to ground through the cap. Turned up, the cap shunts more of the fed-back treble to ground, so less treble is fed back and the top end rises, a few dB around 5 kHz.
.1µ, 100 nF
The presence cap. With the pot it sets where the lift starts.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
1.1k, 1.1 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 2.2k. The resistors are drawn as one part, the two in parallel.
1.1k, 1.1 kΩ
Grid stopper for tube B. Each of the two tubes has its own 2.2k. The resistors are drawn as one part, the two in parallel.
V5, EL34 / 6CA7
EL34, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V6, EL34 / 6CA7
EL34, tube B of the pair. Two tubes in parallel, drawn as one.
220k, 220 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.
220k, 220 kΩ
Grid leak for tube B.
500, 500 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 1k. The resistors are drawn as one part, the two in parallel.
500, 500 Ω
Screen resistor for tube B. Each of the two tubes has its own 1k. The resistors are drawn as one part, the two in parallel.
130, 130 Ω
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
23.5µ, 23.5 µF electrolytic
Filter cap on the screen supply.
15k, 15 kΩ
Dropping resistor from B+ to the node for the inverter, as on the Kitty Hawk M1 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.
33µ, 33 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.
15k, 15 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Kitty Hawk M1 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.
47µ, 47 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, 1.7 kΩ to 8 Ω, 20 H primary, 5 mH leakage
The output transformer, 1.7 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, 380 V ac, 60 Hz
The power transformer’s high-voltage winding, 380 V ac end to end, into a bridge of four silicon diodes with the center tap unused. It’s drawn as two sources of 380 V ac a half cycle apart into two diodes, which charges the reservoir the same way at one diode drop (under a volt) higher. The value is typical of the part, not measured.
PT, 380 V ac, 60 Hz
The same winding on the other half cycle. It runs opposite the first source, so the bridge charges the reservoir on every half cycle.
PT R, 20 Ω
Resistance of the whole winding, plus the primary’s share. In a bridge every charging pulse runs through the whole winding. With the diodes’ drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 20 Ω
The same winding’s resistance, on the other half cycle. 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.
47µ, 47 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.
Bias tap, 60 V ac, 60 Hz
The power transformer’s bias tap, 60 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
4.7k, 4.7 kΩ
Series resistor into the bias rectifier, 4.7k Ω. It limits the charging current.
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.
15k, 15 kΩ
Second resistor of the bias supply, 15k Ω to the bias line. With the cap after it, it filters the ripple a second time.
62k, 62 kΩ
From the bias line to ground, 62k Ω. With the 15k Ω and 4.7k Ω above it, it divides the supply down to the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
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
Roy Electric’s 1987 drawing. ECC83s throughout. The photoresistors are typical values, 5 kΩ lit and open dark. The output transformer is 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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