Two-6AQ5 tremolo practice amp, early 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 GA-5T Skylark.

Two 6EU7 stages with a Loudness pot between them, a cathodyne into two cathode-biased 6AQ5s with no feedback, and a phase-shift tremolo fed straight into the output tubes’ grid leaks. No depth control.

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. Two 6EU7 stages with the Loudness pot between them. The second stage’s plate load is two 47 kΩ in series, and the signal comes from between them, so the cathodyne gets about half the plate’s swing through 250 pF. Two 6AQ5s run in cathode bias with no feedback. The tremolo oscillator is the other section of the cathodyne’s 6EU7. Its plate swing goes through .047 µF to where the output tubes’ grid leaks meet, with 330 kΩ to ground there, so both grids move together and the output stage’s gain rises and falls. There’s no depth control. With the power amp on, a steady note swings about 17 dB at Frequency 5, about 6.3 times a second. Near the top of the Frequency range the oscillator’s swing shrinks, and past about 9 it stops. The 6EU7 runs on the 12AX7 model and the 6AQ5 on the 6V6GT’s.

Download.

Parts and what they do

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

R1A, 100 kΩ
Jack 1’s resistor into the grid, 100 kΩ. Jack 2 has no switch, so with nothing in it its 100 kΩ goes nowhere, and a guitar in jack 1 sees 100 kΩ to the grid and 1 MΩ to ground.
R1B, 100 kΩ
Jack 2’s resistor into the grid, 100 kΩ. With nothing in jack 1, its switch grounds jack 1’s tip, so the two 100 kΩ make a divider: jack 2 is about 6 dB quieter.
RG1, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
V1a, 6EU7
First section of a 6EU7, the input stage.
100k, 100 kΩ
Plate resistor for V1a. The drawing reads +130 V on the plate.
1.5k, 1.5 kΩ
Cathode resistor for V1a, +1 V on the drawing.
20µ, 20 µF electrolytic
Bypass for V1a’s cathode resistor.
.022µ, 22 nF
Coupling cap from V1a to the Loudness pot.
Loud 500k, 500 kΩ pot, audio taper
Loudness, 500 kΩ audio, the only control besides Frequency. Its wiper is V1b’s grid. There’s no tone control and no master.
V1b, 6EU7
Second section of the 6EU7. It drives the cathodyne, which has no gain of its own.
2.2k, 2.2 kΩ
Cathode resistor for V1b, not bypassed, +1.2 V on the drawing.
47k, 47 kΩ
Lower half of V1b’s plate load. The drawing reads +150 V on the plate.
47k, 47 kΩ
Upper half of V1b’s plate load, from the 200 V node. The signal is taken from between the two, so the cathodyne gets about half the plate’s swing.
250p, 250 pF
Only 250 pF into the cathodyne’s grid. The cathodyne’s grid resistor goes to its cathode, so the grid looks like a few megohms and the 250 pF still passes most of the guitar’s range.
470k, 470 kΩ
Plate resistor for the oscillator, from the screen node. The drawing reads +110 V on the plate.
V2b, 6EU7
The other section of V2’s 6EU7, the tremolo oscillator. Its cathode goes straight to ground. The grid biases itself on its 4.7 MΩ leak.
.01µ, 10 nF
First cap of the oscillator’s phase-shift ladder.
330k, 330 kΩ
First ladder leg to ground.
.02µ, 20 nF
Second ladder cap.
Freq, 1 MΩ pot, reverse audio taper
Frequency, 1 MΩ reverse audio, as a variable resistor in the ladder’s middle leg, after the switch on the pot. Turned up, its resistance drops and the oscillator runs faster.
150k, 150 kΩ
In series with the Frequency pot, the rest of the middle leg. It sets the top speed.
.02µ, 20 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.
4.7M, 4.7 MΩ
Grid leak for the oscillator, and the ladder’s last leg.
.047µ, 47 nF
Couples the oscillator’s swing from its plate to where the output tubes’ grid leaks meet, and blocks the plate’s DC.
330k, 330 kΩ
From the grid leaks’ common node to ground. It’s their DC return, and with the .047 µF it sets how much of the swing reaches both output grids. It’s fixed: there’s no depth control.
V2a, 6EU7
6EU7, 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.
100k, 100 kΩ
The cathodyne’s plate resistor, the output to tube B.
1.5k, 1.5 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
100k, 100 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. It’s the grid’s only path to ground, and the cathode follows the grid, so the preamp sees much more than 470k Ω.
.01µ, 10 nF
Coupling cap to tube A’s grid.
.01µ, 10 nF
Coupling cap to tube B’s grid.
22k, 22 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
22k, 22 kΩ
Grid stopper for tube B.
V3, 6AQ5
6AQ5, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V4, 6AQ5
6AQ5, tube B of the pair.
220k, 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.
220k, 220 kΩ
Grid leak for tube B.
270, 270 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.
20µ, 20 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.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the cathodyne and preamp, as on the Gibson GA-5T Skylark 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 cathodyne 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, 10 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.
PT, 218 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 218 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part.
PT, 218 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.
PT R, 100 Ω
Resistance of the other half. Typical of the part.
V5, 6X4
One plate of the 6X4 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.
V5, 6X4
The rectifier’s other plate, on the other half cycle.
20µ, 20 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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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
The 6EU7 and 6AQ5 models are fitted to Tung-Sol plate charts.

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. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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