Pentode tremolo amp with a mic channel, 1956 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 Gibson GA-20T Ranger.

A 5879 pentode instrument channel with a 6SQ7 tremolo on its cathode, and a microphone channel on a 12AY7, mixed straight on the phase inverter’s grid. Two 6V6GTs, a paraphase inverter, 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. Two channels meet at the phase inverter’s grid, each through the top end of its 1 MΩ volume pot, with no mixing resistors. The instrument channel’s 5879 runs starved: 510 kΩ on the plate and its screen held near 28 V by a divider. Its cathode resistor also carries the 6SQ7 oscillator’s current, so as the oscillator swings, the cathode moves and the 5879’s gain moves with it. Five .005 µF caps with 1 MΩ to ground strip the slow swing off the plate, and some bass with it. The mic channel is both halves of a 12AY7 on one plate resistor, one per jack, biased by their 1 MΩ grid leaks. The tone control is a treble cut on the inverter’s grid. Depth sets how hard the oscillator is fed, and Speed runs it from about 3.5 to 7.7 times a second.

Download.

Parts and what they do

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

.02µ, 20 nF
Input cap from mic jack 1 to its half of the 12AY7. With nothing plugged in, the jack’s switch grounds the tip.
1M, 1 MΩ
Grid leak, 1 MΩ. The cathodes are grounded, so the grid biases itself a little negative from the grid current through this resistor.
V1a, 12AY7 / 6072
One half of the 12AY7, for mic jack 1. Cathode straight to ground, plate tied to the other half’s.
.02µ, 20 nF
Input cap from mic jack 2. That jack is empty on the bench, so its switch grounds this end.
1M, 1 MΩ
Grid leak for the second half.
V1b, 12AY7 / 6072
The 12AY7’s other half, for mic jack 2. With nothing in that jack it still pulls current and loads the shared plate, so it lowers the first half’s gain.
220k, 220 kΩ
Plate resistor for both halves, 220 kΩ. The drawing reads +47 V on the plates.
.01µ, 10 nF
Coupling cap to the mic volume’s wiper.
Vol 1M, 1 MΩ pot, audio taper
Mic volume, 1 MΩ. The signal comes in on the wiper and leaves from the top end, which is the mixing node. Turned down, the wiper shorts the signal to ground. The taper isn’t marked, so audio is assumed.
220k, 220 kΩ
From instrument jack 1’s tip to ground.
100k, 100 kΩ
From instrument jack 1 to the 5879’s grid.
220k, 220 kΩ
From instrument jack 2’s tip to ground. These jacks have no switch, so with jack 2 empty this 220 kΩ and its 100 kΩ are the grid’s only leak, 320 kΩ.
100k, 100 kΩ
From instrument jack 2 to the 5879’s grid. With the guitar in jack 1 it forms a divider with the 220 kΩ below it, and the grid gets about 3 dB less than the jack.
V2, 5879
The 5879 pentode, the instrument channel. Its cathode is shared with the oscillator, which is how the tremolo gets in.
510k, 510 kΩ
Plate resistor, 510 kΩ. The drawing reads +126 V on the plate.
1M, 1 MΩ
Top of the screen divider. With the 150 kΩ below it, the screen sits low, +24 V on the drawing, so the plate current is small.
150k, 150 kΩ
Bottom of the screen divider, to ground.
.25µ, 250 nF
Screen bypass cap.
2.2k, 2.2 kΩ
Cathode resistor for both V2 and the oscillator, about +1 V on the drawing. Most of its current is the oscillator’s, so as the oscillator swings the cathode voltage moves, and V2’s gain moves with it.
20µ, 20 µF electrolytic
Bypass cap on the shared cathode. At the tremolo’s few cycles a second it’s about 1.5 kΩ, so it lowers the swing a bit without stopping it.
.005µ, 5 nF
First of five .005 µF caps in a row, each with 1 MΩ to ground after it. Together they cut the slow tremolo swing from V2’s plate, and some bass.
1M, 1 MΩ
Ladder resistor to ground.
.005µ, 5 nF
Ladder cap.
1M, 1 MΩ
Ladder resistor to ground.
.005µ, 5 nF
Ladder cap.
1M, 1 MΩ
Ladder resistor to ground.
.005µ, 5 nF
Ladder cap.
1M, 1 MΩ
Ladder resistor to ground.
.005µ, 5 nF
Last ladder cap, into the instrument volume’s wiper.
Vol 1M, 1 MΩ pot, audio taper
Instrument volume, 1 MΩ, wired like the mic volume, its top end on the mixing node. The taper isn’t marked, so audio is assumed.
.01µ, 10 nF
Tone cap, .01 µF from the mixing node to the tone pot. With the pot it bleeds treble to ground.
Tone, 250 kΩ pot, audio taper
Tone, 250 kΩ as a variable resistor from the cap to ground. Turned down, its resistance drops and the cap bleeds more treble off the inverter’s grid. The drawing marks no taper, so audio is assumed. Up is brighter.
510k, 510 kΩ
One of the oscillator’s two feeds, from V2’s plate. With the .01 µF and the second 510 kΩ it keeps the guitar signal out of the oscillator, but the slow swing gets back to V2’s plate this way.
.01µ, 10 nF
Filter cap in the middle of the feed from V2’s plate.
510k, 510 kΩ
Second half of the feed from V2’s plate.
V3, 12SQ7
The 6SQ7’s triode, the tremolo oscillator. Its cathode is V2’s.
100k, 100 kΩ
From the oscillator’s plate to the Depth pot.
Depth, 500 kΩ pot, reverse audio taper
Depth, 500 kΩ as a variable resistor in the oscillator’s main feed from the 270 V screen node. Turned up, its resistance drops, the oscillator pulls more current and swings harder, and so does the shared cathode. The drawing marks no taper, so reverse audio is assumed, as on the GA-40.
.05µ, 50 nF
First cap of the oscillator’s phase-shift ladder.
47k, 47 kΩ
In series with the Speed pot, the ladder’s first leg to ground. It sets the fastest speed.
Speed, 500 kΩ pot, linear taper
Speed, 500 kΩ as a variable resistor. Turned up, its resistance drops and the oscillator runs faster. Its far end goes through the remote footswitch to ground. The drawing marks no taper, so linear is assumed.
.05µ, 50 nF
Second ladder cap.
240k, 240 kΩ
Second ladder leg to ground.
.05µ, 50 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.
510k, 510 kΩ
Grid leak for the oscillator, and the ladder’s last leg.
V4a, 12AX7 / ECC83 / 7025
12AX7, the first half of the paraphase inverter. It’s a plain gain stage: its plate swings opposite the input and drives tube A. Its grid connects straight to the preamp’s output with no coupling cap, so its path to ground is through the preamp’s volume and tone network.
220k, 220 kΩ
Plate resistor for the first half, the output to tube A.
680, 680 Ω
Cathode resistor for both halves. The tube has one cathode for its two triodes, so this carries both currents and sets both biases.
220k, 220 kΩ
Tube A’s grid resistor, down to the tap. With the resistor under the tap it feeds a small share of tube A’s grid signal to the second half’s grid. The share is about one over the second half’s gain, so the two output grids swing about the same.
4.7k, 4.7 kΩ
The tap, 4.7k Ω to ground under tube A’s grid resistor, and the second half’s grid resistor. The two sides only match while the share times the second half’s gain is 1, so they drift apart as the tube ages.
V4b, 12AX7 / ECC83 / 7025
The second half. It turns the divided signal over again, so its plate swings with the input. It drives tube B.
220k, 220 kΩ
Plate resistor for the second half, the output to tube B.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 nF
Coupling cap to tube B’s grid.
V5, 6V6GT
6V6GT, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V6, 6V6GT
6V6GT, tube B of the pair.
220k, 220 kΩ
Grid leak for tube B.
200, 200 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.
10k, 10 kΩ
Dropping resistor from B+ to the screens. It’s large, so the screens sit about 30 V below the plates and drop further when they draw more current on a hard note.
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 GA-20T Ranger 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.
10µ, 10 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, 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, 320 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 320 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part.
PT, 320 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.
PT R, 60 Ω
Resistance of the other half. Typical of the part.
V7, 5Y3GT
One plate of the 5Y3GT 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.
V7, 5Y3GT
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 5879 is fitted to one published point.

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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