Brownface harmonic vibrato amp with a GZ34, early 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 Fender 6G4-A Super (vibrato channel).

The vibrato channel of the brownface Super, the 6G4-A, into its pair of 5881s, a GZ34 rectifier and two 10 in speakers. The signal splits into a low band and a high band, each with its own triode, and a slow oscillator turns one up while it turns the other down. The preamp is the 6G7-A Bandmaster’s, part for part.

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. Speed runs the oscillator from about 3.2 to 9.2 times a second. Intensity sets how much of its swing reaches the band tubes. A follower copies the swing at the oscillator’s plate, and a split-load stage turns it into two equal swings, on its plate and its cathode, so each band tube’s grid gets the same swing the opposite way. At full Intensity each grid swings about 2 V p-p and each band plate about 57 V. Most of that cancels at the mixing node, and the .001 µF into the inverter cuts what’s left to under half a volt. The high band gets the pad’s signal through 250 pF, the low band through 220 kΩ and .01 µF, and the two grids get equal signal near 300 Hz. The treble pot has a tap 70 kΩ up from its bottom lug: the lows and mids reach the tap and the highs the top. Below the tap the wiper slides toward a .003 µF to ground, so the first fifth of the knob cuts highs. With the footswitch down, the oscillator’s ladder is grounded at its middle and the oscillator stops. The normal channel sits idle, there for its current and the cathode it shares. The power amp is the Bandmaster’s with a GZ34 in place of the silicon diodes and 56 kΩ of feedback in place of 100 kΩ, into 4 Ω. On Preamp only, the phase inverter is only a load and box A is held steady. The tube models are typical of a 12AX7, not measured.

Download.

Parts and what they do

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

R1A, 68 kΩ
The vibrato channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R1B, 68 kΩ
The vibrato channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
RIN, 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.
27k 1W, 27 kΩ
Drops the phase inverter’s supply to box B for the gain stages.
20µ, 20 µF electrolytic
Box B’s filter cap.
27k 1W, 27 kΩ
From box A down to box C, the vibrato supply.
20µ, 20 µF electrolytic
Box C’s filter cap.
27k 1W, 27 kΩ
On down to the oscillator’s supply.
20µ, 20 µF electrolytic
The oscillator’s filter cap.
V1a, 12AX7 / ECC83 / 7025
The vibrato channel’s first gain stage.
100k, 100 kΩ
First stage plate resistor.
1500, 1.5 kΩ
First stage cathode resistor, used by this channel alone.
25µ, 25 µF electrolytic
Bypass cap on the first stage’s cathode.
34k, 34 kΩ
The normal channel’s input resistors, with nothing plugged in. Its grid sits at ground.
V1b, 12AX7 / ECC83 / 7025
The normal channel’s first stage, idle. It’s included for the current it draws from box B.
100k, 100 kΩ
Plate resistor for the normal channel’s first stage. The normal channel’s tone network after it is left out.
1500, 1.5 kΩ
Cathode resistor for the normal channel’s first stage.
25µ, 25 µF electrolytic
Bypass cap for the normal channel’s first stage.
250p, 250 pF
Treble cap, from the plate to the top of the treble pot. There’s no coupling cap ahead of the tone network: every path from the plate to ground goes through a cap.
100k, 100 kΩ
The low path, from the plate down to the two caps below.
.1µ, 100 nF
Carries the lows and mids on to the treble pot’s tap.
.02µ, 20 nF
Bass cap, to the bottom of the bass pot and the 6800 Ω.
Bass, 250 kΩ pot, audio taper
Bass, 250 kΩ audio, a variable resistor between the treble pot’s tap and the bass cap. Turned down, it ties the tap to the 6800 Ω through the bass cap, and the bass drains away. Turned up, it puts more resistance in that path.
6800, 6.8 kΩ
From the bottom of the bass pot to ground.
Treble, 350 kΩ pot, linear taper
Treble, 350 kΩ, tapped 70 kΩ from its bottom lug. The lows reach the tap and the highs the top, and the wiper mixes the two. Below the tap the wiper slides toward the .003 µF. No taper is marked, so it’s assumed to be linear.
.003µ, 3 nF
From the treble pot’s bottom lug to ground. With the wiper below the tap it cuts the highs.
Volume, 1 MΩ pot, audio taper
Volume, 1 MΩ audio.
V2a, 12AX7 / ECC83 / 7025
Second gain stage.
100k, 100 kΩ
Most of the plate load. The vibrato’s signal comes from its bottom end, where it meets the 10 kΩ.
10k, 10 kΩ
The rest of the plate load, down to box B. The signal across it is about an eleventh of the signal at the plate, and that’s what feeds the vibrato.
.003µ, 3 nF
From the plate to box B, across the whole plate load. It rolls off the stage’s highs.
820, 820 Ω
Cathode resistor shared with the normal channel’s second stage, so it carries both triodes’ current.
25µ, 25 µF electrolytic
Bypass cap on the shared cathode.
.05µ, 50 nF
Couples the pad into the vibrato’s two bands.
V2b, 12AX7 / ECC83 / 7025
The normal channel’s second stage, its volume at 0.
100k, 100 kΩ
Its plate resistor.
.05µ, 50 nF
The normal channel’s coupling cap to the mixing node.
220k, 220 kΩ
The normal channel’s mixing resistor. With that channel’s volume at 0 it only loads the mixing node.
250p, 250 pF
Only 250 pF straight into V3b’s grid, so the lows barely get through.
220k, 220 kΩ
Into the low band. With the .01 µF below it, it rolls off the highs.
.01µ, 10 nF
The low band’s cap to ground. With the 220 kΩ it makes a gentle low-pass.
.005µ, 5 nF
Couples the low band to V3a’s grid.
1M, 1 MΩ
V3b’s grid resistor.
1M, 1 MΩ
V3a’s grid resistor.
V3a, 12AX7 / ECC83 / 7025
Low-band stage. The LFO swings its grid, which moves its bias and so its gain.
V3b, 12AX7 / ECC83 / 7025
High-band stage. Its grid gets the LFO inverted, so its gain rises while V3a’s gain falls.
4700, 4.7 kΩ
Cathode resistor shared by the two band tubes.
4µ, 4 µF electrolytic
Shared bypass cap, 4 µF.
100k, 100 kΩ
V3a’s plate resistor, 5 %, from box C.
100k, 100 kΩ
V3b’s plate resistor, 5 %, from box C.
470k, 470 kΩ
Low band to the mixing node.
470k, 470 kΩ
High band to the mixing node, where the two bands add back together.
.001µ, 1 nF
Into the phase inverter. Only .001 µF into the inverter’s 1 MΩ, so it trims the deep bass and cuts the LFO left at the mixing node to under half a volt at full Intensity.
V4a, 12AX7 / ECC83 / 7025
The oscillator. The three caps after its plate turn its signal upside down at one frequency, and it oscillates at that frequency.
470k, 470 kΩ
Oscillator plate resistor.
4700, 4.7 kΩ
Oscillator cathode resistor.
25µ, 25 µF electrolytic
Oscillator bypass cap.
.03µ, 30 nF
Plate cap to ground.
.02µ, 20 nF
First ladder cap.
Speed, 4 MΩ pot, reverse audio taper
Speed, 4 MΩ reverse audio, used as a variable resistor.
100k, 100 kΩ
Sets the top speed. It’s all that’s left with Speed on 10.
.01µ, 10 nF
Second ladder cap.
1M, 1 MΩ
Middle resistor of the ladder, returned to the cathode.
.01µ, 10 nF
Third ladder cap.
1M, 1 MΩ
Oscillator grid leak.
V4b, 12AX7 / ECC83 / 7025
A cathode follower straight off the oscillator’s plate. It copies the plate’s swing to the vibrato network without loading the oscillator.
470k, 470 kΩ
The follower’s cathode resistor.
8.2M, 8.2 MΩ
Couples the LFO from the follower.
.02µ, 20 nF
Blocks the follower’s DC.
.1µ, 100 nF
To ground, with the 8.2 MΩ above it. It cuts the LFO well down and smooths it.
Intensity, 10 MΩ pot, reverse audio taper
Intensity, 10 MΩ reverse audio.
.1µ, 100 nF
Smooths the wiper.
.01µ, 10 nF
Couples the LFO to the splitter’s grid.
V5, 12AX7 / ECC83 / 7025
A split-load stage. The same LFO comes off its plate and its cathode in opposite phase, one for each band tube.
56k, 56 kΩ
The splitter’s plate resistor, from box C.
1500, 1.5 kΩ
The splitter’s bias resistor.
56k, 56 kΩ
The splitter’s cathode load. It matches the plate’s 56 kΩ, so the two outputs swing about the same.
1M, 1 MΩ
The splitter’s grid resistor, returned above the 56 kΩ so the grid sits near the cathode.
.1µ, 100 nF
On the splitter’s plate, to ground. It shunts audio and passes the slow LFO.
.1µ, 100 nF
On the splitter’s cathode, to ground. It does the same job.
.05µ, 50 nF
Couples the plate’s LFO toward V3b’s grid.
1M, 1 MΩ
Plate’s LFO to V3b’s grid.
.05µ, 50 nF
Couples the cathode’s LFO toward V3a’s grid.
1M, 1 MΩ
Cathode’s LFO to V3a’s grid.
V6a, 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.
V6b, 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.
820, 820 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
6.8k, 6.8 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.
.1µ, 100 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.
47p, 47 pF
A small cap from plate to plate. It rolls off the far top end the inverter passes on, which keeps the feedback loop steady.
56k, 56 kΩ
The feedback resistor from the speaker. With the 4.7k Ω below it, it sends about 8 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
4.7k, 4.7 kΩ
The bottom of the feedback loop, 4.7k Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.1µ, 100 nF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 25 kΩ pot, linear taper
Presence, a 25k linear pot as a variable resistor, its resistance falling as it’s turned up, in series with the cap across the leg. Turned up, the cap shunts more of the leg’s treble, so less treble is fed back and the top end rises.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
V7, 5881
n5881, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V8, 5881
n5881, tube B of the pair.
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.
470, 470 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
470, 470 Ω
Screen resistor for tube B.
4 H, 4 H
A choke from B+ to the screen supply, as in the amp this style is based on. It passes DC with little drop, so the screens sit close to B+. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
100, 100 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
4.7k, 4.7 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Fender 6G7-A Bandmaster 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 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, 4 kΩ to 4 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 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, 347 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 346.7 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, 347 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, 50 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, 50 Ω
Resistance of the other half. Typical of the part, not measured.
V9, GZ34
One plate of the GZ34 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.
V9, GZ34
The rectifier’s other plate, on the other half cycle.
20µ, 20 µF electrolytic
The reservoir caps’ first. 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.
20µ, 20 µF electrolytic
Another reservoir cap, in parallel with the first.
Bias tap, 51 V ac, 60 Hz
The power transformer’s bias tap, 51.3 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
tap R, 100 Ω
The bias tap’s own resistance, 100 Ω, typical, not measured. The drawing has no resistor between the tap and the diode.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
8µ, 8 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.
8µ, 8 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
56k, 56 kΩ
From the bias line to ground, 56k Ω. With the 15k 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, 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. The feedback holds the output steadier against that than it would be without feedback.
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. The feedback holds the output steadier against that than it would be without feedback.
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
Every gain tube is a 7025, a low-noise 12AX7, and the oscillator and its splitter are 12AX7s, all fitted to the Brimar 12AX7 data sheet. Typical of the type, not measured.

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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