Two-channel reverb amp with a 7199 pentode and cathodyne, 1965 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 Ampeg Gemini II G-15.

Two 12AX7 channels, each a stage, the volume, a second stage and a bass and treble network, mixed into the pentode half of a 7199. Reverb and an optical tremolo on Channel 1. The 7199’s triode is a cathodyne driving two 6L6GCs.

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. Each channel is two stages of a 12AX7 with the volume between them and the bass and treble network after. The first stage’s cathode isn’t bypassed, and the second has only .1 µF on its cathode, so the lows get less gain. Channel 1’s Treble wiper also drives a 6CG7 reverb driver. The springs are a measured tank. The return comes back through Dimension to the mix. The 7199’s pentode makes up the level lost in the networks and the mix, and the feedback from the speaker goes to its cathode. Its triode, in the power amp, is the cathodyne. The tremolo works on Channel 1: half a 12AX7 runs as a phase-shift oscillator, about 5.5 to 7.5 times a second, and its plate lights the TM-1 module’s neon through 390 kΩ. The module’s photocell sits in series in the channel with the 4 MΩ Intensity pot across it. When the neon goes out the cell goes dark and the note dips. The neon and cell figures are typical.

Download.

Parts and what they do

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

47k, 47 kΩ
Jack 1’s resistor to the grid, 47 kΩ.
47k, 47 kΩ
Jack 2’s resistor to the grid, 47 kΩ. With the guitar in jack 2 and nothing in jack 1, jack 1’s 47 kΩ and 100 kΩ hang from the grid to ground, so the note comes in about 2.4 dB lower.
100k, 100 kΩ
On jack 1’s switch: 100 kΩ from its tip to ground with nothing plugged in. A plug opens the switch.
5.6M, 5.6 MΩ
Grid leak, 5.6 MΩ.
V4a, 12AX7 / ECC83 / 7025
Half of a 12AX7, the Channel 1 input stage. Its cathode isn’t bypassed, so its gain is about 35.
4.7k, 4.7 kΩ
Cathode resistor, 4.7 kΩ, not bypassed. The factory drawing reads +2 V here.
270k, 270 kΩ
Plate resistor, 270 kΩ, from node E. The factory drawing reads +200 V on the plate.
.1µ, 100 nF
Coupling cap into the Volume pot.
Volume 1, 1 MΩ pot, audio taper
Channel 1 volume, 1 MΩ, between the two stages. Its wiper is the second stage’s grid. The drawing marks no taper, so audio is assumed.
.00025µ, 250 pF
The Ultra High cap, 250 pF, from the volume pot’s top to its wiper when the switch is on. With the volume down, treble goes around the pot. At full it does nothing. Piazza’s redraw shows 25 pF here. The factory drawing shows .00025 µF.
V4b, 12AX7 / ECC83 / 7025
The other half of the 12AX7, the second stage, ahead of the tone network.
3.3k, 3.3 kΩ
Cathode resistor, 3.3 kΩ. The factory drawing reads +2 V here.
.1µ, 100 nF
Only .1 µF across the cathode resistor, so the stage has its full gain only from about 500 Hz up. The lows get less gain.
150k, 150 kΩ
Plate resistor, 150 kΩ. The factory drawing reads +210 V on the plate.
.1µ, 100 nF
Coupling cap into the bass and treble network.
TM-1, Ampeg TM-1 cell
The photocell in the TM-1 tremolo module, in series in Channel 1’s path, with the Intensity pot across it. Lit by the module’s neon its resistance falls and the note passes. Dark, it rises toward 35 MΩ and the note goes through the pot only. Its figures are typical of the TM-1 class.
Intensity 4M, 4 MΩ pot, audio taper
Intensity, 4 MΩ, a variable resistor across the photocell (its wiper is tied to one end). At 0 it shorts the cell and nothing moves. Turned up, more resistance is across the cell, so the note dips deeper each time the cell goes dark. The taper isn’t marked, so audio is assumed.
220k, 220 kΩ
Feeds the Bass pot’s top. With the 22 kΩ under the pot it sets how far the network drops the lows.
Bass 1, 1 MΩ pot, linear taper
Bass, 1 MΩ, with a cap across each half. Its wiper feeds the Treble wiper through 120 kΩ. Toward the top the lows from the 220 kΩ get through, toward the bottom they’re pulled to the 22 kΩ. The taper isn’t marked, so linear is assumed.
.001µ, 1 nF
From the Bass pot’s top to its wiper. It passes the mids around the upper half of the pot.
.01µ, 10 nF
From the Bass pot’s wiper to its bottom. It passes the mids around the lower half.
22k, 22 kΩ
From the Bass pot’s bottom to ground, the bottom of the network’s divider.
270k, 270 kΩ
From the Bass pot’s wiper to ground. It drops the level slightly more.
.00047µ, 470 pF
Into the Treble pot’s top, 470 pF. Only the top end gets through it. Piazza’s redraw shows 47 pF. The factory drawing shows .00047 µF.
Treble 1, 4 MΩ pot, linear taper
Treble, 4 MΩ. Its top gets the treble through the 470 pF, its bottom goes to ground through 4700 pF, and its wiper is the channel’s output. Up, more treble from the top. Down, the 4700 pF pulls treble off. The taper isn’t marked, so linear is assumed.
.0047µ, 4.7 nF
Under the Treble pot to ground, 4700 pF. With the knob down it pulls the treble off. Piazza’s redraw shows 470 pF. The factory drawing shows .0047 µF.
120k, 120 kΩ
From the Bass wiper to the Treble wiper. The lows and mids reach the output this way.
270k, 270 kΩ
Mixing resistor, 270 kΩ, from the Channel 1 Treble wiper to the 7199’s grid, where the other channel and the reverb meet it.
47k, 47 kΩ
Jack 1’s resistor to the grid, 47 kΩ.
47k, 47 kΩ
Jack 2’s resistor to the grid, 47 kΩ. With the guitar in jack 2 and nothing in jack 1, jack 1’s 47 kΩ and 100 kΩ hang from the grid to ground, so the note comes in about 2.4 dB lower.
100k, 100 kΩ
On jack 1’s switch: 100 kΩ from its tip to ground with nothing plugged in. A plug opens the switch.
5.6M, 5.6 MΩ
Grid leak, 5.6 MΩ.
V5a, 12AX7 / ECC83 / 7025
Half of a 12AX7, the Channel 2 input stage. Its cathode isn’t bypassed, so its gain is about 35.
4.7k, 4.7 kΩ
Cathode resistor, 4.7 kΩ, not bypassed. The factory drawing reads +2 V here.
270k, 270 kΩ
Plate resistor, 270 kΩ, from node E. The factory drawing reads +200 V on the plate.
.1µ, 100 nF
Coupling cap into the Volume pot.
Volume 2, 1 MΩ pot, audio taper
Channel 2 volume, 1 MΩ, between the two stages. Its wiper is the second stage’s grid. The drawing marks no taper, so audio is assumed.
.00025µ, 250 pF
The Ultra High cap, 250 pF, from the volume pot’s top to its wiper when the switch is on. With the volume down, treble goes around the pot. At full it does nothing. Piazza’s redraw shows 25 pF here. The factory drawing shows .00025 µF.
V5b, 12AX7 / ECC83 / 7025
The other half of the 12AX7, the second stage, ahead of the tone network.
3.3k, 3.3 kΩ
Cathode resistor, 3.3 kΩ. The factory drawing reads +2 V here.
.1µ, 100 nF
Only .1 µF across the cathode resistor, so the stage has its full gain only from about 500 Hz up. The lows get less gain.
150k, 150 kΩ
Plate resistor, 150 kΩ. The factory drawing reads +210 V on the plate.
.1µ, 100 nF
Coupling cap into the bass and treble network.
220k, 220 kΩ
Feeds the Bass pot’s top. With the 22 kΩ under the pot it sets how far the network drops the lows.
Bass 2, 1 MΩ pot, linear taper
Bass, 1 MΩ, with a cap across each half. Its wiper feeds the Treble wiper through 120 kΩ. Toward the top the lows from the 220 kΩ get through, toward the bottom they’re pulled to the 22 kΩ. The taper isn’t marked, so linear is assumed.
.001µ, 1 nF
From the Bass pot’s top to its wiper. It passes the mids around the upper half of the pot.
.01µ, 10 nF
From the Bass pot’s wiper to its bottom. It passes the mids around the lower half.
22k, 22 kΩ
From the Bass pot’s bottom to ground, the bottom of the network’s divider.
270k, 270 kΩ
From the Bass pot’s wiper to ground. It drops the level slightly more.
.00047µ, 470 pF
Into the Treble pot’s top, 470 pF. Only the top end gets through it. Piazza’s redraw shows 47 pF. The factory drawing shows .00047 µF.
Treble 2, 4 MΩ pot, linear taper
Treble, 4 MΩ. Its top gets the treble through the 470 pF, its bottom goes to ground through 4700 pF, and its wiper is the channel’s output. Up, more treble from the top. Down, the 4700 pF pulls treble off. The taper isn’t marked, so linear is assumed.
.0047µ, 4.7 nF
Under the Treble pot to ground, 4700 pF. With the knob down it pulls the treble off. Piazza’s redraw shows 470 pF. The factory drawing shows .0047 µF.
120k, 120 kΩ
From the Bass wiper to the Treble wiper. The lows and mids reach the output this way.
270k, 270 kΩ
Mixing resistor, 270 kΩ, from the Channel 2 Treble wiper to the 7199’s grid, where the other channel and the reverb meet it.
V6a, 6CG7
Half of a 6CG7, the reverb’s first stage. Its grid is Channel 1’s Treble wiper, so only Channel 1 reaches the springs.
2.2k, 2.2 kΩ
Cathode resistor for V6a, not bypassed. The factory drawing reads +4.5 V here.
120k, 120 kΩ
Plate resistor for V6a, from node C. The factory drawing reads +130 V on the plate.
.047µ, 47 nF
Coupling cap to the reverb driver.
47k, 47 kΩ
In series into the driver’s grid. With the 47 kΩ to ground it halves the signal.
47k, 47 kΩ
The reverb driver’s grid resistor.
V6b, 6CG7
The other half of the 6CG7, the reverb driver. It drives the reverb unit through .47 µF with no transformer, on a 10 kΩ plate resistor.
330, 330 Ω
Cathode resistor for the driver, 330 Ω 2 W, not bypassed. The factory drawing reads +5.5 V here.
10k, 10 kΩ
Plate resistor for the driver, 10 kΩ 5 W, from node F. It’s low so the driver can push current into the reverb unit’s coil. The factory drawing reads +255 V on the plate.
.02µ, 20 nF
From the driver’s plate to ground. It trims the far top end sent to the springs.
.47µ, 470 nF
Into the reverb unit’s input coil, .47 µF. It keeps the plate’s DC off the coil.
Unit in R, 190 Ω
Resistance of the reverb unit’s input coil. The current through it drives the springs, which play from a measured tank’s impulse response. Typical of a 1,475 Ω input coil, not on the drawing.
Unit in L, 235 mH
Inductance of the reverb unit’s input coil, 0.235 H, about 1,475 Ω at 1 kHz. Typical, not on the drawing.
Unit out L, 350 mH
The reverb unit’s output coil. The springs come back in series with it. Its figures are a measured long tank’s, typical, not on the drawing.
Unit out R, 181 Ω
Resistance of the unit’s output coil. Typical, not on the drawing.
22k, 22 kΩ
Across the reverb unit’s output, 22 kΩ to ground.
10k, 10 kΩ
From the reverb unit’s output to the recovery grid, 10 kΩ. With the .005 µF from the grid to the cathode it trims the top of the return.
V7a, 12AX7 / ECC83 / 7025
Half of a 12AX7, the reverb recovery stage.
.005µ, 5 nF
From the recovery grid to its cathode, .005 µF, as both drawings show. At the top end it ties the grid to the cathode, so the stage’s gain falls there. The springs’ hiss gets less gain.
1k, 1 kΩ
Cathode resistor for the recovery stage, not bypassed. The factory drawing reads +.8 V here.
270k, 270 kΩ
Plate resistor for the recovery stage, from node E. The factory drawing reads +110 V on the plate.
.005µ, 5 nF
Into the Dimension pot.
Dimension 500k, 500 kΩ pot, linear taper
Dimension, the reverb level, 500 kΩ. Turn it up for more of the springs. The taper isn’t marked, so linear is assumed.
.002µ, 2 nF
From the Dimension wiper toward the mix, .002 µF. Into the 150 kΩ pair it passes from about 500 Hz up, so the reverb that comes back is thin.
150k, 150 kΩ
From the reverb’s return node to ground, 150 kΩ.
150k, 150 kΩ
The reverb’s mixing resistor, 150 kΩ, to the 7199’s grid, where the two channels’ 270 kΩ meet it.
V7b, 12AX7 / ECC83 / 7025
The other half of V7, a 12AX7, the tremolo’s oscillator. A ladder of three .05 µF caps from its plate back to its grid turns the phase round at a few hertz. Its plate also lights the TM-1’s neon.
470k, 470 kΩ
The oscillator’s plate resistor, 470 kΩ from node F. The factory drawing reads 130 V on the plate with the tremolo on and 210 V with it off.
270k, 270 kΩ
The oscillator’s grid resistor, 270 kΩ to ground, the last leg of the ladder.
3.3k, 3.3 kΩ
The oscillator’s cathode resistor, 3.3 kΩ, grounded through the tremolo foot switch. The factory drawing reads 1.2 V on the cathode with the tremolo on and 3 V with it off.
25µ, 25 µF electrolytic
Cathode bypass, 25 µF 25 V, straight to ground, so it stays across the cathode with the foot switch open.
.05µ, 50 nF
First cap of the ladder, from the oscillator’s plate.
120k, 120 kΩ
The ladder’s first leg to ground, 120 kΩ.
.05µ, 50 nF
Second cap of the ladder.
120k, 120 kΩ
The ladder’s second leg, 120 kΩ into the Speed pot’s wiper. It sets the top speed.
Speed 300k, 300 kΩ pot, linear taper
Speed, 300 kΩ, a variable resistor from the 120 kΩ to ground (its top lug isn’t connected). Turned up, there’s less track in the ladder’s leg and the oscillator runs faster. Piazza’s redraw shows 500 kΩ. The taper isn’t marked, so linear is assumed.
.05µ, 50 nF
Last cap of the ladder, into the oscillator’s grid.
390k, 390 kΩ
From the oscillator’s plate to the neon, 390 kΩ. As the plate swings up the neon’s current rises and it glows brighter.
Neon, NE-2 neon
The neon lamp in the TM-1 module, from the 390 kΩ to ground. It strikes at about 76 V and once lit holds about 58 V, and its light follows its current. NE-2 class figures. The tick a real one can put in the signal isn’t modeled.
1M, 1 MΩ
Across the neon, 1 MΩ.
.2µ, 200 nF
From the neon to the 22 kΩ and the intensity switch, .2 µF. With the switch closed it shunts the oscillator’s swing at the neon.
1.5M, 1.5 MΩ
Across the .2 µF, 1.5 MΩ, so the cap’s charge leaks off with the switch open.
22k, 22 kΩ
In series to the intensity switch, 22 kΩ.
V3, 7199 pentode (RCA sheet)
The pentode half of a 7199, pins 2, 3, 6 and 7. It makes up the level lost in the tone networks and the mix and drives the cathodyne, the 7199’s triode half, in the power amp.
270k, 270 kΩ
Plate resistor for the pentode, 270 kΩ from node C, as on the factory drawing, which reads +150 V on the plate. Piazza’s redraw shows 47 kΩ 1 W.
.0001µ, 100 pF
From the pentode’s plate to ground, 100 pF. With the plate resistor it rolls off the far top end.
470k, 470 kΩ
Screen resistor, 470 kΩ from node C. With the 150 kΩ to ground it holds the screen low. The factory drawing reads +60 V.
150k, 150 kΩ
From the screen to ground, 150 kΩ, the bottom of the screen’s divider.
.1µ, 100 nF
From the screen to the cathode, .1 µF, so the screen moves with the cathode and holds still against it.
150, 150 Ω
The unbypassed part of the pentode’s cathode resistor, 150 Ω. The feedback from the speaker comes in on top of it. The factory drawing reads +2 V on the cathode.
2.2k, 2.2 kΩ
The rest of the cathode resistor, 2.2 kΩ, bypassed.
100µ, 100 µF electrolytic
Bypass across the 2.2 kΩ, 100 µF 12 V.
15k, 15 kΩ
Feedback, 15 kΩ from the output transformer’s 16 Ω tap to the pentode’s cathode. With the 150 Ω under it, about 1 % of the tap’s voltage comes back, enough to cut the gain and flatten the speaker’s bumps.
16 Ω tap, 1 V
The output transformer’s 16 Ω tap, with the speaker on 8 Ω. It’s an ideal tap on the same winding, 1.414 times the speaker voltage. On Preamp only or another power amp the feedback resistor goes to ground.
.1µ, 100 nF
Coupling cap into the cathodyne.
V8, 7199 (triode section)
n7199t, 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.
47k, 47 kΩ
The cathodyne’s cathode load, the same value as its plate resistor, straight to ground, and the output to tube A. There’s no separate bias resistor. The grid’s divider sets the cathode a volt or two above the grid.
1.5M, 1.5 MΩ
Grid resistor for the cathodyne, moved from the junction of its cathode resistors to ground. With the resistor from the supply it makes a divider that holds the grid about where the cathode junction held it.
10M, 10 MΩ
Resistor from the supply to the cathodyne’s grid. With the grid resistor to ground it sets the grid’s DC voltage, so the grid no longer rides up and down with the cathode.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
1k, 1 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1k, 1 kΩ
Grid stopper for tube B.
V9, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V10, 6L6GC
6L6GC, tube B of the pair.
270k, 270 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.
270k, 270 kΩ
Grid leak for tube B.
1k, 1 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
40µ, 40 µF electrolytic
Filter cap on the screen supply.
2.2k, 2.2 kΩ
Dropping resistor from B+ to the node for the reverb driver and the tremolo, as on the Ampeg Gemini II G-15 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.
40µ, 40 µF electrolytic
Filter cap at the node for the reverb driver 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.
5.6k, 5.6 kΩ
Dropping resistor from the node for the reverb driver and the tremolo to the node for the 7199 and the reverb driver, as on the Ampeg Gemini II G-15 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.
40µ, 40 µF electrolytic
Filter cap at the node for the 7199 and the reverb driver. 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 7199 and the reverb driver to the node for the preamp, as on the Ampeg Gemini II G-15 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.
40µ, 40 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, 4 kΩ to 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 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, 360 V ac, 60 Hz
The power transformer’s high-voltage winding, 360 V ac end to end, into a bridge of four silicon diodes. It’s drawn as two sources of 360 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.
PT, 360 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, 40 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.
PT R, 40 Ω
The same winding’s resistance, on the other half cycle. Typical of the part.
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.
30µ, 30 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.
Pump, 203 V ac, 60 Hz
Stands in for the bias pump with the part below it. On the drawing, a .047µF cap from one leg of the high-voltage winding, 56k Ω to ground and the diode make the bias. The operating point can’t be solved through a cap in series, so this 203.11 V ac source and its resistor replace it. They were fitted to a simulation of the real pump on this winding: the same bias, and the same shift when the grids pull current on a hard note. It moves with the wall voltage, as the pump does.
pump R, 85.3 kΩ
Stands in for the pump with the source beside it, 85.3k Ω, fitted. It sets how far the bias moves under grid current. The amp has a .047µF cap here and 56k Ω from the diode end to ground.
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.
68k, 68 kΩ
The bias supply’s load, 68k Ω to ground. With the 85.3k Ω above it, it sets 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. 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
Three 12AX7s, a 6CG7 and the 7199’s pentode. The 7199 pentode model is fitted to the RCA sheet’s two operating points. The reverb unit’s coils are typical, not on the drawing.

Sources

  • Joe Piazza, redrawn Gemini II G-15 schematic, 1996

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.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

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