Two-channel 6SL7 bass amp, tone between the halves, 1964 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 B-12-NC Portaflex (1964).

Two channels, each a 6SL7 with a bass and treble network between its halves and a volume after, mixed into a 6SL7 floating paraphase with feedback and two cathode-biased 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. This is a bass amp, but the note played into it is the same guitar as everywhere else on the bench, so the low notes here are a guitar’s. Each channel is a 6SL7 with both cathodes unbypassed, so the gain stays low. The bass and treble network sits between the two halves, so the second half sees what it lets through. The volume comes last, with .005 µF across its top half. The two channels meet through 270 kΩ each at V3’s first grid, in the power amp. The tube models are typical of the types, not measured.

Download.

Parts and what they do

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

120k, 120 kΩ
Input 1’s resistor to the grid, 120 kΩ. With the 5.6 MΩ grid leak it makes a small divider, and with the tube’s input capacitance it trims the far top end.
120k, 120 kΩ
Input 2’s resistor to the grid, 120 kΩ. With nothing plugged in, this jack’s switch puts the 1 MΩ under it, so it hangs 1.12 MΩ from the grid to ground.
1M, 1 MΩ
On input 2’s switch: 1 MΩ from the jack’s tip to ground with nothing plugged in. A plug opens the switch.
5.6M, 5.6 MΩ
Grid leak, 5.6 MΩ, as drawn. It ties the grid to ground for DC.
V1a, 6SL7 (RCA)
The first half of the channel one’s 6SL7, pins 1, 2 and 3. Its cathode resistor isn’t bypassed, so the gain is about 30 and it stays clean on a bass.
2.2k, 2.2 kΩ
Cathode resistor, 2.2 kΩ, not bypassed. The drawing reads +1.2 V here.
470k, 470 kΩ
Plate resistor, 470 kΩ. The drawing reads +110 V on the plate.
.02µ, 20 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 1, 1 MΩ pot, linear taper
Bass, 1 MΩ, with a cap across each half. Its wiper feeds the second half’s grid 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.
47p, 47 pF
Into the Treble pot’s top, 47 pF. Only the top end gets through it.
Treble 1, 4 MΩ pot, linear taper
Treble, 4 MΩ. Its top gets the treble through the 47 pF, its bottom goes to ground through 470 pF, and its wiper is the second half’s grid. Up, more treble from the top. Down, the 470 pF pulls treble off the grid. The taper isn’t marked, so linear is assumed.
470p, 470 pF
Under the Treble pot to ground. With the knob down it pulls the treble off the grid.
120k, 120 kΩ
From the Bass wiper to the Treble wiper. The lows and mids reach the second half’s grid this way.
V1b, 6SL7 (RCA)
The second half of the 6SL7, pins 4, 5 and 6. It makes up the level lost in the tone network. Its cathode isn’t bypassed either.
5.6k, 5.6 kΩ
Cathode resistor, 5.6 kΩ, not bypassed. The drawing reads +2.2 V here.
470k, 470 kΩ
Plate resistor, 470 kΩ. The drawing reads +175 V on the plate.
.02µ, 20 nF
Coupling cap into the top of the volume pot.
Volume 1, 1 MΩ pot, audio taper
The channel one’s volume, 1 MΩ, after both halves of the 6SL7. The drawing marks no taper, so audio is assumed.
.005µ, 5 nF
From the volume pot’s top to its wiper. With the volume below full it lets the mids and treble around the pot, so they drop less than the lows.
270k, 270 kΩ
Mixing resistor, 270 kΩ, from the channel one’s volume wiper to V3’s grid, where the other channel’s 270 kΩ meets it. Each channel loads the other through the pair.
2.2M, 2.2 MΩ
Grid leak, 2.2 MΩ, as drawn. It ties the grid to ground for DC.
V2a, 6SL7 (RCA)
The first half of the channel two’s 6SL7, pins 1, 2 and 3. Its cathode resistor isn’t bypassed, so the gain is about 30 and it stays clean on a bass.
2.2k, 2.2 kΩ
Cathode resistor, 2.2 kΩ, not bypassed. The drawing reads +1.2 V here.
470k, 470 kΩ
Plate resistor, 470 kΩ. The drawing reads +110 V on the plate.
.02µ, 20 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 second half’s grid 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.
47p, 47 pF
Into the Treble pot’s top, 47 pF. Only the top end gets through it.
Treble 2, 4 MΩ pot, linear taper
Treble, 4 MΩ. Its top gets the treble through the 47 pF, its bottom goes to ground through 470 pF, and its wiper is the second half’s grid. Up, more treble from the top. Down, the 470 pF pulls treble off the grid. The taper isn’t marked, so linear is assumed.
470p, 470 pF
Under the Treble pot to ground. With the knob down it pulls the treble off the grid.
120k, 120 kΩ
From the Bass wiper to the Treble wiper. The lows and mids reach the second half’s grid this way.
V2b, 6SL7 (RCA)
The second half of the 6SL7, pins 4, 5 and 6. It makes up the level lost in the tone network. Its cathode isn’t bypassed either.
5.6k, 5.6 kΩ
Cathode resistor, 5.6 kΩ, not bypassed. The drawing reads +2.2 V here.
470k, 470 kΩ
Plate resistor, 470 kΩ. The drawing reads +175 V on the plate.
.02µ, 20 nF
Coupling cap into the top of the volume pot.
Volume 2, 1 MΩ pot, audio taper
The channel two’s volume, 1 MΩ, after both halves of the 6SL7. The drawing marks no taper, so audio is assumed.
.005µ, 5 nF
From the volume pot’s top to its wiper. With the volume below full it lets the mids and treble around the pot, so they drop less than the lows.
270k, 270 kΩ
Mixing resistor, 270 kΩ, from the channel two’s volume wiper to V3’s grid, where the other channel’s 270 kΩ meets it. Each channel loads the other through the pair.
V3a, 6SL7 (RCA)
6SL7GT, the driven half of the floating paraphase inverter. Its plate swings opposite the input and drives tube A. The feedback from the speaker goes to its cathode. Its grid connects straight to the preamp’s mixing resistors, which are its path to ground.
V3b, 6SL7 (RCA)
The other half. Its grid is fed from the middle of the divider, so its plate swings the other way and drives tube B.
470k, 470 kΩ
Grid resistor for the other half, to ground.
1k, 1 kΩ
Cathode resistor for both halves. It sets their bias, a couple of volts.
25µ, 25 µF electrolytic
Bypass cap across the shared cathode resistor. The cathodes hold still, so the second half gets its signal only from the divider.
47k, 47 kΩ
Plate resistor for the driven half.
47k, 47 kΩ
Plate resistor for the other half.
470k, 470 kΩ
Top of the divider, from tube A’s plate. With the resistor below it, from tube B’s, the middle sits at the average of the two sides. With the two swinging equal and opposite, the middle holds still. Whatever is left goes to the second half’s grid, which corrects it, so the inverter keeps itself even as the tube ages.
510k, 510 kΩ
Bottom of the divider, to tube B’s plate.
.02µ, 20 nF
From the middle of the divider to the second half’s grid. It keeps the plates’ DC off that grid.
10k, 10 kΩ
The feedback resistor from the speaker. With the 220 Ω below it, it sends about 2 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
220, 220 Ω
The driven half’s own cathode resistor, 220 Ω, between its cathode and the shared cathode resistor, not bypassed. It’s also the bottom of the feedback loop: the feedback resistor from the speaker connects at the top of it, so the fed-back speaker voltage moves that cathode. It lowers that half’s gain too.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 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.
V4, 6L6GC
6L6GC, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, 6L6GC
6L6GC, tube B of the pair.
270k, 270 kΩ
Grid leak for tube A, to ground. 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.
270k, 270 kΩ
Grid leak for tube B.
1.5k, 1.5 kΩ
One screen resistor shared by both tubes, with no cap after it. It carries both screens’ current. The two screens’ signal currents are opposite and cancel in it, so its drop follows the even harmonics and grows as the stage is driven harder.
250, 250 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.
50µ, 50 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.
40µ, 40 µF electrolytic
Filter cap on the screen supply.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Ampeg B-12-NC Portaflex 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.
20µ, 20 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.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for both channels, as on the Ampeg B-12-NC Portaflex 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 both channels. 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, not measured.
PT, 360 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 360 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, 360 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, not measured.
PT R, 60 Ω
Resistance of the other half. Typical of the part, not measured.
V6, 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.
V6, GZ34
The rectifier’s other plate, on the other half cycle.
40µ, 40 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. 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.
16 Ω tap, 1 V
The 16 Ω tap the feedback comes from, assumed to be an ideal tap on the same winding: 1.414 times the speaker voltage. The load the feedback network puts on the winding is left out. It’s tens of kilohms on a winding of a few ohms.

About this version

As drawn
Two 6SL7s, one a channel. The 6SL7 model is fitted to the RCA plate curves. The B-12-N of 1962 has the same preamp.

Sources

  • Joe Piazza, redrawn B-12-NC schematic, 1999

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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