Push-pull 6L6 amp with a paraphase inverter and four inputs, 1953 style

Parts and what they do

Stock values for the 5C5 Pro version. Tap a part on the schematic to change it.

.1µ, 100 nF

Input cap from channel 1’s first jack to its grid, .1 µF 200 V. The grid has no cathode resistor to sit against, so the cap keeps the jack’s DC off it.

5M, 5 MΩ

Grid leak, 5 MΩ. The cathode is grounded, so the grid biases itself: a trickle of grid current through 5 MΩ holds it near −1 V. Big peaks pull more grid current, and the bias walks colder for a moment.

V1a, 6SC7

Channel 1’s first half of the 6SC7, the one the first jack feeds. One cathode serves both halves, tied straight to ground. A mu of about 70.

.1µ, 100 nF

Input cap from channel 1’s second jack, .1 µF 200 V. That jack is empty, so its switch grounds the tip and the grid sits at signal ground.

5M, 5 MΩ

The second half’s 5 MΩ grid leak.

V1b, 6SC7

Channel 1’s second half, its plate tied to the first half’s. With its jack empty it still pulls current through the shared plate resistor and loads the plate, so the stage gains slightly over half what one half alone would.

250k, 250 kΩ

Plate resistor for both halves of channel 1’s 6SC7, 250 kΩ from the preamp node.

.1µ, 100 nF

Coupling cap from channel 1’s plates into its volume’s wiper, .1 µF 600 V.

Vol 1M, 1 MΩ pot, audio taper

Channel 1 volume, 1 MΩ. The signal comes in on the wiper. One end goes to ground and the other to V3a’s grid, so up moves the wiper toward the grid. Down, the wiper grounds the channel and leaves the whole 1 MΩ from V3a’s grid to ground. The drawing doesn’t mark the taper, so audio is assumed.

.1µ, 100 nF

Input cap from channel 2’s first jack to its grid, .1 µF 200 V. The grid has no cathode resistor to sit against, so the cap keeps the jack’s DC off it.

5M, 5 MΩ

Grid leak, 5 MΩ. The cathode is grounded, so the grid biases itself: a trickle of grid current through 5 MΩ holds it near −1 V. Big peaks pull more grid current, and the bias walks colder for a moment.

V2a, 6SC7

Channel 2’s first half of the 6SC7, the one the first jack feeds. One cathode serves both halves, tied straight to ground. A mu of about 70.

.1µ, 100 nF

Input cap from channel 2’s second jack, .1 µF 200 V. That jack is empty, so its switch grounds the tip and the grid sits at signal ground.

5M, 5 MΩ

The second half’s 5 MΩ grid leak.

V2b, 6SC7

Channel 2’s second half, its plate tied to the first half’s. With its jack empty it still pulls current through the shared plate resistor and loads the plate, so the stage gains slightly over half what one half alone would.

250k, 250 kΩ

Plate resistor for both halves of channel 2’s 6SC7, 250 kΩ from the preamp node.

.1µ, 100 nF

Coupling cap from channel 2’s plates into its volume’s wiper, .1 µF 600 V.

Vol 1M, 1 MΩ pot, audio taper

Channel 2 volume, 1 MΩ. The signal comes in on the wiper. One end goes to ground and the other to V3a’s grid, so up moves the wiper toward the grid. Down, the wiper grounds the channel and leaves the whole 1 MΩ from V3a’s grid to ground. The drawing doesn’t mark the taper, so audio is assumed.

500p, 500 pF

The bright cap, .0005 µF (500 pF) on the 5C5, from channel 1’s volume wiper to the tone pot. It carries treble around volume 1. On the 5B5 it’s .00025 µF from the inverter’s grid.

Tone 1M, 1 MΩ pot, audio taper

Tone, 1 MΩ. On the 5C5 the wiper is on the inverter’s grid with both volumes: clockwise moves it toward the bright cap from channel 1, counterclockwise toward the cap to ground, which cuts treble from both channels. On the 5B5 the wiper is on volume 2’s wiper, so it shapes channel 2. The drawing marks no taper or direction. Audio taper, clockwise brighter.

5n, 5 nF

The cut cap from the tone pot’s far end to ground, .005 µF on the 5C5, .01 µF on the 5B5.

V3a, 6SC7

6SC7, 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.

250k, 250 kΩ

Plate resistor for the first half, the output to tube A.

2.5k, 2.5 kΩ

Cathode resistor for both halves. The tube has one cathode for its two triodes, so this carries both currents and sets both biases.

25µ, 25 µF electrolytic

Bypass cap across the shared cathode resistor, for full gain above a few hertz.

250k, 250 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.

6.8k, 6.8 kΩ

The tap, 6.8k 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.

V3b, 6SC7

The second half. It turns the divided signal over again, so its plate swings with the input. It drives tube B.

250k, 250 kΩ

Plate resistor for the second half, the output to tube B.

100p, 100 pF

A small cap from plate to plate. It rolls off the far top end the inverter passes on.

.05µ, 50 nF

Coupling cap to tube A’s grid.

.05µ, 50 nF

Coupling cap to tube B’s grid.

V4, 5881

n5881, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.

V5, 5881

n5881, tube B of the pair.

250k, 250 kΩ

Grid leak for tube B.

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.

25µ, 25 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.

16µ, 16 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 Fender 5C5 Pro 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.

16µ, 16 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, 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, 350 V ac, 60 Hz

One half of the power transformer’s high-voltage winding, 350 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, 350 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.

V6, 5U4G

One plate of the 5U4G 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, 5U4G

The rectifier’s other plate, on the other half cycle.

16µ, 16 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.

16µ, 16 µF electrolytic

Another reservoir cap, in parallel with the first.

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.

Versions

5C5 Pro

The 5C5 drawing: 6SC7s in both channels and the inverter. The 6SC7 model is fitted only to the type’s published operating point, with curves borrowed from the 6SL7 away from it. The 6L6Gs run on the 5881 model, the closest model on the bench. The transformers are typical figures, not measured.

5C5, late (12AY7, 12AX7)

The drawing’s note: the latest 5C5s use 12 V glass miniatures in place of the 6SC7s, a 12AY7 in each channel and a 12AX7 in the inverter, on the same parts. The 12AY7 has about two thirds the mu of the 6SC7 and the 12AX7 about one and a half times. The tube models are fitted to their data sheets, not measured.

5C4 Super, modeled after the Fender 5C4 Super

The 5C4 Super’s drawing is the 5C5’s part for part, with two speakers in parallel on the secondary, a 2x10. On the bench it stays on the 5C5’s matched 8 Ω tap, so the plates see the same load. The output transformer is typical, not measured.

5B5 Pro, modeled after the Fender 5B5 Pro

The 5B5 drawing: .05 µF input and coupling caps, the tone network on channel 2 (.00025 µF from the inverter’s grid to the tone pot, .01 µF to ground, the wiper on volume 2’s wiper), tube A’s tap 7000 Ω, every filter cap 20 µF. The transformers are typical figures, not measured.

5A5 Pro, field coil, modeled after the Fender 5A5 Pro

The 5A5 drawing, marked corrected: the 5B5 with a field-coil speaker, its coil from B+ to ground. 7 kΩ is typical of a 15 in field coil of the period, not on the drawing. It pulls about 50 mA on top of the tubes, so B+ sits lower. The transformers are typical figures, not measured.