Two-EL84 tremolo amp with bass and treble, late 1950s

Parts and what they do

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

R1A, 47 kΩ

Jack 1’s resistor into the grid, 47 kΩ. Jack 2 has no switch, so with nothing in it its 47 kΩ goes nowhere, and a guitar in jack 1 sees 47 kΩ to the grid and 1 MΩ to ground.

R1B, 47 kΩ

Jack 2’s resistor into the grid, 47 kΩ. With nothing in jack 1, its switch grounds jack 1’s tip, so the two 47 kΩ make a divider: jack 2 is about 6 dB quieter. Jack 3, not drawn here, is wired the same.

RG1, 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.

V1a, 6EU7

First half of a 6EU7, the input stage. The 1 MΩ from its output back to its grid holds its gain down and lowers what the guitar sees.

100k, 100 kΩ

Plate resistor for V1a. The GA-8’s drawing reads +168 V on the plate.

2.2k, 2.2 kΩ

Cathode resistor for V1a, +1.3 V on the GA-8’s drawing.

20µ, 20 µF electrolytic

Bypass for V1a’s cathode resistor.

.01µ, 10 nF

Coupling cap from V1a to the tone network.

1M, 1 MΩ

From the tone network’s top back to V1a’s grid. The plate swings opposite the grid, so this is negative feedback: with the 47 kΩ in front, the stage’s gain from the jack is held near 15 and its distortion down.

Bass, 2 MΩ pot, linear taper

Bass, 2 MΩ linear, a variable resistor between the tone network’s top and the volume, with 500 pF across it. The 500 pF carries the treble past it either way. Turned up, its resistance drops and the bass comes through too.

500p, 500 pF

Across the bass pot. It passes the treble around the pot’s resistance.

Vol 500k, 500 kΩ pot, audio taper

Volume, 500 kΩ audio. Its wiper is V1b’s grid, tied to the treble pot’s wiper. There’s no master.

500p, 500 pF

The treble pot’s bright cap, from the tone network’s top.

Treble, 1 MΩ pot, audio taper

Treble, 1 MΩ audio. One end gets the signal through 500 pF, the other goes through .01 µF to ground, and the wiper is V1b’s grid. Turned up, the wiper moves toward the 500 pF, so treble skips the bass pot and volume. Turned down, it moves toward the .01 µF, which bleeds treble off the grid. Up is assumed to be brighter.

.01µ, 10 nF

The treble pot’s cut cap, to ground.

V1b, 6EU7

Second half of the 6EU7. It drives the cathodyne, which has no gain of its own.

100k, 100 kΩ

Plate resistor for V1b. The GA-8’s drawing reads +167 V on the plate.

2.2k, 2.2 kΩ

Cathode resistor for V1b, not bypassed, +1.2 V on the GA-8’s drawing.

500p, 500 pF

Only 500 pF into the cathodyne’s grid. The cathodyne’s grid resistor goes to its cathode, so the grid looks like a few megohms and the 500 pF still passes most of the guitar’s range.

150k, 150 kΩ

Plate resistor for the oscillator, from the screen node.

V2b, 6EU7

The other half of V2’s 6EU7, the tremolo oscillator. Its cathode goes straight to ground. The grid biases itself on its 4.7 MΩ leak.

.01µ, 10 nF

First cap of the oscillator’s phase-shift ladder.

330k, 330 kΩ

First ladder leg to ground.

.02µ, 20 nF

Second ladder cap.

Freq, 1 MΩ pot, reverse audio taper

Frequency, 1 MΩ reverse audio, as a variable resistor in the ladder’s middle leg, after the foot switch. Turned up, its resistance drops and the oscillator runs faster.

150k, 150 kΩ

In series with the Frequency pot, the rest of the middle leg. It sets the top speed.

.02µ, 20 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.

4.7M, 4.7 MΩ

Grid leak for the oscillator, and the ladder’s last leg.

.047µ, 47 nF

Couples the oscillator’s swing from its plate to the Depth pot and blocks the plate’s DC.

470k, 470 kΩ

In series with the Depth pot. With the pot it divides the swing down and keeps the output grids’ network from loading the oscillator much.

Depth, 250 kΩ pot, reverse audio taper

Depth, 250 kΩ reverse audio, to ground. Its wiper is where the output tubes’ two grid leaks meet, so it sets how much of the swing reaches both grids together. Turned up, more of it does. On Preamp only, the wiper drives nothing.

V2a, 6EU7

6EU7, 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.

100k, 100 kΩ

The cathodyne’s plate resistor, the output to tube B.

3.3k, 3.3 kΩ

The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.

100k, 100 kΩ

The cathodyne’s cathode load, the same value as its plate resistor. The cathode above the bias resistor is the output to tube A.

470k, 470 kΩ

Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage. It’s the grid’s only path to ground, and the cathode follows the grid, so the preamp sees much more than 470k Ω.

.01µ, 10 nF

Coupling cap to tube A’s grid.

.01µ, 10 nF

Coupling cap to tube B’s grid.

V3, EL84 / 6BQ5 (Philips curves)

EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.

V4, EL84 / 6BQ5 (Philips curves)

EL84, tube B of the pair.

220k, 220 kΩ

Grid leak for tube A, returned with tube B’s to the tremolo’s Depth pot, so the oscillator swings both grids together. 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.

220k, 220 kΩ

Grid leak for tube B.

130, 130 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.

1k, 1 kΩ

Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.

20µ, 20 µF electrolytic

Filter cap on the screen supply.

22k, 22 kΩ

Dropping resistor from the screen supply node to the node for the inverter and preamp, as on the Gibson GA-18T Explorer 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 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, not measured.

PT, 240 V ac, 60 Hz

One half of the power transformer’s high-voltage winding, 240 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, 240 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, 100 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, 100 Ω

Resistance of the other half. Typical of the part, not measured.

V5, 6CA4

One plate of the 6CA4 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.

V5, 6CA4

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 6EU7 uses the 12AX7 model. The drawing prints no voltages. The voltages are checked against the GA-8’s drawing, the same amp without the tremolo.