Two-channel 6L6 reverb amp with a pentode mixer, mid 1960s

Parts and what they do

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

R1A, 100 kΩ

The chan 1 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 50 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.

R3A, 100 kΩ

The chan 1 channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 100 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 200 kΩ.

R2A, 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.

R1B, 100 kΩ

The chan 2 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 50 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.

R3B, 100 kΩ

The chan 2 channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 100 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 200 kΩ.

R2B, 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

One half of the first 6EU7, channel 1’s input stage. Both halves share one cathode resistor.

V1b, 6EU7

The other half, channel 2’s input stage.

100k, 100 kΩ

Plate resistor for V1a. The drawing reads +190 V on the plates.

100k, 100 kΩ

Plate resistor for V1b.

1.5k, 1.5 kΩ

The one cathode resistor for both halves of V1, +1.6 V on the drawing.

20µ, 20 µF electrolytic

Bypass for V1’s shared cathode resistor, full gain.

.047µ, 47 nF

Coupling cap to channel 1’s tone network.

.047µ, 47 nF

Coupling cap to channel 2’s tone network and the Reverb pot.

330p, 330 pF

Channel 1’s treble cap, into the top of the Treble pot. Only the treble gets through it.

Treble, 250 kΩ pot, reverse audio taper

Channel 1’s Treble, 250 kΩ reverse audio as marked. Its wiper is the network’s output. Turned up, the wiper moves toward the treble cap and the treble comes up.

100k, 100 kΩ

From the network’s input to the bottom of the Treble pot and the top of the Bass pot. The bass and mids come this way.

Bass, 250 kΩ pot, audio taper

Channel 1’s Bass, 250 kΩ audio, its far end grounded. Turned up, its wiper moves toward ground, so more of the track sits between the 100 kΩ and ground and more bass gets through. Turned down, the .01 µF and 10 kΩ pull the bass down.

.01µ, 10 nF

From the top of the Bass pot to its wiper. It passes the mids and treble around the bass part of the track.

10k, 10 kΩ

From the Bass wiper to ground. It sets how far the bass can be cut.

250p, 250 pF

Channel 2’s treble cap, into the top of the Treble pot. Only the treble gets through it.

Treble, 250 kΩ pot, reverse audio taper

Channel 2’s Treble, 250 kΩ reverse audio as marked. Its wiper is the network’s output. Turned up, the wiper moves toward the treble cap and the treble comes up.

100k, 100 kΩ

From the network’s input to the bottom of the Treble pot and the top of the Bass pot. The bass and mids come this way.

Bass, 250 kΩ pot, audio taper

Channel 2’s Bass, 250 kΩ audio, its far end grounded. Turned up, its wiper moves toward ground, so more of the track sits between the 100 kΩ and ground and more bass gets through. Turned down, the .01 µF and 10 kΩ pull the bass down.

.01µ, 10 nF

From the top of the Bass pot to its wiper. It passes the mids and treble around the bass part of the track.

10k, 10 kΩ

From the Bass wiper to ground. It sets how far the bass can be cut.

Vol 1, 500 kΩ pot, audio taper

Channel 1’s Volume, 500 kΩ audio, on the treble wiper. Its wiper is V3a’s grid.

V3a, 6EU7

One half of the third 6EU7, channel 1’s second stage.

3.3k, 3.3 kΩ

Cathode resistor for V3a, not bypassed, so the stage has less gain.

180k, 180 kΩ

Between V3a’s plate and the node its signal comes from. Written over on this copy of the drawing (it reads like 190 k, 1 W), so 180 k is assumed. With the 68 kΩ it makes a divider, so only about a quarter of the plate’s swing goes on.

68k, 68 kΩ

From the supply to V3a’s output node. With the 180 kΩ it’s the stage’s plate load.

.047µ, 47 nF

From channel 1 to the mixing node.

Vol 2, 500 kΩ pot, audio taper

Channel 2’s Volume, 500 kΩ audio, wired the other way round: the treble wiper drives the pot’s wiper, one end is grounded and the other end is the pentode’s grid. Turned up, less of the track sits between the signal and the grid and more of it sits to ground.

1M, 1 MΩ

Brings the reverb return to the pentode’s grid.

V2 pentode, 7199 (pentode section)

The 7199’s pentode, channel 2’s second stage, where the reverb comes back in.

47k, 47 kΩ

Plate resistor for the pentode. The drawing reads +87 V on the plate.

150k, 150 kΩ

Screen resistor for the pentode, +95 V on the drawing.

10µ, 10 µF electrolytic

Screen bypass cap for the pentode.

330, 330 Ω

Cathode resistor for the pentode, not bypassed, +1.5 V on the drawing.

.047µ, 47 nF

From the pentode’s plate to the mixing node.

220k, 220 kΩ

The mixing node’s resistor to ground.

.01µ, 10 nF

Coupling cap into the cathodyne’s grid.

Reverb 2M, 2 MΩ pot, audio taper

Reverb, 2 MΩ audio, from channel 2’s tone input to ground. Its wiper feeds the reverb send. Turn it up to send more to the springs.

V2 triode, 7199 (triode section)

The 7199’s triode, the reverb send’s gain stage.

100k, 100 kΩ

Plate resistor for the 7199’s triode, +54 V on the drawing.

1k, 1 kΩ

Cathode resistor for the 7199’s triode, not bypassed, +1.8 V on the drawing.

.01µ, 10 nF

Coupling cap to the reverb driver.

470k, 470 kΩ

The reverb driver’s grid resistor.

V4a, 12AU7 / ECC82

One half of the 12AU7A, the reverb driver. Its cathode isn’t bypassed. The drawing reads +9 V on it, about 9 mA. The other half belongs to the optical tremolo, not modeled.

1k, 1 kΩ

Cathode resistor for the reverb driver.

Reverb T, 10 kΩ to 8 Ω, 10 H primary, 50 mH leakage

The reverb transformer, from the driver’s plate to the spring unit. It loads the driver and the supply. Typical of the part (the GA-19RVT’s figures), not measured.

Unit in R, 800 mΩ

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 long spring tank, not measured.

Unit in L, 1.3 mH

Inductance of the reverb unit’s input coil. Typical of a long spring tank, not measured.

Unit out L, 360 mH

The reverb unit’s output coil, from the recovery grid to ground. The springs come back in series with it. Typical of a long spring tank, not measured.

Unit out R, 200 Ω

Resistance of the unit’s output coil. Typical, not measured.

.001µ, 1 nF

From the recovery grid to ground. It trims the springs’ top end.

220k, 220 kΩ

Grid resistor for the recovery stage.

V3b, 6EU7

The other half of the third 6EU7, the reverb recovery stage. It brings the springs back up, into the pentode’s grid.

100k, 100 kΩ

Plate resistor for the recovery stage, +170 V on the drawing.

1.5k, 1.5 kΩ

Cathode resistor for the recovery stage, not bypassed, +1.1 V on the drawing.

.01µ, 10 nF

From the recovery plate to the 1 MΩ into the pentode’s grid.

V5a, 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.

56k, 56 kΩ

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

2.2k, 2.2 kΩ

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

56k, 56 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.

.02µ, 20 nF

Coupling cap from the inverter to side A’s driver.

.02µ, 20 nF

Coupling cap from the inverter to side B’s driver.

1.5k, 1.5 kΩ

Side A’s driver cathode resistor, 1.5k Ω to ground with no bypass cap. Each half has its own, so each stage has less gain and the two sides don’t act on each other.

1.5k, 1.5 kΩ

Side B’s driver cathode resistor, the same.

220k, 220 kΩ

Grid leak for side A’s driver, to ground.

V6a, 6EU7

6EU7, side A’s driver: a gain stage that raises the inverter’s signal about 30 times on its way to tube A’s grids. The other half drives side B.

100k, 100 kΩ

The driver’s plate resistor, 100k Ω from the node for the drivers.

.02µ, 20 nF

Coupling cap from the driver’s plate to tube A’s grids.

220k, 220 kΩ

Grid leak for side B’s driver, to ground.

V6b, 6EU7

The other half of the 6EU7, side B’s driver.

100k, 100 kΩ

The driver’s plate resistor, 100k Ω from the node for the drivers.

.02µ, 20 nF

Coupling cap from the driver’s plate to tube B’s grids.

V7, 6L6GC

6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.

V8, 6L6GC

6L6GC, 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

The choke from the reservoir cap to B+, as on the amp’s drawing. The output transformer’s center tap, the screens and everything after them hang past it, so all of their current runs through it. It passes DC with little drop. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.

50, 50 Ω

The choke’s winding resistance, typical, not measured. With the output tubes’ whole current through it, it’s most of the DC drop between the reservoir and B+.

20µ, 20 µF electrolytic

Filter cap on B+ after the choke, where the output transformer’s center tap and the screens hang.

10k, 10 kΩ

Dropping resistor from the screen supply node to the node for the drivers, as on the Gibson GA-77RVT Vanguard 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 drivers. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.

10k, 10 kΩ

Dropping resistor from the node for the drivers to the node for the inverter and preamp, as on the Gibson GA-77RVT Vanguard 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.

10µ, 10 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.

Tremolo, 22 kΩ

Stands for the optical tremolo (not modeled) on the screen supply node. It pulls about the current worked out from the voltages on the drawing. That part of the amp isn’t modeled here. Typical, not measured.

OT, 6.6 kΩ to 8 Ω, 40 H primary, 10 mH leakage

The output transformer, 6.6 kΩ plate to plate into 8 Ω, center tap on B+ after the choke. 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, 362 V ac, 60 Hz

One half of the power transformer’s high-voltage winding, 362 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, 362 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.

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.

20µ, 20 µF electrolytic

The reservoir cap. It feeds the choke, and past it the output transformer’s center tap and everything else. 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.

Bias tap, 51 V ac, 60 Hz

The power transformer’s bias tap, 51 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.

50µ, 50 µF electrolytic

Filter cap on the bias supply, its plus side to ground.

7.5k, 7.5 kΩ

Second resistor of the bias supply, 7.5k Ω to the bias line. With the cap after it, it filters the ripple a second time.

50µ, 50 µF electrolytic

Second filter cap, on the bias line, its plus side to ground.

22k, 22 kΩ

From the bias line to ground, 22k Ω. With the 7.5k 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, 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 12AU7A the 12AU7 model, the 7199 the bench’s 7199 presets. The tremolo, an optical one, isn’t modeled. The reverb transformer and unit are typical, not on the drawing.