The middle circuit of the piggyback Bassman's three-revision run: a
50-watt head-and-cabinet rig that replaced the tweed 4x10 combo entirely.
Fender moved the Bassman name onto a genuinely different amplifier here —
two full input channels (Bass, Normal), each with its own Bass/Treble/Volume
network, mixing through a pair of 470 kΩ resistors just ahead of a
long-tailed-pair phase inverter and a fixed-bias 5881 pair.
The 6G6 (late 1960) used a GZ34 tube rectifier; the 6G6-A (early 1961)
moved to solid-state rectification; the 6G6-B carried that forward with
further circuit changes through 1963, running in blonde Tolex until the
blackface AA864 piggyback head replaced it in 1964 —
carrying this circuit's two-channel split, its solid-state supply and its
long-tailed-pair inverter with it.
Circuit walkthrough (short form)
Bass channel: two inputs (68 kΩ stoppers, 1 MΩ leak) → V1A (2.7 kΩ
cathode, 220 kΩ plate off the Bass row's +230 V lane, printed +135 V/+1.1 V)
→ V1B, a cathode follower direct-coupled to that plate: its own plate
lead goes straight to the same +230 V lane with no plate resistor in it, and
its 100 kΩ cathode load is the stage's output, printed +136 V. That cathode
drives the Bass/Volume network: a 250 pF + 47 kΩ branch up to the top of the
250 kΩ-L Volume pot, and a 1 MΩ down to a node from which two 0.25 µF caps
rise — one to the 10 kΩ's foot (the 10 kΩ hangs from the Volume top), which
is the 25 kΩ-L Bass rheostat's wiper, the other to the rheostat's far lug,
where an 820 Ω foot goes to ground. The Volume wiper feeds V3A (below).
Normal channel: two inputs → V2A (1.5 kΩ cathode, 100 kΩ plate load
climbing to that channel's own +355 V lane, printed +230 V/+1.9 V) → a
two-knob ladder on a tapped Treble pot (100 kΩ slope; 250 pF into the hot
lug of a 350 kΩ Treble pot with a 70 kΩ tap; 0.1 µF from the slope foot to
that tap, where the 250 kΩ-A Bass rheostat also hangs; 0.05 µF from the
slope foot to the Bass foot and 6.8 kΩ from there to ground; the Treble
pot's cold lug bled to ground through 0.005 µF) → a 1 MΩ-A
Volume pot → V2B recovery (2.7 kΩ cathode, 220 kΩ plate off the same
+355 V lane, printed +190 V/+1.8 V) → a 470 kΩ resistor into the mixing
node described below.
Bass channel, continued: the Bass Volume's wiper feeds V3A's grid
(100 kΩ plate off +230 V, 1.5 kΩ cathode, printed +150 V/+1.2 V) → the Bass
channel's own 250 kΩ-A Treble control (the front panel's BASS/TREBLE/VOLUME
group ahead of the Bass jacks reads 25k-L/250k-A/250k-L, so this pot sits in
V3's signal path rather than beside the channel's other two knobs) → V3B
(1.5 kΩ cathode, 100 kΩ plate off +230 V with 0.002 µF across it, printed
+150 V/+1.3 V) → a 470 kΩ resistor into the mixing node.
Mixing node: V3B's 470 kΩ and the Normal channel's 470 kΩ from V2B's
plate meet at one node, and a 500 pF capacitor carries it into the phase
inverter. V3 is therefore the Bass channel's own second and third gain
stages, not a driver both channels share: the Normal channel joins the Bass
channel's path only here. No capacitor stands between either plate and the
node, so the two 470 kΩ resistors are also a DC path from V2B's plate to
V3B's, about 40 µA, and the simulated deck includes it.
Phase inverter: long-tailed pair, 82 kΩ (hot) / 100 kΩ (cold) 5%
plates off a +410 V lane (printed +300 V/+280 V), both 1 MΩ grid leaks
returned to the tail junction. The shared cathodes sit 820 Ω above that
junction — printed +30 V at the cathodes, +28 V at the junction. Below the
junction the sheet letters the whole foot: 6.8 kΩ down to a
presence/feedback node, 4.7 kΩ from there to ground, the 56 kΩ speaker
feedback returning to the same node (a second path to ground through the
output transformer's secondary), and the front panel's 25 kΩ-L Presence pot
bridging the 4.7 kΩ end to end with a 0.1 µF · 200 V cap on its wiper.
Output: two 5881s, fixed-biased through 220 kΩ leaks from a −54 V
supply (grounded cathodes, no cathode resistor). Their plates hang on the
output transformer's primary, whose centre tap the sheet letters +430 V —
the same node as the reservoir — and the +428 V printed at the plate leads
is the primary's own resistance. The 470 Ω 1 W screen stoppers come off the
other end of the choke: node B, the +428 V node the dropper chain also
leaves from.
Power: a centre-tapped HT winding feeds two three-diode series legs — a
solid-state full-wave rectifier; there is no rectifier tube. Two 20 µF ·
600 V cans in parallel are the reservoir, and past the standby switch that
+430 V node does two things: it runs up to the output transformer's primary
centre tap, and it feeds the choke (TR2, 125C1A). The choke's far end is
node B, +430 V less the choke's own resistance, printed +428 V — and every
other lane comes off it: 4.7 kΩ · 1 W → +410 V (phase inverter) → 27 kΩ ·
1 W → +355 V (the Normal channel's whole lane), with the Bass row's
+230 V lane on its own 56 kΩ · 1 W. Each of those nodes carries a
20 µF · 600 V filter, and the two 470 Ω screen stoppers sit on node B
alongside the droppers. A small silicon diode off an AC tap, 1 kΩ dropping,
a 27 kΩ bleeder and a 25/50 µF dual can produce −54 V for the output bias.
Only the two rectifier legs and the bias parts are on the eyelet board; the
cans, the choke and the three droppers are chassis parts the layout page
does not draw, and the board takes each lane as a lettered lead.
Reading this drawing
The E-FB drawing (both the board-layout and schematic pages) prints its own
title block reading "FENDER 'BASSMAN' LAYOUT / SCHEMATIC — MODEL 6G6-B" —
confirmed against the target amp before any value was read.
Two places on this sheet are easy to misread, and both reward a lug-level
look at high magnification.
V1B is a cathode follower. Its plate lead runs straight down to the
Bass row's +230 V lane with no plate-load resistor in it; its grid takes
V1A's plate directly, with no coupling capacitor and no grid leak of its
own; and its 100 kΩ cathode load is what feeds the tone network. The
printed figure at that cathode is +136 V, not +13.6 V — the mark
between the 3 and the 6 is the +230 V lane's own wire crossing the
lettering, and it runs the full height of the text rather than sitting on
the baseline like the decimal points elsewhere on the sheet. A follower
riding about a volt above the +135 V plate that drives it is exactly what
+136 V beside +135 V describes; read the crossing wire as a decimal point
and the resulting 13.6 V across 100 kΩ is a current no self-biased 12AX7
stage can pass at any rail on this drawing.
The sheet's third "+230 V." is a plate voltage. It letters +230 V
twice on the Bass row's own supply lane — a rail — and once more in the
Normal channel, at V2A's plate. That plate's 100 kΩ load climbs to the
+355 V lane, the same lane V2B's 220 kΩ uses. Hang it on a shared
+230 V rail instead and the stage's current halves, leaving the cathode a
third under the printed 1.9 V — the sheet's own figures rule that reading
out.
The Normal channel's tone network is the blackface ladder, on a pot
with a tap. The Treble control is lettered 350K-70K TAP and has four
connections: the 250 pF on its hot lug, the wiper to the Volume pot, the
0.1 µF from the 100 kΩ slope's foot landing on the tap, and the cold lug
going to ground through 0.005 µF. The Bass control is a rheostat from that
tap node down to the 6.8 kΩ foot, with the 0.05 µF from the slope foot
landing on the same foot — the same six-part shape the AB763 amplifiers
draw, except that the AB763's bass capacitor lands on the Treble pot's
cold lug and this one lands on a tap part-way up the element, with the
cold lug bled separately. An earlier drawing here ran the 0.05 µF from the
plate, had no slope resistor and no 0.1 µF, and tied the tap, the slope
foot and the Bass foot into one node; the sheet was re-read lug by lug and
the network redrawn with all six parts.
Both readings are corroborated by arithmetic already on the sheet.
136 V across 100 kΩ is 1.36 mA; add V1A's 0.43 mA and V3's
two 0.8 mA sections and the +230 V lane draws about 3.4 mA, which across its
own 56 kΩ · 1 W dropper falls 190 V from the +428 V node — landing on the
printed +230 V. Read as a common-cathode stage instead, V1B passes about
30 µA, the lane draws 2 mA, and the same dropper would leave it above
+310 V. In the Normal channel, the 125 V across V2A's 100 kΩ is 1.25 mA, and
1.9 V across its 1500 Ω cathode resistor is 1.27 mA — the two printed
figures agreeing to 2%.
V3A's cathode figure (+1.2 V beside its 1.5 kΩ) and the phase-inverter
tail's whole foot (6.8 kΩ, 4.7 kΩ, the 56 kΩ feedback and the Presence pot
across the foot) are likewise printed on the sheet and read directly:
nothing in the simulated deck is an estimate.
The Presence pot itself is the one drawn DC path left out of the deck — pots
are omitted corpus-wide — and including its 25 kΩ element would move the
tail junction from 28.4 V to about 27.8 V, both inside the chart's ±20%.
With the circuit the sheet actually draws, every gated node but one lands
inside the chart's own ±20% convention, and the three derived rails land
within 2.8% of their printed figures. The exception is V1A's cathode:
1.32 V simulated against a printed 1.1 V, 20.1% — a tenth of a percent
outside. Its own lane is derived rather than driven and sits 2.8% above the
printed +230 V; hold that lane at the printed figure instead and the same
cathode reads 1.29 V, 17.4%. The entry stays a draft because that node is
out, and because a verified badge is a maintainer's to grant.
The board drawing follows the E-FB layout page, and it draws each channel's
Bass, Treble and Volume network in full. The Normal channel's slope resistor
and its three capacitors stand on the board; its 6.8 kΩ foot and 0.005 µF
bleed hang at the Bass and Treble pots, which is where the layout page puts
them, and the Treble pot is drawn with its fourth, tap terminal. The two
470 kΩ mixing resistors meet at their own eyelet, which carries the 500 pF
to the phase inverter, as the layout page draws them. The drawn wiring is
proved electrically equivalent to the simulated circuit.
The Bass/Normal split, and why V3 carries a Treble pot
This is a genuinely two-channel amplifier, not a single voice with a bright
switch: each channel gets its own preamp and its own tone network, and they
mix only after both have been shaped, at the node that feeds the phase
inverter. The Bass channel's Treble control sits inside V3's signal path
(between its two triode sections) rather than beside the channel's Bass and
Volume pots, and that is a printed fact of this drawing, not a
simplification — the front-panel silkscreen order (Presence, Bass, Treble,
Volume, then the Normal jacks; Bass, Treble, Volume, then the Bass jacks) is
what fixes which knob belongs to which channel, and the schematic draws the
Bass channel's Treble pot between V3A and V3B, upstream of the mixing
point.
The Normal channel, by contrast, carries its Treble control in its own
first-stage network (350 kΩ/70 kΩ tap) before mixing — the two channels'
tone-shaping paths are not identical twins of each other, only broadly
parallel.
