← Reference

6SJ7

also 6SJ7GT · 6SJ7-GT (glass octal) · 12SJ7 (12.6 V heater)

RCA metal sharp-cutoff pentode — the octal preamp valve of the wide-panel tweed era, before the 9-pin 12AX7 took the front end over. A pentode input stage gives several times the gain of a triode in the same place, and the metal envelope is its own shield: grid No.1 comes out on a base pin rather than a top cap, which is what let Fender put it on a chassis-mounted board at all. Grid No.3 (the suppressor) has its own pin and is strapped to the cathode.

Basing (pinout)

Bottom view — the socket as you see it from the wiring side. Hover, tap, or focus a pin to name it. Read from the datasheet basing diagram (how to read these →).

1 2 3 4 5 6 7 8

Hover, tap, or focus a pin to name it.

  • High voltage (B+)
  • Control grid
  • Cathode
  • Heater / filament
  • Internal connection

Viewed from below (socket / wiring side) · octal · basing 8N

⚡ Oxblood pins sit at B+ — hundreds of volts in service, lethal, and still charged after power-off.

Datasheets

Primary manufacturer data, via public tube-data archives. The model below is fitted to the tabulated "average characteristics" in these sheets; the datasheet files themselves are linked, never rehosted.

PublisherTypeSource
RCA 6SJ7 / 6SJ7-GT frank.pocnet.net ↗
General Electric 6SJ7 / 12SJ7 (ET-T1400) frank.pocnet.net ↗

Circuit Codex model

A CC0 ngspice model in the Koren form, fitted here to the published anchor — no parameters copied from other model collections. Methodology →

Anchor
Va=250 V, Vg2=100 V, Vg3=0 V, Vg1=-3 V → Ia=3.0 mA, Ig2=0.8 mA, gm=1650 µmho
Source
RCA 6SJ7 / 6SJ7-GT data sheet, June 15, 1948 — Amplifier Class A1, pentode connection, typical operation and characteristics
Note
Two parameters here are not the usual project defaults, and both are documented rather than assumed. µ=19 is the tabulated TRIODE-connection amplification factor (grids No.2 and No.3 tied to the plate) standing in for the grid-No.1-to-grid-No.2 factor, which neither RCA nor GE prints for this tube — the same substitution the KT66 model makes. RCA's own gm·rp at that point (2500 µmho × 7600 Ω = 19.0) confirms the printed 19. KVB=8.57 is fitted, not defaulted: this is the one pentode in the corpus whose sheet tabulates plate current at two plate voltages at the same screen and grid (Va=250 V → 3.0 mA and Va=100 V → 2.9 mA at Vg2=100 V, Vg1=-3 V), and in this model form that pair measures the plate-voltage dependence directly. Both plate voltages are CI-gated. EX=1.5 is the project default per METHODOLOGY.md.
File
models/6sj7.inc ↗

See these parameters drawn: the load line explorer plots the 6SJ7's plate-curve family from this model and solves an operating point on it.

Maximum ratings

Design-centre values, pentode connection. Two and a half watts of plate dissipation and a screen ceiling of 125 V put this firmly in small-signal service: the screen must be fed through a dropper from the preamp rail, never tied to it. The triode-connection column is the same tube with grids No.2 and No.3 on the plate, and it is rated lower still (250 V).

Plate voltage
300 V
Grid-No.2 (screen) voltage
125 V
Grid-No.2 supply voltage
300 V
Plate dissipation
2.5 W
Grid-No.2 dissipation
0.7 W
Grid-No.1 voltage, positive bias value
0 V
Peak heater-cathode voltage
90 V, either polarity
Grid-No.1 circuit resistance
1 MΩ
Plate voltage, triode connection
250 V
Plate dissipation, triode connection
2.5 W
Rating system
Design centre — carries an allowance for manufacturing spread
Source
RCA 6SJ7 / 6SJ7-GT data sheet, June 15, 1948 — Amplifier Class A1, maximum ratings (design-centre values); the same figures appear on General Electric ET-T1400, November 1956 ↗