The 6K7G (or 6K7GT) is a remote-cutoff (variable-mu) pentode designed for RF/IF amplification, but can be used in audio preamplifiers, often praised for its "smooth" or "vintage" sound when biased correctly. As a variable-mu tube, careful selection of the loading line is required to achieve high fidelity, or it can be used for compression effects.

6K7G Pentode Preamplifier Configuration

A typical high-gain pentode preamplifier stage using a 6K7G tube has the following connections:

Grid (Cap): Input signal (usually via a grid bleeder resistor, e.g., 1 Mohms).

Pin 3 (Plate): Output, through a plate load resistor (e.g., 100K/220K=AB+).

Pin 4 (Screen Grid, G2): Connected to a lower voltage than the plate via a dropout 

resistor and bypassed to ground with a 0.2uF DA capacitor to reduce noise.

Pin 5 (Suppressor Grid, G3): Connected directly to the cathode.

Pin 8 (Cathode): Connected to ground via a bias resistor (e.g., 220 Ohms to 600 Ohms), 

typically bypassed by a large electrolytic capacitor (e.g., 10uF/100uF) 

for higher gain.

Heater (pins 2 and 7) = 6.3V

Key Design Notes

Noise Management: Since 6K7 pentodes can be microphonic, short, 

shielded wiring and a metal enclosure (or good shielding for the G version) are recommended.

Point Operation: For linear (audio) operation, the screen voltage must be kept stable.

Input signal: Due to its remote-cutoff nature, this tube handles larger signals on the grid better than sharp-cutoff tubes like the 6J7.