Photo by Roadside Guitars
Amped guest blogger Kurt Prange, of Amplified Parts, puts 16 separate 6L6GC tubes to the test and comparatively rates their low, mid and high-end response.
RCA introduced the 6L6 to the world in the 1930 s. It had a metal envelope which was popular at that time for use in radio sets since it was less likely to break than a glass envelope.
Revisions to the original metal 6L6 such as increased power handling and glass envelopes yielded new designations such as 6L6G, 6L6GA, 6L6GB and finally 6L6GC (introduced by GE in the late 1950's).
Today, the 6L6GC is one of the most common power tubes used for electric guitar amplification and there is an almost overwhelming supply of current production options to choose from.
The purpose of this comparison is simply to provide guitar players with a frame of reference to help in finding the best current production 6L6GC for their needs. Also included in this comparison are direct substitutes for the 6L6GC including: KT66 and 5881.
The 6L6GC is an octal, beam power tube. What does that mean? The octal part tells us what kind of socket the tube plugs into. It has eight pins arranged around the circumference of a circle with a diameter of about 11/16 of an inch. There is a key between pins 1 and 8 to ensure that the tube can only be plugged in one way. The beam power part tells us that the tube, as RCA put it, "is a tetrode or pentode in which directed electron beams are used to increase substantially the power-handling capability of the tube.
A feature of a beam power tube is its low screen-grid current. Because of the effective suppressor action provided by space charge and because of the low current drawn by the screen grid, the beam power tube has the advantages of high power output, high power sensitivity, and high efficiency." The 6L6GC is a type of beam power tube employing a beam-confining electrode at cathode potential in place of an actual suppressor grid.
The Measured Voltage Gain Test Setup
Each tube in this comparison was installed in the same custom built, fixed bias single-ended guitar amplifier in order to obtain the voltage gain measurements and conduct listening tests. Each tube sample used for comparison was chosen because it measured in the middle range of idle plate current draw at the same operating point set by a Maxi-Matcher Digital Tube Tester:
An audio frequency generator was used to sweep from 10Hz to 20kHz with the input voltage measured at the 6L6GC control grid (g1) and the output voltage measured at the output transformer's secondary winding while connected to an 8 ohm load. The 50Hz measurement was used for Lows, 700 Hz for mids and 6kHz for highs.
Three frequencies were chosen from the frequency plots to represent the lows, mids and highs: 50Hz, 700Hz and 6kHz, respectively, for each tube brand (for a quicker visual reference). Each tube sample used for comparison was chosen because it measured in the middle range of idle plate current draw at the same operating point set by a Maxi-Matcher Digital Tube Tester: Va = 400VDC, Vg1 = -48VDC, Ia 24mA.
The Listening Test Setup
A custom 6L6GC switching box was constructed to allow for a quick switch comparison of two tubes at a time while plugged into the V2 socket of the same fixed bias, single-ended guitar amplifier used for the frequency sweep. Listening tests were then conducted while playing guitar and switching between the tubes at various amplifier control settings to come up with "guitarist's opinion" descriptions for each tube.