# 6L6GC Comparison of Current Made Tubes

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:

The "mid-range" DC plate current value measured in Maxi-Matcher

$$I_a = 24\text{ mA}$$

Maxi-Matcher given plate voltage

$$V_a = 400\text{ VDC}$$

Maxi-Matcher given control grid voltage

$$V_{g1} = -48\text{ VDC}$$

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 secondary winding while connected to an 8 ohm load. The 50Hz measurement was used for Lows, 700Hz for mids and 6kHz for highs.

Tube Brand / Electrical RatingsMeasured Voltage Gain (dB)Physical Comparison
Drawn to Scale
T-6L6GC-WC
6L6GC Winged C / SED - Made in Russia
 Maximum Ratings $V_a$ 500V $V_{g2}$ 500V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-6L6GC-WC

Seated Height: 3.66"
Glass Diameter: 1.53"
Guitarist's Opinion: This tube has a straightforward, focused tone with an emphasis on highs for sizzling leads.
T-6L6GC-STR-T
6L6GC STR Tung-Sol Reissue - Made in Russia
 Maximum Ratings $V_a$ 500V $V_{g2}$ 500V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-6L6GC-STR-T

Seated Height: 3.19"
Glass Diameter: 1.44"
Guitarist's Opinion: This tube is well balanced, smooth and warm.
T-6L6GC-JJ
6L6GC JJ Electronic - Made in Slovakia
 Maximum Ratings $V_a$ 500V $V_{g2}$ 450V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-6L6GC-JJ

Seated Height: 3.88"
Glass Diameter: 1.44"
Guitarist's Opinion: This tube is fat and smooth with some sparkle on the top-end. It offers more bite than most.
6L6GC Tube Amp Doctor - Made in China
 Maximum Ratings $V_a$ 550V $V_{g2}$ 450V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

Seated Height: 3.71"
Glass Diameter: 1.49"
Guitarist's Opinion: This black plate tube has a lot of grit in overdrive. It's bright, focused and chimey.
6L6WGC Tube Amp Doctor - Made in China
 Maximum Ratings $V_a$ 550V $V_{g2}$ 450V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

Seated Height: 3.09"
Glass Diameter: 1.50"
Guitarist's Opinion: 6L6WGC black plate is slightly smoother than TAD 6L6GC with reduced bite and brightness.
T-KT66-TUNG
KT66 Tung-Sol Reissue - Made in Russia
 Maximum Ratings $V_a$ 500V $V_{g2}$ 450V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-KT66-TUNG

Seated Height: 4.07"
Glass Diameter: 2.03"
Guitarist's Opinion: This big bottle tube is tight, smooth and punchy. It offers a lot of sizzle for overdrive leads.
T-6L6-EH
6L6GC Electro-Harmonix - Made in Russia
 Maximum Ratings $V_a$ 500V $V_{g2}$ 450V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-6L6-EH

Seated Height: 3.68"
Glass Diameter: 1.46"
Guitarist's Opinion: This tube has a straightforward tone that's strong and tight with a chimey top-end.
T-6L6WXT+SOVT
6L6WXT+ Sovtek - Made in Russia
 Maximum Ratings $V_a$ 500V $V_{g2}$ 450V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-6L6WXT+SOVT

Seated Height: 3.63"
Glass Diameter: 1.47"
Guitarist's Opinion: This tube is similar to its EH cousin, but with a little more warmth and low-end.
T-6L6-CHP-MP-GT
6L6-CHP Groove Tubes - Made in China
 Maximum Ratings $V_a$ 500V $V_{g2}$ 450V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-6L6-CHP-MP-GT

Seated Height: 3.53"
Glass Diameter: 1.39"
Guitarist's Opinion: This black plate tube is focused and tight with crisp, chimey highs.
T-6L6GE-MP-GT
6L6-GE Groove Tubes - Made in China
 Maximum Ratings $V_a$ 500V $V_{g2}$ 450V $W_a$ 30W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-6L6GE-MP-GT

Seated Height: 3.44"
Glass Diameter: 1.50"
Guitarist's Opinion: This reproduction of a GE 'cleartop' is similar to the CHP, but smoother and sweeter in the highs.
T-KT66-JJ
KT66 JJ Electronic - Made in Slovakia
 Maximum Ratings $V_a$ 550V $V_{g2}$ 550V $W_a$ 25W Filament Data $V_f$ 6.3V $I_f$ 1.3A
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-KT66-JJ

Seated Height: 3.92"
Glass Diameter: 2.06"
Guitarist's Opinion: This tube is tight, smooth, crisp and punchy with bigger lows than other KT66s.
T-6L6WGC-SINO
6L6WGC China - Made in China
 Maximum Ratings $V_a$ 400V $V_{g2}$ 400V $W_a$ 25W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-6L6WGC-SINO

Seated Height: 3.69"
Glass Diameter: 1.47"
Guitarist's Opinion: This black plate tube is focused, loose and easy on the highs.
T-5881WXT-SOVT
5881WXT Sovtek - Made in Russia
 Maximum Ratings $V_a$ 500V $V_{g2}$ 450V $W_a$ 23.5W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-5881WXT-SOVT

Seated Height: 3.65"
Glass Diameter: 1.45"
Guitarist's Opinion: This tube is similar to its EH cousin, having a focused, straightforward tone.
T-5881-SOVT
5881 Sovtek - Made in Russia
 Maximum Ratings $V_a$ 500V $V_{g2}$ 450V $W_a$ 23.5W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-5881-SOVT

Seated Height: 3.38"
Glass Diameter: 1.46"
Guitarist's Opinion: This tube is the same 5881WXT, but with a smaller 'wafer' base.
T-6L6WGS-SINO
6L6WGS China - Made in China
 Maximum Ratings $V_a$ 400V $V_{g2}$ 400V $W_a$ 23W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-6L6WGS-SINO

Seated Height: 3.15"
Glass Diameter: 1.52"
Guitarist's Opinion: This black plate tube is warm, fat and loose with a smooth top-end.
T-5881-TUNG
5881 Tung-Sol Reissue - Made in Russia
 Maximum Ratings $V_a$ 400V $V_{g2}$ 400V $W_a$ 23W Filament Data $V_f$ 6.3V $I_f$ 900mA
-9-8-7-6-5-4-3-2-10
Lows
Mids
Highs

#### Drawing of T-5881-TUNG

Seated Height: 2.91"
Glass Diameter: 1.45"
Guitarist's Opinion: This tube is focused and bright with top-end sparkle.

## A Comparison of Current Production 6L6GC Tubes

RCA introduced the 6L6 to the world in the 1930s. 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 1950s). 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.

## 6L6GC Basics

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 sample was installed in the same custom built, fixed bias single-ended guitar amplifier (see schematic) with a Fender black face style preamp circuit. A frequency generator was used to measure voltage gain and plot the frequency response from 10Hz to 20kHz. The input voltage was measured at the 6L6GC control grid (g1) and the output voltage was measured at the output transformer secondary winding while connected to an 8 ohm load.

### Schematic of the power amp used for 6L6GC voltage gain measurements and listening tests

#### 6L6GC Voltage Gain Schematic

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:

$$V_a = 400\text{ VDC}$$$$V_{g1} = -48\text{ VDC}$$$$I_a = 24\text{ mA}$$

### The Listening Test

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.