TUBE CONVERTERS

Yellow Jackets® tube converters allow EL84 power tubes to be used in place of the most common guitar amp power tubes including 6L6, EL34, 6V6, 7027, 6550 and 7591. Most Yellow Jackets® screen No provide a substantial output power grid Connection reduction and a “self-bias” Class A 9 1 No EL84 configuration for the EL84 so that no Connection control bias adjustment is required. Yellow 8 2 grid Jackets® are like getting a whole new amp. plate 7 3 cathode + suppressor grid

Yellow Jackets® Types No 6 4 Connection 5 filament YJS p. 2 filament YJSHORT p. 3 YJC p. 3 YJ20 p. 4 YJUNI p. 4 1 9 YJ7591 p. 4 YJR p. 5

1 8

screen control grid 4 5 grid

plate 3 6 No Connection

2 7 filament filament

No 1 8 cathode Connection + suppressor grid

1 Why would I want to convert to EL84's using Yellow Jackets®? Every power tube type offers a different characteristic sound and feel. EL84's have a very tight and focused sound which has become world renown by their use in the British VOX™ AC30 guitar amplifiers. Additionally, most Yellow Jackets® converters will produce a substantial maximum power reduction (50% to 90%) making it easier to find that sweet, warm mix of preamp and power amp distortion at a lower volume.

Yellow Jackets also convert the power tube bias to “self-bias” Class A so that no bias adjustment is necessary. You can switch back and forth between EL84's and your amplifier’s original power tubes without rebiasing.

How do you know which one to pick? If you are in doubt, you may e-mail [email protected] and we will help you decide. Yellow Jackets® are not recommended for use in amplifiers that have a rated power output greater than 100 watts or plate voltages greater than 519 VDC. YJS (Converts 6L6, EL34, 6550, 7027 or 6V6 to EL84) The YJS is for amplifiers with fixed bias power tubes. Fixed bias power tubes have their cathode connected directly to ground and a negative DC voltage applied to their control grid. The negative bias voltage is often designed to be adjustable by way of a bias pot.

BASING DIAGRAMS FOR COMMON TUBES THAT CAN BE CONVERTED BY THE YJS

screen control screen control screen control grid 4 5 grid grid 4 5 grid grid 4 5 grid

control plate plate plate 3 6 No 3 6 No 3 6 grid Connection Connection

2 7 2 7 filament 2 7 filament filament filament filament filament

screen No 1 8 cathode 1 8 cathode 1 8 cathode grid Connection + suppressor grid suppressor grid + suppressor grid 6L6, 5881, 6V6, 6550 EL34, 6CA7 7027

The 1959 narrow panel tweed Fender® Bassman (5F6-A) FENDER® BASSMAN 5F6-A OUTPUT STAGE SCHEMATIC is a historically significant 40 watt guitar amplifier. It is well known that this amplifier circuit served as the design 5881 3 base for the very first Marshall™ amplifiers, JTM45. .1µ 5 4

This partial schematic of the 5F6-A output stage is 8 1W presented here as an example of fixed-bias power tubes. 220K Notice the power tube cathodes (pin 8) are connected directly to ground (represented by the triangle) and the 1W

8 470 470 negative voltage connected to the control grids (pin 5) 220K 5 4 through the 220K resistors. .1µ 3

5881 Some amplifiers (e.g. Fender® Hot Rod Deluxe) use a 1Ω resistor and a 8µ + - +430V +432V diode connected in parallel from the power tube cathodes to ground. -48V These components are added to help when taking bias measurements in 20µ 20µ 20µ fixed bias amps and do not present any problems for the YJS. 56K

2 YJSHORT (Converts 6L6, EL34, 6550, 7027 or 6V6 to EL84) The YJSHORT is simply a short version of the YJS. It is ½” shorter allowing it to fit into more amplifiers. Before ordering Yellow Jackets® for your amp, it’s a good idea to make sure you’ll have enough clearance.

The YJUNI is the only other Yellow Jackets® tube converter that comes in the short style.

22 mm

22 mm

1.5" (38 mm) 2.82" mm 71.4

6L6GC 2.82" 71.4 mm JJ Electronic 5" 4.5" YJS 3.94" (100 (100 mm) YJC YJ20 YJSHORT YJ7591 YJUNI 2.18" 1.68"

YJC (Converts 6L6, EL34, 6550, 7027 or 6V6 to EL84) The YJC is for amplifiers with cathode biased power tubes. (This is often referred to as “self-bias” or “auto- bias” because it does not require a bias adjustment when tubes are changed). Cathode biased power tubes will have a resistor and bypass capacitor connected in parallel from their cathode to ground, as seen with most preamp 12AX7 triodes. The YJC is the YJS with an added ground wire. The ground wire is used to bypass the amp’s original cathode bias components so that the Yellow Jacket’s internal bias components are used instead.

These two guitar amp output stage schematics are provided as examples of cathode biased power tubes that would require the YJC converter. The 5E5 (1955, 26W) has a push-pull output, FENDER® PRO-AMP 5E5 OUTPUT STAGE while the AA764 (1964, 4W) has a single-ended output. 6L6GB 3 .1µ 1500 FENDER® VIBRO-CHAMP AA764 OUTPUT STAGE 5 4 25µ 8

.02µ 220K 6V6GT +342V 3 +26V 250 5 4 8 8 220K 5 4 1500 +21V .1µ 220K 3 6L6GB µ 25

470 1K 2500

+340V +355V +370V +395V

20µ 20µ 16µ 16µ

3 What would happen if I didn’t connect the ground wire in my cathode biased amp? The amp would bias itself too cold to sound good, but no damage would occur. If you don’t know how your amp is biased (fixed or cathode), you can just try connecting and disconnecting the ground wire and listen for which way sounds the best. (Connecting the ground wire in a fixed bias amp won’t cause damage, either).

YJ20 (Converts 6V6 and similar based/low power relatives like 6F6, 6G6, 6K6, 6Y6, etc. to EL84) The YJ20 is like a YJC that does not drop the plate and screen voltages. It is meant for use in lower power output amps (20W or less) when no power reduction is desired. It comes with a ground wire for cathode biased amps.

YJUNI (Converts 6V6 and similar based/low power relatives like 6F6, 6G6, 6K6, 6Y6, etc. to EL84) The YJUni is a direct pin to pin converter with no internal voltage dropping or bias components. It is meant for use in lower power output amps (20W or less) when no power reduction is desired and you would prefer to use the amp’s original bias circuit. If used in a fixed-bias amp, the YJUni is the only Yellow Jackets® converter that would require a bias adjustment.

The YJUni was designed by THD Electronics® for use in their UniValve®, BiValve-30™ and Flexi-50™ amplifiers with the Hi V/Lo V switch set to Lo V (plate voltage = 300 to 320V).

YJ7591 (Converts 7591 to EL84) The YJ7591 is for amplifiers that use 7591 power tubes. Only the triode version will provide a power reduction. It comes with a ground wire for cathode biased amps.

This output stage schematic of the Ampeg® J-12R cathode screen + suppressor grid “Reverbojet” (1967, 15W) is presented here as an grid 4 5 example of a cathode-biased amp using 7591 power control plate tubes. Notice how the pin-outs differ from the other 3 6 grid common octal power tubes found in guitar amps.

2 7 AMPEG® J-12R OUTPUT STAGE filament filament

7591 No 1 8 screen 3 Connection grid .1µ 6 4-8 7591 25µ 5 270K +14V 140

5 270K

6 4-8 .1µ 3 7591 1K +345V +365V

40µ 70µ

4 YJR (Converts 5AR4/GZ34, 5U4 or 5Y3 to solid-state) The YJR is used to convert full-wave tube rectifiers to solid-state. This is usually done to give the amplifier a tighter, more aggressive sound and feel. Avoid tube cathode stripping by keeping your amp on standby for at least 30 seconds when using a solid-state rectifier. Tubes require some warm-up time before their electrons are ready to flow.

plate No 4 plate Connection 4 5

No plate' Connection 6 plate' 3 6

2 filament 2 7 No filament Connection filament + 1 8 No 1 8 Connection cathode No filament Connection 5AR4/GZ34 5U4GB, 5Y3GT

One of the more significant conversions from tube to solid-state rectification came in 1965 when Marshall™ created their 100W “Plexi” Super Lead JTM100.

MARSHALL™ JTM45 MODEL 1987 (RECTIFIER CIRCUIT) MARSHALL™ JTM100 MODEL 1959 (RECTIFIER CIRCUIT)

GZ34 Standby Standby 8 6

2 4

Valve Valve Heaters Heaters 6.3VCT 6.3VCT

5 What’s the difference between the normal and triode versions? The triode version provides a greater power reduction and more linearity than the normal (pentode) version.

Most guitar amp power tubes are pentodes, meaning they have five electrodes: 1) cathode 2) control grid 3) screen grid 4) suppressor grid (or beam confining electrode) 5) plate

Power tube triodes like the 300B (more common to hi-fi stereo) have three electrodes: 1) cathode 2) control grid 3) plate

Pentodes can be wired to behave like triodes by connecting the suppressor grid directly to the cathode and the screen grid to the plate through a current limiting resistor.

Do certain amps perform better with the triode? This is a matter of personal taste. The triode version will provide a greater power reduction and is more linear (less distortion) than the pentode. If you’re looking for the maximum power reduction and want to tame the response of your amp a little, then go for the triode. If you’d like a power reduction, but want a response with more bite and attitude, then go for the pentode.

Amplifiers with Pentode/Triode Switch Some amps, including Marshall™ JCM900, are designed with an “Output Mode Switch” that changes the power tube configuration from “low” power (triode) to “high” power (pentode). For these amps, you will want to use the Yellow Jackets® pentode version and let the amp’s switch convert them to triode.

Tremolo FENDER® PRINCETON REVERB AA1164 OUTPUT STAGE Some amps have a tremolo effect that is 6V6GT 3 achieved by varying the bias voltage of the .1µ power tubes as in the 1964 Fender® 5 4 Princeton Reverb (AA1164). This 8 particular type of tremolo effect will not 220K work with Yellow Jackets® installed.

-34V 8 220K Plate Voltage Too High 5 4 .1µ Yellow Jackets® are NOT to be used in 3 6V6GT amplifiers with extremely high plate voltages such as Marshall™ Majors, +400V 1K +420V Ampeg® SVTs, HiWatt® 200's and 20µ 20µ Orange 120's. These amplifiers have too high a plate voltage and will damage the From Tremolo EL84's and possibly the Yellow Jackets® Oscillator Tube 250KL 100K .1µ INTENSITY - themselves. While the Yellow Jackets® 25µ 22K reduce plate and screen voltages each by + From Power 100 volts, this would still make the plate Transformer voltage too high in certain amps. 6 Cathode Drive vs. Grid Drive MUSIC MAN® RD-50 The vast majority of guitar amps use 6L6GC power tubes that are grid driven, like all of 3 the other amps mentioned previously. 100 5 4 470p Some amps like the Music Man® RD-50 75K 8 (1982, 50W) use a cathode drive method. 20µ 680 + JE1692 Notice how the tube cathodes (pin 8) are 470 connected to transistors. Yellow Jackets® 680 will not work with cathode driven amps. 680 6.8

Output Stage Symmetry 6.8 680 680 It is important to keep your output stage symmetric for best results when using JE1692 470 + 680 Yellow Jackets®. An asymmetric output 20µ 470p stage will present the cold, gritty sound of 75K 8 cross-over distortion. 5 4 100 3 6L6GC Output stage symmetry refers to push-pull outputs (as +30V opposed to “single-ended”). All but one of the amps +290V +585V discussed have push-pull outputs. This can be seen by the output transformer having power tubes connected to both “ends” and a “center-tap” connected to some high voltage power supply. We can think of the tubes on one end pushing, while the tubes on the other end are pulling (and FENDER® TWIN REVERB AB763 OUTPUT STAGE SCHEMATIC vice-versa). The output 6L6GC 3 transformer primary connects to the tubes 1500 5 4 push end The output transformer secondary connects to 8 Secondary the speaker(s) side of output 470 center-tap transformer is NOT shown

pull end

8 470

5 4

.1µ BLUE The output stage of the Fender® Twin Reverb AB763 1500 3 (1963, 85W) is shown here in simplified schematic and 6L6GC

layout form to illustrate output stage symmetry. The 220K layout drawing shows where the tubes are positioned in the actual amplifier chassis. The two tubes connected to

220K 6L6GC the brown wire will push together, while the two 3 connected to the blue wire will pull together. 1500 .1µ 5 4

8 BROWN

FENDER® TWIN REVERB AB763 POWER TUBE LAYOUT 470

Output BROWN Transformer 8 470 Wires BLUE 5 4 1500 5 6 5 6 5 6 5 6 3 4 7 4 7 4 7 4 7 6L6GC 8 8 8 3 3 3 8 3 2 1 2 1 2 1 2 1 -52V +458V +460V

7 OUTPUT STAGE SYMMETRY

YELLOW JACKETS® ON THE OUTSIDE YELLOW JACKETS® ON THE INSIDE

OUTPUT STAGE ASYMMETRY

YELLOW JACKETS® OPTIONS FOR AMPS WITH FOUR POWER TUBES

Two YJ’s and Two Empty Sockets vs. Four YJ’s In a 100-watt amp like a Twin Reverb or Marshall 100, either two or four Yellow Jacket converters can be used. If only two are used and the other two (inside or outside) sockets are left empty, the output is dropped to about 20 watts. With four in place, the amp puts out about 40 watts and takes on a whole new warmth and richness.

P(out) = 100W P(out) = 20W P(out) = 40W

8