Why would I want to convert to EL84s using Yellow Jackets®?
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 EL84s and your amplifier's original power tubes without rebiasing.
How do you know which one to pick?
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.
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 Yellow Jacket
This partial schematic of the 5F6-A output stage is presented here as an example of fixed-bias power tubes. Notice the power tube cathodes (pin 8) are connected directly to ground (represented by the triangle) and the negative voltage connected to the control grids (pin 5) through the 220K resistors.
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.
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 a examples of cathode biased power tubes that would require the YJC converter. The 5E5 (1955, 26W) has a push-pull output, while the AA764 (1964, 4W) has a single-ended output.
What would happen if I didn't connect the ground wire in my cathode biased amp?
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.
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).
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 "Reverbojet" (1967, 15W) is presented here as an example of a cathode-biased amp using 7591 power tubes. Notice how the pin-outs differ from the other common octal power tubes found in guitar amps.
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.
What's the difference between the normal and triode versions?
Most guitar amp power tubes are pentodes, meaning they have five electrodes:
- control grid
- screen grid
- suppressor grid (or beam confining electrode)
- control grid
Do certain amps perform better with the triode?
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
Cathode Drive vs. Grid Drive
Output Stage Symmetry
Output stage symmetry refers to push-pull outputs (as opposed to "single-ended". All but one of the amps 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 vice-versa).