Yellow JacketYellow Jacket converters allows amplifiers using common power tubes to use EL84s without modification or rebiasing.
Tube Converter - Yellow Jacket® YJUni, for low-power amps
The YJUni is a simple pin-converting adapter for use in most amps with 6V6 or similar based tubes like 6F6, 6G6, 6K6, 6Y6, etc. The YJUni is shorter than standard Yellow Jacket® converters. It is designed for use in lower power output amps (20 watts or less) when no power reduction is desired.