The PCB has a jumper for EL84x tubes. What does the “X” stand for?
Answer: We use a tube that is a modern equivalent of the EL84. It has a slightly different pin-out from the standard EL84. In most cases you can just plug them in, but with the correct external circuit, the EL84X can give more power than a standard EL84.
For modern tubes that are all based on the same design (Russian, Chinese and Czech) and quite a few older EL84 this is no issue, but for example the Mullard EL84 (and some others too) have an internal connection on a pin that is normally “NC” (no connection) that could lead to fireworks.
Some commercial EL84 amplifiers actually use this pin as a wiring point (in line with the EL84X pin-out, incidentally or intentionally), but they cause trouble with some old tubes. We like to avoid that.
Pulling the jumper makes the Stereo 50 compatible with all original old stock EL84 (in case you can find any), with the jumper in place is works with the EL84X and delivers deliver more power.
