I've been having some fun playing with tubes models in LTspice. The good old pcl805 used in the Audiovalve RKV particularly attracted me. They're cheap and plentiful but mostly they're powerful ! The pentode section can withstand 8W. Their triode section is medium ยต and said to be decently linear.
As long as one doesn't go crazy with the rail voltage, it's easy to get 70ma of SE class A out of a pair of PCL805. This gets us 0.9W into 300ohms, 100mW into 32ohms. At 600ohms... 50Vp-p.
In the typical tubes applications I quickly checked however (cathode follower, white cathode follower, etc), the output impedance achieved wasn't that great for low impedance cans. So I decided to go the Audiovalve way: adding sand to the mix to increase significantly open loop gain and then apply feedback. And so, here's the crazy beast. A jfet-bjt-tube amplifier with a total power consumption of 50W
The input jfets are cascoded by the two triodes. A ccs in the tail and a current mirror at the top improve gain and matching. The output tubes are triode strapped and sit on top of a bjt CCS. It could be servoed, but I choose not to, for safety reasons.
The HV power supply value isn't that critical: around 140-150V is fine. An isolation transformer is perfect and cheap for the job. It must provides at least 200ma.
The LV voltages supplies (heaters and negative rail) depends on the tubes you get, the value of the negative rail is actually non critical (but regulate it). 6.3V for ECL805, 18V for PCL805. A dual secondary xformer is the best choice.
As long as one doesn't go crazy with the rail voltage, it's easy to get 70ma of SE class A out of a pair of PCL805. This gets us 0.9W into 300ohms, 100mW into 32ohms. At 600ohms... 50Vp-p.
In the typical tubes applications I quickly checked however (cathode follower, white cathode follower, etc), the output impedance achieved wasn't that great for low impedance cans. So I decided to go the Audiovalve way: adding sand to the mix to increase significantly open loop gain and then apply feedback. And so, here's the crazy beast. A jfet-bjt-tube amplifier with a total power consumption of 50W
The input jfets are cascoded by the two triodes. A ccs in the tail and a current mirror at the top improve gain and matching. The output tubes are triode strapped and sit on top of a bjt CCS. It could be servoed, but I choose not to, for safety reasons.
The HV power supply value isn't that critical: around 140-150V is fine. An isolation transformer is perfect and cheap for the job. It must provides at least 200ma.
The LV voltages supplies (heaters and negative rail) depends on the tubes you get, the value of the negative rail is actually non critical (but regulate it). 6.3V for ECL805, 18V for PCL805. A dual secondary xformer is the best choice.
