The reason why I am using this tube is, simply affordable. I cannot afford to get 300B but this tube is around $5 and good to try. The type 38 is very beginning of pentode in the history, but already heater voltage is 6.3, as same as more modern tube. 

The configuration is a type of  "audio maniacs will get angry". Heater voltage is generated by tiny AC/DC, and plate voltage is generated by AC 100V direct-rectified. So none transformer (except for the one in AC/DC) for power supply. It's easy!

On the top side, one output transformer and one choke coil for power supply. YES, that' all!

Actual operation can be seen in the following movie...