Western Electric WE91A type tube amplifier * No NFB * AC heater * 8W power

This is special order made item. Price for order the same one is 4000USD.　Tubes and shipping fee are not included.

Original WE91A type tube amplifier used 2 penthode 310A tube, both are penthode connnection and the gain of driver stage is too much with 2 penthode tube, each tubes has gain around 1:70-100. The purpose of using 2 step gain is for deep NFB in schematic. Due to very high gain so even adopting a deep NFB, the total gain of amp is still very high and it makes the hum level of original amp go up to 3-5mV which is not acceptable.

One more another problem is 310A is very hard to get and very expensive tube. Instead of using 310A I used WE348A which is same quality and easier to get. You can use 6S7G, EF37 tube which is same as WE348A.

Generally in other WE91A replica, I will use DC heater for 300B to reduce the hum level to 1mV, but in this amp, I used AC heater for better result becuase even with AC heater, the hum level is only 0.8mV which is acceptable for almost speakers even horn speaker.

In original schematic, there is no filter choke, they used the coil of field coil speaker working as a choke, it is not as good as filter choke.

In this new schematic, first tube is penthode connection, secondary is triode connection, working as a buffer tube to drop the output impedance of driver stage to make high frequency is better without singal loss. NFB is no need because technica data is good enough, so in this amp NFB is zero.

Specification:

Response frequency : 30Hz ~ 30K Hz ( -1db) @8W
Noise : 0.8mV
THD : 5% 8W

Tubes are not included.