29-05-2017, 03:23 PM
While looking for a replacement O/P valve for my Eddystone 358 receiver the idea crossed my mind that a high voltage MOSFET might just be able to do the job. Usually when I get these brilliant ideas they never work but this time it seems to have worked out OK. The first version of the circuit used the set's own fully decoupled cathode resistor but that was found to have far too much gain and this version seems to be very similar in output to the original valve. Another advantage is that it can instantly be swapped back to valve operation.
The FET chosen was simply the cheapest 800V fully insulated device I could find on Ebay. Most of these devices require around 4V between the gate and source to get them conducting so the zener voltage and R2 were chosen in conjunction with the original 1Meg grid leak resistor to give around 5V on the drain. R2 does also affect the frequency response of the audio so needs to be kept reasonably high.The ferrite bead on the gate lead was essential to stop the circuit oscillating at around 50 Mhz.
In the finished version the FET will be on short leads and attached to a spare hole in the chassis adjacent to the output valve socket. An earth tag on the mounting screw will serve as the ground connection. I will add a 100mA fuse in the drain lead just in case the FET pops off.
Al
The FET chosen was simply the cheapest 800V fully insulated device I could find on Ebay. Most of these devices require around 4V between the gate and source to get them conducting so the zener voltage and R2 were chosen in conjunction with the original 1Meg grid leak resistor to give around 5V on the drain. R2 does also affect the frequency response of the audio so needs to be kept reasonably high.The ferrite bead on the gate lead was essential to stop the circuit oscillating at around 50 Mhz.
In the finished version the FET will be on short leads and attached to a spare hole in the chassis adjacent to the output valve socket. An earth tag on the mounting screw will serve as the ground connection. I will add a 100mA fuse in the drain lead just in case the FET pops off.
Al
Tuned for maximum smoke .