Solid state output valve

[ekjdm14]

Thanks all for the advice and suggestions. I think I've grasped the basic gist of what I'm after and have drawn up a "starter for 10" circuit using elements of the 3 designs I've seen. Waiting on a couple of resistor values & got to grab a suitable trimmer from the spares box, if I get a moment I'll draw the circuit on the computer and see if anyone can spot any "gotchas" I've fallen for. (It's just on paper at the moment, I work better with pencil/paper than Paint!

When I've got it together, regardless of if it's this design or not, I think to begin with I'll bring the set up slowly on variac & using a solid-state rectifier so I can monitor voltages and current flow on the FET circuit.

Cheers all, Dan.

[ekjdm14]

Chassis is together and working! Thank you everyone who's helped out with advice and ideas. There is still an issue with instability/very touchy on bias (over about 30mA it starts to run away with itself) and at one point the 7N80 FET smoked, but didn't blow. (I have 315mA fuses in circuit as that's the lowest I had in stock)

I do need to address my potential divider side of things as the values are not what I wanted just the closest I had in at the moment (post is verrrry slow!) and hopefully that will rectify the instability problem, but for the moment it's working at a lower bias voltage and doesn't sound too bad at all. Video of the circuit in action here- https://www.youtube.com/watch?v=e5WfigqFGqI


Thanks again to all, Dan

[ekjdm14]

Instability still present in current (third) guise of this circuit, but I have some clues and am still waiting the values of component I wanted. That didn't stop me from listening to Winston Churchill's speech (amongst other things) on Absolute '40s today though! Got the chassis back out of the set again now just having a play round to see if I can improve stability further.

[ppppenguin]

When you say instability do you mean DC conditions or HF oscillation? These devices can easily oscillate at many MHz. You may not have a scope that can see this. You may well need a gate stopper resistor, mounted immediately on the gate lead.

[ekjdm14]

I meant instability as in source/drain runaway, caused (I know now) by the adjustment pot being the wrong side of the source resistor. It's that way in the Dave Allen drawing, not sure if it's a mistake or if it worked that way for him but with the bias pot return going to the bottom of the source resistor now it's stable at 50mA no matter what I do with the volume control.

I did have some issues early on with oscillation although mainly AF, but all seems good now & have been running the set most of the afternoon pulling in Absolute '40s on the long wire admirably well. I'll be redrawing the circuit as it stands now & will post it here as well as on VRRR in case it's of use to others.

Cheers, Dan.