rg keen tube wah question

[numpty]

http://www.geofex.com/index.htm
Hi
  I've read about a couple of successful builds of this project on this forum so built one up recently.Although the circuit layout checks out OK and the components are correct a get a ht voltage drop down to 50v with a 200v supply as  if the mostfet is regulating the ht.
if the tube is pulled or the mosfet disconnected the voltage remains at 200v. My question is have there been any alteration to the schematic
since its introduction or have I a faulty component?.As this is a test bed project built on perf board using 250v rated caps, if its a flop the components can be re utilised, I am using a bench supply PSU also. I can rebuild it on tag board if i have made a mistake which isn't apparent to me on the perf board, however i need to find out whether this circuit works properly as it appears on the geofex site.

[R.G.]

My proto of that circuit worked as shown. Others have also told me that it works as well.

From your description, it sounds like your high voltage supply is sagging down to 50V; this would indicate that maybe the power supply can't supply quite enough current. If it were mine, I would measure the voltage across the filter capacitors both before and after the 1K filter resistor, and also with and without the circuit hooked up. That will help tell you what the power supply is doing.

There is a possible modification that you could do to lower the circuit's current drain if you find that it's the power supply sagging. The 10K source resistor of the MOSFET uses most of the DC current in the circuit. It's low to keep the output impedance of the MOSFET source follower low, but you can raise that resistor up to as much as 100K. That will lower the DC current drain, with some modest decrease in the ability of the circuit to drive the inductor/cap resonator. This may or may not be a problem in tone. It depends on what you hear.

Let us know what you find.

[Paul Marossy]

There is a possible modification that you could do to lower the circuit's current drain if you find that it's the power supply sagging. The 10K source resistor of the MOSFET uses most of the DC current in the circuit. It's low to keep the output impedance of the MOSFET source follower low, but you can raise that resistor up to as much as 100K. That will lower the DC current drain, with some modest decrease in the ability of the circuit to drive the inductor/cap resonator. This may or may not be a problem in tone. It depends on what you hear.

Yeah, I am interested to know how this fares as well. I still need to build mine. I have PCB layouts designed - I just need to etch them and get the project rolling. I just moved a few weeks ago, so that has dominated all of my spare time for the moment... 

[numpty]

Thanks for your advise - I will keep you posted, I will check it out this weekend. At least i know the project should work from what you say.