VVR not doing anything

[bancika]

I decided to install a VVR circuit in my AX84 firefly (12AU7 selft split push pull) that is still too loud for my apartment. I used IRF840 for the mosfet and the schematic below. At first I followed pin numbers from the schematic but then I realized those were wrong (input voltage goes to drain pin 2, not pin 1, etc), so I rewired according to the layout below. In both arrangements, VVR pot doesn't do anything. My output voltage is identical to input voltage (265V). Voltage on the middle lug of the pot changes as I turn it from 265V to 130V which sounds weird, I'd expect to get 1/10th of the voltage at minimum setting, not 1/2.

Any hints what could be the issue? I double checked the wiring. IRF840 should be sufficient for a 1W amp, right? I'm not really sure how the circuit is supposed to work. Why would I get the full voltage at the output? Zener is oriented correctly. The only thing I added is a filter capacitor right after the 10ohm resistor and zener, to filter the voltage before it gets to the output stage. I included the polarity diode D2 because I plan to switch between fully VVR'd amp and output stage only.





Thanks!

[xtian]

You had your MOSFET wired wrong, so maybe it's fried. Use ohmmeter to check resistance between pins. It should show high resistance.

[bancika]

Thanks. Must be it, reads very low between each two pins

[R.G.]

Yep, dead MOSFET. A good MOSFET reads diode one way, open the other between the source and drain. The gate would read open both ways to both source and drain if you could read it with an ohmmeter without putting its life in danger. Most ohmmeters use a 9v battery for the ohmmeter voltage source, and that's pretty safe for MOSFETs, but your hand can carry a static charge to the gate that will puncture the generally-20V rating of the gate. You can try metering the gate pretty safely if you have a protection zener from gate to source and drain. but otherwise you risk killing the MOSFET to see if it's alive.

[bancika]

Thanks RG. Didn't know that. I bought a new IRF820 today and it works pretty good. Also re-purposed the standby switch to control if VVR is supplied to the while amp of just the power amp. Make a huge difference in character. Will post more details in a separate thread early next week

Cheers