Hi,
I built a power MOSFET-based variable voltage circuit (VVR) for an 18 watt home-made tube amp I built. This allows one to crank the amp so it's "working", without being incredibly loud. This is a known-working circuit, and I have a couple of questions regarding some trouble I've experienced with it.
In use, after a few hours of operation I started to pick up an annoying but pretty low hum (60Hz). Not too bad, but maybe symptomatic of something not quite right.
Then, months later, while tweaking components, I shifted the voltage up and down a few times pretty fast - maybe not good to roll the pot quickly with these voltages? My tube rectifier arced and the fuse blew. Found a blob of solder under the preamp board the fell out, so maybe I shorted something when soldering. Anyway, removing the whole VVR board showed the 12V zener diode was blown. I have spares so plan to simply re-do the whole thing as I've torn it apart. The rectifier survived the arcing.
I am using an SK3675, which is replacement for the NTE 2377 MOSFET that I couldn't get. The pre-filter coming from the tube rectifier (EZ-81) is shown on some schemas and not others....I suspect if might have been the source of the trouble. It shows up on Si rectifier circuits mostly, and I have a tube recto. I thought I'd use it to try to get some additional smoothing before the VVR, but maybe a bad idea?
What I'm looking for is sort of a small explanation of how this circuit operates (like a variable resistance?), what the zener does, why it blew (due to a transient short farther "down the line" that I knocked out after the failure?), and should I eliminate the pre-filter components when I rebuild it (the 220K thru the 22uF cap)?
The input voltage to the MOSFET circuit is in the neighborhood of 310V ripply DC from the tube rect., and output varies down to as low as about 100V. After the output, the adjusted DC heads to the filter caps in the amp as normal.
Thanks for any insight into this question; this is a great forum where you're not treated like an idiot if you don't immediately grasp the subtleties of how things like this work!!
Circuit below:
I built a power MOSFET-based variable voltage circuit (VVR) for an 18 watt home-made tube amp I built. This allows one to crank the amp so it's "working", without being incredibly loud. This is a known-working circuit, and I have a couple of questions regarding some trouble I've experienced with it.
In use, after a few hours of operation I started to pick up an annoying but pretty low hum (60Hz). Not too bad, but maybe symptomatic of something not quite right.
Then, months later, while tweaking components, I shifted the voltage up and down a few times pretty fast - maybe not good to roll the pot quickly with these voltages? My tube rectifier arced and the fuse blew. Found a blob of solder under the preamp board the fell out, so maybe I shorted something when soldering. Anyway, removing the whole VVR board showed the 12V zener diode was blown. I have spares so plan to simply re-do the whole thing as I've torn it apart. The rectifier survived the arcing.
I am using an SK3675, which is replacement for the NTE 2377 MOSFET that I couldn't get. The pre-filter coming from the tube rectifier (EZ-81) is shown on some schemas and not others....I suspect if might have been the source of the trouble. It shows up on Si rectifier circuits mostly, and I have a tube recto. I thought I'd use it to try to get some additional smoothing before the VVR, but maybe a bad idea?
What I'm looking for is sort of a small explanation of how this circuit operates (like a variable resistance?), what the zener does, why it blew (due to a transient short farther "down the line" that I knocked out after the failure?), and should I eliminate the pre-filter components when I rebuild it (the 220K thru the 22uF cap)?
The input voltage to the MOSFET circuit is in the neighborhood of 310V ripply DC from the tube rect., and output varies down to as low as about 100V. After the output, the adjusted DC heads to the filter caps in the amp as normal.
Thanks for any insight into this question; this is a great forum where you're not treated like an idiot if you don't immediately grasp the subtleties of how things like this work!!
Circuit below: