R.G. amplified zener

[lowell]

Hi RG,
Or anyone good w/ solid state devices. I've inserted the amplified zener in series the center of my OUTPUT TRANSFORMER (not power transformer) to lower only the plate voltages on power tubes. I have used a 51v 1/2w zener and 88w power mosfet w/ same resistors as geofex picture. for some reason this setup is only dropping 30v off the 330v. I then tried a 75v 1/2w zener and nothing changed. Is it possible I'm using the wrong mosfet or that I need to tweak these resistor values too? This is the mosfet I'm using.

Mouser part # 781-SUP45N03-13L-E3
http://www.mouser.com/Search/Product...UP45N03-13L-E3

[Dave H]

I think you need a different MOSFET. That MOSFET has an absolute max. drain-source voltage of only 30V.

[lowell]

ooohhh... was thinking that might be a problem. So I'd need that spec to be more than the voltage that I'd want to drop... so that number should safely be at least 100v if I'm to use a 75v zener?

[Rick Erickson]

I don't know your amp circuit configuration, but: if you lower your plate voltage too much below the screen voltage the screens may try to pull excessive current which can damage the tubes. Also if your bias voltage remains constant, then you will be running the tubes very cold. I suppose this would work in a triode-mode cathode biased amp.

RE

[lowell]

RE,
thanks for the tips... the bias actually naturally increases when I engage the amplified zener. Maybe this is cause I have not tended to the screens? Or maybe it's because the voltage divider created by the power tubes and their cathode resistor has changed allowing for more current w/ lower plate voltage? It's a 5E3X2. What do you think I should do w/ the screens? They are still being fed by the power supply 2.7k dropping resistor, seem ok?

[tubeswell]

RE,
thanks for the tips... the bias actually naturally increases when I engage the amplified zener. Maybe this is cause I have not tended to the screens? Or maybe it's because the voltage divider created by the power tubes and their cathode resistor has changed allowing for more current w/ lower plate voltage? It's a 5E3X2. What do you think I should do w/ the screens? They are still being fed by the power supply 2.7k dropping resistor, seem ok?

Yep the screen voltage affects the bias because it affects all the voltages in the tube. If you want to knock down the screen voltage you need to either do a similar thing to what you've done to the plate voltage, or put in a chunky power resistor from the screen-node decoupling/filter cap to the screens, or put the zener regulator in the B+ line before the plate supply so that if knocks down all the voltages in the amp (by which point you might need to lower the supply resistors for the preamp to bring the pre-amp voltages back up).

[lowell]

thanks, so do you think I NEED to adjust the screen resistor? cause as of now it's allowing the tubes to naturally increase current when the plate voltage is decrease, which seems like a nice coincidence to me. but if there is something I should be concerned about w/ the screens please let me know. also, can you shed some light on screens and screen resistors and what is good and bad?

[tubeswell]

The screen is a wire grid and acts to enhance the power output of the tube. It does this by increasing the voltage of the tube, thereby increasing current flow (acting kind of like an 'electron particle accelerator'). They reduce the inter-electrode capacitance between the cathode and the plate, allowing electrons to flow more easily to the plate. They are situated closer to the plate than the signal grid or the cathode.

If the screen voltage is too low, it won't enhance the tube's output power.

If the screen voltage is too much higher than the plate voltage, then the electrons will get attracted to the screen (instead of to the plate), which will lead to overheating/meltdown and destruction of the screen (as it is merely a grid and is not designed to absorb too much energy, and doesn't have a grid-leakage path for the excess electrons to escape to). And it will also likely take out the tube (and maybe other stuff).

The idea is to have the screen sitting at somewhere between 0V to 50V (or so) below the plate voltage. The valve wizard site has a few helpful articles on the theory and practice of setting up pentodes for proper operation.

[lowell]

yes! what a great simple explanation. thank you!