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Using Zener Diode to Drop B+

Ever want to drop just 20 volts of B+? How about 40 volts? Here is an example to use a 25-watt zener diode to achieve just that.

While this method is relatively easy to install and implement, it is imperative that you understand the importance of device power dissipation and the need for adequate heat sinking. The zener is good for 25 watts of dissipation, but only if it is well connected to a heat sink. That includes its bracket and the amp chassis the bracket is bolted to. Together, they will get the heat out of the zener diode allowing it to function properly. 

Bottom line is, you must use plenty of heat sink compound (you can get this at Radio Shack), spread it evenly on all metal surfaces where shown in the pictorial, and the hardware must be level and tight. I don’t have a torque spec for the zener or bracket nuts, so you’ll need to use your own judgement. Following these suggestions will allow the zener to work properly and last a long time.

It is no problem at all to use several zeners with a rotary switch to have different B+ voltages, providing you have made arrangements to adjust the bias if it is a fixed bias amp. 

Since all zener diodes are reverse biased, there’s always a potential for them to generate some electrical noise. Generally, low voltage diodes do not have that problem, but it’s always a good plan to take an inexpensive precaution. Use a .1uF capacitor to attach to the zener diode in an effort to quench any noise that might occur. Make sure you mount the capacitor as shown and use very short leads. 

The following is a pictoral with descriptions of what the zener device looks like and how it is installed for maximum safety, reliability, and performance.

 

Zener with heat sink compound applied

Zener mounted on a bracket
(Weber does not have this.)

Bracket mounted on chassis
Apply heat sink compound to chassis or bottom of bracket to ensure a good thermal connection between chassis and bottom surface of bracket.

Capacitor soldered in place
Scrape off some of the plating to solder the capacitor to the zener body.

Some info concerning the mounting and operation of the zener. The silicon heatsink compound (we call it bird poop) is messy and is difficult to get off your skin. It is harmless, it’s just a pain in the butt to deal with. You can use alcohol to remove it from the skin. It is almost impossible to get out of clothes. So, I would suggest you wear your crummiest T shirt and put an old towel on your lap before you start. If you have some surgical or other tight rubber gloves to wear, all the better.

Electrical considerations. Zeners are unusual because they are purposely reverse biased until they break down and that’s the voltage where they regulate. Unfortunately, that is dependent on the current through them, and that gap or tolerance seems to be more narrow as the power is increased. For instance, let’s say you want a 20 volt drop. You order our 20 volt model, but it only drops 18 volts. Yet, if you increase the current in the B+, the drop changes a little. That’s normal operation. The factory specs a zener to have a certain breakdown voltage at a certain current, and for a 25 watt zener, that would be pretty high current compared to the normal idling current in an amp. Just something to keep in mind when selecting and implementing the zener diode. 

One more consideration. If your amp has a solid state rectifier and also has a very high value of filter capacitance, there might be a rather large spike of charging current when you turn on the amp or when you come off of standby. In that case, to protect the zener, you might consider putting an NTC in-rush thermistor in series with the zener. That would go between the center-tap of the power transformer high voltage winding and the terminal on the top of the zener diode. Thermistors are available on our site (here) as well as from Mouser and Digi-key.