Voltage Dropping Zener Diodes

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

Locked
User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 2289
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Voltage Dropping Zener Diodes

Post by JMPGuitars »

With modern wall voltages our B+ voltages tend to be higher than the 345VDC (+/- 5V) that we ideally like to see. Using a solid state rectifier will make that even higher. The ideal way to correct this is with Zener voltage dropping diodes.

Typically 5 watt Zeners are what you want in an 18 watter.

The position of the Zeners effects their efficiency and how many volts will be dropped. If you place a 10V Zener before the first filter cap, you can expect it to drop much less than the rated voltage (usually about 50% in my experience).

The ideal placement for the Zeners is between the 1st and 2nd filter cap. Since the DC voltage is filtered in this position, the Zeners work efficiently and will drop the rated voltage.

I recommend using more than 1 Zener if dropping any significant amount of voltage. For example, I needed to drop 40VDC on my recent Tremolo TMB build. I used two 5W 20V Zener diodes in series. Take a look:
zener-dropping-diode-at-1st-filter-cap.jpg

Here's the layout drawing:
zener-dropping-diode-at-1st-filter-cap-layout.jpg

As you can see, the Zener diode stripes must always point towards the voltage source. I marked the heat shrink tubing so I know which direction the stripes are going.

Another thing to note is that by connecting it in this position, you can also connect the output transformer after the diodes. This will help ensure that your output tube plate voltages aren't too high.

If you're only dropping 10V, then just use a 5W 10V Zener. 25V or more, and I would use 2 or 3 5W Zeners, or switch to a higher wattage.

Here's some examples of what I use (but use whatever suits your needs safely):

5.1V: 1N5338BRLG
10V: 1N5347B-TP
15V: 1N5352B-TP
20V: 1N5357B-TP

Thanks,
Josh
You do not have the required permissions to view the files attached to this post.
6 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

Locked