Zener diodes in the center tap

[phsyconoodler]

So what are the maximum amount of Zener diodes one can use to lower voltages?For example can you use a string of 20 volt zeners or do you have to use a smaller voltage and sneak up on it?
 And is it good practice to use a capacitor as well?Not to sure on this as I've not done it before.

[dude]

I was told long ago over at 18.com to use a string consisting of 5 watt, 15volts off CT.

I have used up to three and dropped approx 45 volts. They get hot so leave the leads long, no heat shrink.

20 volts would just create more heat, I assume. Came from Zappoid Phil

al 

[Fresh_Start]

The only time I used zeners to drop B+, I used one to drop 50 volts.  The key is max power dissipation.  For an SE 6V6 amp with one 12AX7, I think I used a single 25 watt zener.  No capacitor involved.  IMHO any "switching noise" ought to get filtered out with the ripple in your B+.

Instead of me typing more, you'll get different opinions reading these threads:

http://www.el34world.com/Forum/index.php?topic=11956.msg110751#msg110751

That MOSFET Follies circuit looks simple & cheap BTW.

http://www.el34world.com/Forum/index.php?topic=12327.0

However, one big zener mounted to your chassis as a heat sink is pretty simple too.  Just costs more, like $12 or more.

If you order one, make sure that you get the "cathode to case" variant.

http://www.solidstateinc.com/specsearch/specs/1N3305-ssi.pdf

Hope that helps,

Chip

P.S.  If using a string of 5 watt zeners, I would stick with either 5 or 10 volts per zener to manage power dissipation.  Depends on your amp.

[sluckey]

Beware of gremlins if you put zeners on the CT of a fixed bias amp whose negative bias voltage has an AC source that also depends on that CT.

[Geezer]

I've used them on cathode biased units w/ good results.

Used a string of 10v/5W zeners for several reasons:
1) They allow you to select a final voltage by tapping along the string to get any voltage drop you want
2) stringing them out with long leads and distance between the diodes helps dissipate the heat (I "zig-zagged" them on a turret board)
3) They are cheap, ~$0.37 ea from Mouser

I've run these on 2x of my main amps (HoSo variants of 12w & 25w) for 2 years now without failure or even a burp.

G

[Fresh_Start]

Beware of gremlins if you put zeners on the CT of a fixed bias amp whose negative bias voltage has an AC source that also depends on that CT.

Sorry I forgot to mention that potential problem.  Glad sluckey did.  It gets complicated fast.  I think that you can still take your negative bias voltage off the PT secondaries, but if the zener on the center tap lowers the unregulated/unfiltered secondary voltage then you have to make changes to the bias circuit.  You also have to pay attention to which "ground" you use for the bias circuit.  That simple & cheap MOSFET circuit avoids those pitfalls.

Chip

[phsyconoodler]

Well I went ahead and did it.I used 4-20v 5watt zeners in a 36 watt lite amp and it's working just fine and no noise at all.The voltage dropped about 55v in total.
  It's perfect for this cathode biased amp.
I have used a large wattage resistor in another amp and it worked too.There are a few ways to skin the same cat I guess.
  Thanks for the replies!

[worth]

Wouldn't four 20V Zeners drop 80V ?

[kagliostro]

In franch language 80 is quatre-vingts    (four times twenty), so YES they will do

All jokes aside, it sure is

Kagliostro

[sluckey]

Wouldn't four 20V Zeners drop 80V ?

Yes,,, if,,, you're feeding them with pure DC through a current limiting resistor. But the stuff on the CT of the transformer looks more like a bunch of 60Hz (actually 120Hz) half wave unipolar pulses. In this case the actual dc voltage dropped will always be less than the rating(s) of the zener(s).