How to drop B+ with zener, explained to a 5 years old child?!

[tube.tone]

Hi all,

I have done some research, I understood that this can be done with a diode solder between power transformer center tap and ground.

What should be connect to ground, anode or the cathode?
What value would you recommend in order to drop 20V?
Anyone has pictures of a zener installed on a 5E3?

Thanks for any imput

[sjhusting]

Hi, I've done this. Here's a link which explains it:

http://music-electronics-forum.com/t563/#post52097

steven

[celeste]

The zener needs to be really big, take the desired voltage drop X the max current then double it for safety margin. Likely a 5 watt zener is not going to do it. Larger zeners come in odd form factors and here Weber sells them on brackets that are easiely mounted to a chassis. This is not my favorite way to drop voltage, but it works, it is relieable if done correctly and it is easy, all good things. I am not connected to Weber, I just know that for the fisrt timer, their kits make success easier to achieve.

You choose a zener of the voltage you want to drop. A 20v zener will drop 20v from the B+

[limbe]

Usually not a good idea! U=IR is more reliable.Power resistors waste heat but work.

[TNO]

Never, never, never let a 5 yr old drop B+ with a zener.

[marshman]

...unless you don't like the 5-year-old.

[limbe]

Of course I like my 5-year-old granddaughter!That´s exactly the reason I don´t let her get near my tube amps!300 volts is dangerous.´Hasn´t anybody told you that?

[tube.tone]

[celeste]

Originally Posted by celeste

The zener needs to be really big, take the desired voltage drop X the max current then double it for safety margin.

After I posted that, it struck me it was not right. A zener before a smoothing cap is going to be seeing 3X the DC current because of the ripple current, so for safety's sake, it needs to be at least 6X the DC current draw.

[tubeswell]

Originally Posted by celeste

The zener needs to be really big, take the desired voltage drop X the max current then double it for safety margin. Likely a 5 watt zener is not going to do it. Larger zeners come in odd form factors and here Weber sells them on brackets that are easiely mounted to a chassis. This is not my favorite way to drop voltage, but it works, it is relieable if done correctly and it is easy, all good things. I am not connected to Weber, I just know that for the fisrt timer, their kits make success easier to achieve.

You choose a zener of the voltage you want to drop. A 20v zener will drop 20v from the B+

No it doesn't need to be really really big at all. 5W will do fine for a 9V to 15V drop in most geetar tube amp Pts (taking the example of a 200mA peak current draw for say a 5F6A into account). Remember that you are not asking the zener to drop hundreds of volts, it is only dropping a few volts off the tip of the mountain. So in the example of a 15V drop @200mA, that is 3W. If you want a bigger margin, use 2 x 9V 5W zeners in series for an 18V drop in total - each zener would see 1.8W @ 200mA. I have done this with several amps and they work fine. The banded (cathode) end(s) go towards the ground return path.

(If anyone knows what they are talking about, its Bruce Collins)