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Old 1st February 2012, 08:50 PM   #1
cedricb is offline cedricb  Europe
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Default Reduce B+ with zener

Hi,

I'm trying to reduce my B+ for a PP-EL84 app. I've got a raw ~323VAC on my secondary (center tap) which give 385V for the EL84. So I bought a 20W power Zener to reduce to 360V. I've connected the Zener on the center tap and I don't have any voltage drop! Across the Zener I'm getting around 9VAC.

Any suggestion? ...maybe I've got a faulty multimeter...
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Old 1st February 2012, 09:00 PM   #2
BZed is offline BZed  United States
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Need to see diagram of that. That's not where the zenor needs to be if your discription is correct.
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Old 1st February 2012, 09:06 PM   #3
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If you put the zener on the center tap you would need two zeners back to back to reduce the voltage correctly.
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Old 1st February 2012, 09:09 PM   #4
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Uh what is the part number of the Zener? Sounds like you need some resistors to go with your zener, and it will have to be in a diferent spot, me thinks. Schematic?
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Old 1st February 2012, 09:14 PM   #5
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Quote:
Originally Posted by nigelwright7557 View Post
If you put the zener on the center tap you would need two zeners back to back to reduce the voltage correctly.
No. The current in the circuit consisting of half secondary, rectifier and load flows in only one direction all the time. That's what the rectifier is for. Only the two half secondaries change their role.

The Zener should work in the CT as well as in the B+ lead, provided its polarity and the remaining wiring is correct. I assume a wiring error.

Diagram please!

Greetings,
Andreas

PS. Keep in mind that a Zener diode needs to be put in 'backwards' to operate. In the forward direction, it hasn't much of a voltage drop.
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Old 1st February 2012, 09:53 PM   #6
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by cedricb View Post
Hi,

I'm trying to reduce my B+ for a PP-EL84 app. I've got a raw ~323VAC on my secondary (center tap) which give 385V for the EL84. So I bought a 20W power Zener to reduce to 360V. I've connected the Zener on the center tap and I don't have any voltage drop! Across the Zener I'm getting around 9VAC.

Any suggestion? ...maybe I've got a faulty multimeter...
What happens if you put it in the B+ rail? ie on the DC..after the caps..in series with the load..

Regards
M. Gregg
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Old 1st February 2012, 10:19 PM   #7
Jebem is offline Jebem  Portugal
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Instead of that Zenner that will introduce additional unneeded and unwelcome noise in the circuit, I would drop the AC voltage at the secondary windings before the rectifier, by using series resistors of adequate value.
Dropping 20Volts at something like 100mA per PP channel should be easy done in this way. Try 220Ohm / 5Watt resistors to start with.

Other option, and assuming that the amp uses a vacuum tube as rectifier, sometimes it is possible to replace it with an equivalent one but having a larger voltage drop (check the tube datasheets).

Also, if you are using a solid state rectifier, I would replace it with a vacuum tube rectifier if one have space in the chassis, and if the transformer has a secondary winding able to cope with the extra filament current.
This one fixes the excessive high voltage and will give the amplifier a better tone as well.

Have fun.
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Old 2nd February 2012, 01:22 AM   #8
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Zeners are shunt regulators which you don't want. Use a large resistor or a choke to drop some voltage. Oh and get PSUD2, its invaluable.

PSUD2
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Old 2nd February 2012, 01:28 AM   #9
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Quote:
Originally Posted by cedricb View Post
Hi,

I'm trying to reduce my B+ for a PP-EL84 app. I've got a raw ~323VAC on my secondary (center tap) which give 385V for the EL84. So I bought a 20W power Zener to reduce to 360V. I've connected the Zener on the center tap and I don't have any voltage drop! Across the Zener I'm getting around 9VAC.

Any suggestion? ...maybe I've got a faulty multimeter...
Where did you get this crazy idea?

Even if Zener is double and have symmetric curve, you will add extra hum, and increase dependence of B+ from mains voltage. If it is single, you loose 0.68 volts only adding 60 Hz ripples (50 Hz in Europe) and DC bias of transformer core.

Try to drop using MOSFET source follower, it is simple and filters ripples a lot.
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Old 2nd February 2012, 08:52 AM   #10
cedricb is offline cedricb  Europe
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Originally Posted by Wavebourn View Post
Where did you get this crazy idea?
...from here -> https://taweber.powweb.com/store/vdump.htm

Here is what I've done so far, from what I've read in different sources; a zener give you a constant voltage drop whatever the current load (in comparison of a resistor)

Click the image to open in full size.

Quote:
Originally Posted by Wavebourn View Post
Even if Zener is double and have symmetric curve, you will add extra hum, and increase dependence of B+ from mains voltage. If it is single, you loose 0.68 volts only adding 60 Hz ripples (50 Hz in Europe) and DC bias of transformer core.

Try to drop using MOSFET source follower, it is simple and filters ripples a lot.
Do you have an example?

Last edited by cedricb; 2nd February 2012 at 09:04 AM.
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