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Old 1st March 2008, 08:57 PM   #1
Klimon is offline Klimon  Belgium
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Default Screen grid zeners; which?

Hi there,

Which zeners are best suited to string together as 250V / 5mA screen grid supply? I understand low noise (reduced further by bypassing) is a major concern.

Thanks! Simon
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Old 2nd March 2008, 02:49 PM   #2
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250V @ 5mA, the diode needs to be a 2W device, Or if you string two or more together, you can reduce the wattage of each device, Do they make 250V Zeners....?

Where are you using these...?--In a straight 'screen-feed' from PSU or from UL taps on the Transformer...?

For that sort of voltage, perhaps a Gas/Glow-Regulator/ 'VR' tube regulator may be better--They look good too.....
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Old 2nd March 2008, 04:10 PM   #3
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O-oooh!

Klimon, I must rain on the parade. A high-voltage zener is attractive, but has a severely positive temperature co-efficient. I once bit the dust with 100V zeners. They drifted up to 110V with temperature rise - not boiling either.

"Neutral" zeners are around 5,6V, but I cannot fancy you wiring some 40+ of them in series (though I know someone who did)! This may not be serious for you, but you might consider at least a few in series - say 5 x 50V. And yes, noise will be of concern; a large capacitor will help.

If you are aiming for a signal-neutral supply (i.e. not UL), it is better by far to make a regulator using a few 5V - 6V zeners. Since you do not mind zeners, such a regulator can conveniently be transistorised - it is not part of the signal path (for the puritans).
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Old 3rd March 2008, 09:19 PM   #4
Klimon is offline Klimon  Belgium
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I guess my description wasn't the best; here's what I'm referring to: http://members.home.nl/triode.dick/tekeningen/se807.gif ;just to keep the screen voltage +- constant in a SE amp; I love the looks of those VR tubes (especially the american purple ones) but this one's a rebuilt = no space nor time for esthetics. I was thinking of 50V zeners in series but am wondering how audible different types (with different noise specs) of the quoted voltage would be.

Edit; Johan, do you have a schematic for a very simple regulator? I've looked at the maida regulator but the schematic is enough to put me off (effort-wise).

Regards, Simon
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Old 3rd March 2008, 09:36 PM   #5
Jeb-D. is offline Jeb-D.  United States
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What's wrong with the good old RC screen grid supply? Why the need to add noisy Zeners to an otherwise all tube design.
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Old 3rd March 2008, 10:26 PM   #6
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A good adjustable screen supply can be found here:
The Red Light District - another PP EL84 amp

Cheers
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Old 4th March 2008, 11:22 AM   #7
Klimon is offline Klimon  Belgium
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Quote:
What's wrong with the good old RC screen grid supply? Why the need to add noisy Zeners to an otherwise all tube design.
Nothing I know of yet; it just would be nice to compare both ways. I found I prefer using zeners for triode-strapping a pentode (bill perkins triode trick). As far as my ears are concerned 'adding noisy zeners to an otherwise all tube design' can be an excellent Idea.

Simon
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Old 4th March 2008, 10:38 PM   #8
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Simon,

Mind giving me a website for that triode-trick if there is one? (All I find under Bill Perkins are a politician and a saxophone player!)
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Old 4th March 2008, 10:45 PM   #9
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Quote:
Originally posted by Klimon
I guess my description wasn't the best; here's what I'm referring to: http://members.home.nl/triode.dick/tekeningen/se807.gif ;just to keep the screen voltage +- constant in a SE amp; I love the looks of those VR tubes (especially the american purple ones) but this one's a rebuilt = no space nor time for esthetics. I was thinking of 50V zeners in series but am wondering how audible different types (with different noise specs) of the quoted voltage would be.

Edit; Johan, do you have a schematic for a very simple regulator? I've looked at the maida regulator but the schematic is enough to put me off (effort-wise).

Regards, Simon
You may use a transistor capable to work on 250V dissipating 2W, connecting Zeners between base and collector. In such case you don't need high-Watt Zeners.
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Old 4th March 2008, 10:46 PM   #10
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here's Tom's simple take on triode strapping... http://www.tubes.mynetcologne.de/roe...atics_e_o.html
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