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-   -   Screen grid zeners; which? (http://www.diyaudio.com/forums/tubes-valves/118497-screen-grid-zeners.html)

Klimon 1st March 2008 07:57 PM

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

Alastair E 2nd March 2008 01:49 PM

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.....:D

Johan Potgieter 2nd March 2008 03:10 PM

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).

Klimon 3rd March 2008 08:19 PM

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

Jeb-D. 3rd March 2008 08:36 PM

What's wrong with the good old RC screen grid supply? Why the need to add noisy Zeners to an otherwise all tube design.

Vek 3rd March 2008 09:26 PM

A good adjustable screen supply can be found here:
http://www.diyaudio.com/forums/showt...5&pagenumber=1

Cheers

Klimon 4th March 2008 10:22 AM

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

Johan Potgieter 4th March 2008 09:38 PM

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!)

Wavebourn 4th March 2008 09:45 PM

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.

aardvarkash10 4th March 2008 09:46 PM

here's Tom's simple take on triode strapping... http://www.tubes.mynetcologne.de/roe...atics_e_o.html


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