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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Screen grid zeners; which?

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#2
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.....😀
 
#3
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).
 
#4
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
 
#7
What's wrong with the good old RC screen grid supply? Why the need to add noisy Zeners to an otherwise all tube design.
Click to expand...

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
 
#9
Klimon said:
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
Click to expand...

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.
 
#16
And then there is this:
 

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#17
mach1 said:


Johan,

I think this is what you were referring to:

http://www.t-linespeakers.org/tubes/triode-trick.html
Click to expand...

Thanks, Mach1, that seems to be it.

...the advantage again appearing unclear. Not to belabour, but there seem to be certain guys who are constantly in fear of too much screen dissipation. When the data sheets show that Va=Vg2 is safe, and in fact give the g2-current, it can be easily seen that screen dissipation will be safe. With triode connection there is no "wasted energy" - it all goes to the anode.

Just for those new to this field of bewildering claims.
 
#19
Confessing I don't know the second thing about electronics but this little (despised 😀 ) circuit clearly sounds better to me than simple resistor: better bass-attack and better pace overall. The amp is an el84 set; I now actually use only three zeners + a single 1000µF cap this sounds smoother than the published circuit. These are just my findings.

Simon
 
#20
Hi Simon,

Perhaps it was not said; I didn't check (I see that it is quite acceptable in some circles to say one is not feeling so well!). But yes, you have perhaps gathered that a screen grid can draw a quite varying current. Thus, using any resistor dropping more than a few millivolt (higher than the typical 100 ohm), will cause extra distortion. Triode operated pentodes are only that if the screen exactly follows the anode, otherwise an extra factor is introduced. Such a "close-coupled" condition is achieved by using zeners (or whacking big capacitors).

Thanks for that reference. I am greatly temted to be naughty and reply with some of Eddie's own advice: Do not listen to all the guru's! No further comment.
 
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