Jon Yaeger wrote:

As far as 5W zeners are concerned (for screen regulation) is there anything
to be gained by using, say (4) 75V zeners as opposed to (2) 150V devices?

When I have bought 5W zeners they come in different sized packages and I doubt
they all really are rated for 5W.

But if you have a 5W x 75v zener, the safe operating current is about 27 mA
max,
which generates 2W of heat.
If you have 4 such ZDs in a string, some 8 watts of dissipation is possible,
but with 2 x 5W x 150v, you'd only be able to dissipate 4 watts.

The zeners can be coaxed into handling more power by cutting a little strip of
copper,
wrapping it round the zener, and bolting it to the chassis, with heatpaste
used.
But I'd still never operate a zener continuously at more than 2/5 of the power
diss rating.
The cylindrical bodied zeners can be treated like this; some
have sperical bodies, and I don't like them,
but they are also OK at a watt.


If zeners over heat, they screw up, and the zener voltage can reduce,
or they fuse into another glum SS short circuit.

I use a string of zeners for shunt regulating screen voltages in some of my
power amps.

The regulation by the zeners is totally adequate for the purpose,
and another way is to use voltage reg tubes, but I don't know if they reg
better,
because I have never used any.

I have 470 uF to also bypass the screen supply, and the series feed R is
large enough to bring up the screen supply when the B+
gets turned on in amps where I do that after the heaters have come on.
I used never do this.
But with the SE35 I recently made I got "whooop" noise during power up
just at the point where the cathodes just began to conduct.
After installing a delay circuit for the B+, and Eg2 reg, with a six second
period for B+ to rise to full value with pre-warmed cathodes, no more "whoops"
during warm up.
And no sudden LF voltage lurches at the output in excess of perhaps 1/2 a volt,
all the more likely in an SE amp
with NFB.

The shunt regulation of G2 supplies is conveneniently simple.
Before the screens draw current, the zeners should be able to take the
full amount of screen current *and* the amount of current when the screens
ar drawing current, so hence using say 5 multiple 5W zeners to allow short
term
full diss of 25 watts won't stress them too much.

The screen supply should be only just regulated well enough to allow
full power testing with a sine wave into the rated load with less than 5% drop
in Eg2.
Should clipping be sustained, or a load 1/2 the rated load be used,
the screen supply should be allowed to sag 20%,
due to current in the supply resistor.
With music, and the good C bypass, and zener reg, with furious rock and roll
signals
up to occasional clipping, Eg2 will stay rock steady.

But if a tube goes beserko, and starts conducting too much Ig2, the series feed
R
will allow the G2 voltage to sag, which is a good thing.
The sagging g2 supply tends to turn off the plate current.
And better you burn out a $2 resistor than a $400 OPT!.

People say they don't like the sound of zeners.

I doubt they have a clue about what they are talking about.
The zeners in my circuits improve the glorious sound I hear.

Patrick Turner.