Hi RATs,

How close should I expect the regulation from zener diodes to be? I
have two 110V 5W zeners in series and apparently depending on line
voltage, I'm getting between 240V and 250V. This doesn't seem right to
me. I have a 1.2K 3W resistor feeding the zeners and the 50K load.
Suggestions appreciated.

TIA,

Raymond

You'll get better regulation if you use the zeners to regulate the base
voltage of a pass transistor -- you can just set the zener current to a
pretty minimal level. this also keeps the zeners cool -- Audio Research
used this technique in a couple of their amps.

Should be very stable. I use a BU208a in my Fairchild 255 mod.
http://www.tech-diy.com/fairchild_255a.htm The example shown uses a floating
LM317HV with zener string and BU208A.

There was no choice in modding the Fairchild, it was damaged in a flood.


"Raymond Koonce" wrote in message
...
Hi RATs,

How close should I expect the regulation from zener diodes to be? I
have two 110V 5W zeners in series and apparently depending on line
voltage, I'm getting between 240V and 250V. This doesn't seem right to
me. I have a 1.2K 3W resistor feeding the zeners and the 50K load.
Suggestions appreciated.

TIA,

Raymond

Hi John,

Thanks for your comments. What I'm working on is a bias supply which
will need to supply -155V. I'll study your scheme and give it a try.

Best regards,

Raymond

John Walton wrote:

You'll get better regulation if you use the zeners to regulate the base
voltage of a pass transistor -- you can just set the zener current to a
pretty minimal level. this also keeps the zeners cool -- Audio Research
used this technique in a couple of their amps.

Should be very stable. I use a BU208a in my Fairchild 255 mod.
http://www.tech-diy.com/fairchild_255a.htm The example shown uses a floating
LM317HV with zener string and BU208A.

There was no choice in modding the Fairchild, it was damaged in a flood.


"Raymond Koonce" wrote in message
...


Hi RATs,

How close should I expect the regulation from zener diodes to be? I
have two 110V 5W zeners in series and apparently depending on line
voltage, I'm getting between 240V and 250V. This doesn't seem right to
me. I have a 1.2K 3W resistor feeding the zeners and the 50K load.
Suggestions appreciated.

TIA,

Raymond

Raymond Koonce wrote:

Hi RATs,

How close should I expect the regulation from zener diodes to be? I
have two 110V 5W zeners in series and apparently depending on line
voltage, I'm getting between 240V and 250V. This doesn't seem right to
me. I have a 1.2K 3W resistor feeding the zeners and the 50K load.
Suggestions appreciated.

TIA,

Raymond

The 5 watt zeners will be happy if they have 1 watt in each of them
which means a theoretical current of 9mA max in each.
They will take up to 45 mA before the tubes they regulate start conducting.

But I don't allow the rated max current for 5W to ever flow.
Don't tempt fate.
But with 45mA, the voltage will be above 110v each,
but closer to 110v with only 9 mA.
I sometimes have only 4 mA flowing in 75v x 5w zeners
when I regulate the supply to V1 of a power amp.
I also have 100 uF across the zeners.
The zeners extends the low impedance of the supply down to DC, which a cap
cannot.
The cap shunts the zener diode noise.
Strapping the zeners agianst the chassis with neat fitting strips of copper

is a good idea, to help cool them.

Patrick Turner

John Walton wrote:

You'll get better regulation if you use the zeners to regulate the base
voltage of a pass transistor -- you can just set the zener current to a
pretty minimal level. this also keeps the zeners cool -- Audio Research
used this technique in a couple of their amps.

Should be very stable. I use a BU208a in my Fairchild 255 mod.
http://www.tech-diy.com/fairchild_255a.htm The example shown uses a floating
LM317HV with zener string and BU208A.

The BU208A is a really good rugged device to use.

But I have never liked LM317 low voltage chips floating to reg a high voltage
as a series pass element.

But another way to *shunt* regulate with a HV transistor is to
1, have an R feed to the reg voltage,
2, have the string of zeners taken from the reg voltage to the base of the
BU208A base,
3, have the BU's emitter taken to 0V,
4, between the reg voltage and BU's collector, have about 220 ohms, 5 watts,
which prevents more than 1 amp flow if the BU is turned on hard.

Thus the zeners need only pass a relatively small current, ie, the base current
of the transistor,
and the main shunt current passes through the transistor.
One still needs a shunt cap to shunt the noise of the zeners.
If any rise above the reg voltage occurs, there is more current in the zeners,
which turns on the
transistor, which prevents the rise in voltage.
I use such simple techniques to regulate heater DC supplies, but with generic
LV transistors.


There was no choice in modding the Fairchild, it was damaged in a flood.

The alternative to regulated input stage supplies is to simply use 470 uF caps.

If that Fairchild worked OK with 30 and 40 uF caps originally without LF
instability,
then 470 uF as replacements for the two 30 uF will stop any tendency to
towards instability, even if the amp was just stable to begin with.

I think the Fairchild looks like it was used by Noah, and
it accidently fell into the bilge of the ark and was lost for a few years during

the travels on the Flood.

Patrick Turner.



"Raymond Koonce" wrote in message
...
Hi RATs,

How close should I expect the regulation from zener diodes to be? I
have two 110V 5W zeners in series and apparently depending on line
voltage, I'm getting between 240V and 250V. This doesn't seem right to
me. I have a 1.2K 3W resistor feeding the zeners and the 50K load.
Suggestions appreciated.

TIA,

Raymond