Skip -

If you are build an amplifier .. I would use Ian's boards -
great protection, easy to assemble and takes your guess work out of older
designs

Greg
w9gb


"Ian White, G3SEK" wrote in message
...
Skip wrote:
Hello List,

I am looking for a source for a Zener Diode. I am building an HF
Amplifier using a GS-35b and the schematic I'm using to build the amp
calls for a 36v 50w zoner diode for bias. Here in New Zealand there
does not seem to be anyone with anythings close to what I need.

Also, would it be possible to string a few lower volt watt zeners
together to come up with a close value to what is needed?

A simple shunt regulator using a TL431 and a pass transistor will give
adjustable voltage and better regulation than a large zener diode, and
is actually cheaper.

There's a lot of information on my website, at:
http://www.ifwtech.co.uk/g3sek/boards [some commercial content]


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'

The question wasn't so much ZENER so much as it was BIAS -- the LM317HV or
TL783 will go much higher than an LM317.

Jack
"Ian White, G3SEK" wrote in message
...
Bill Turner wrote:
On Sun, 7 Dec 2003 07:24:04 -0500, "John Walton"
wrote:

or an LM317HVT with PASS transistor

The original question was about a zener diode, ie a shunt regulator. I
wasn't aware that a 317 plus a transistor could be configured to emulate
a zener... can it?

The TL431 certainly can.


_________________________________________________ ________

If you're going to use such things, you must protect them against
inadvertent arcs. One good arc and they will be history.


Normally an arc from the B+ supply of a grounded-grid triode is no
threat to the cathode bias device. Arcs inside the tube will ground out
to the grid, and external arcs to the chassis; and then the current
flows back to B-minus. The cathode and the bias device are not part of
that current path, so the bias device is not at risk.

The exception is if an internal arc is severe enough to burn right
though the grid (I've seen that in a mesh-grid tube) or if the current
is high enough to drive the local grid potential positive. Then some of
the arc current - perhaps tens of amps, peak - will flow down through
the cathode. In such extreme cases, no bias device is likely to
survive... at least, not without some additional protection.

That's why, whatever bias device you use, an 80-cent varistor connected
from cathode to chassis is a very good investment.


A big, fat
zener on the other hand, is relatively immune to such abuse.

"Relatively" in this context is hard to judge. In most cases, the bias
device survives because it wasn't actually under threat - see above. If
a device does fail, it's only guesswork to say how a different bias
device would have fared in the same amplifier.

A more reliable picture emerges from a larger number of amplifiers.
There are more than 350 Triode Boards out in the field, with tubes
ranging from the 2C39 to the 3CXP5000, and they all use basically the
same TL431/TIP147 circuit protected by a varistor. Between them, these
boards have seen a lot of arcs, and there's no indication that the bias
circuit is notably fragile. Certainly there are occasional failures -
but in those cases it seems likely that a big, fat zener wouldn't have
survived either.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

The question wasn't so much ZENER so much as it was BIAS -- the LM317HV or
TL783 will go much higher than an LM317.

Jack
"Ian White, G3SEK" wrote in message
...
Bill Turner wrote:
On Sun, 7 Dec 2003 07:24:04 -0500, "John Walton"
wrote:

or an LM317HVT with PASS transistor

The original question was about a zener diode, ie a shunt regulator. I
wasn't aware that a 317 plus a transistor could be configured to emulate
a zener... can it?

The TL431 certainly can.


_________________________________________________ ________

If you're going to use such things, you must protect them against
inadvertent arcs. One good arc and they will be history.


Normally an arc from the B+ supply of a grounded-grid triode is no
threat to the cathode bias device. Arcs inside the tube will ground out
to the grid, and external arcs to the chassis; and then the current
flows back to B-minus. The cathode and the bias device are not part of
that current path, so the bias device is not at risk.

The exception is if an internal arc is severe enough to burn right
though the grid (I've seen that in a mesh-grid tube) or if the current
is high enough to drive the local grid potential positive. Then some of
the arc current - perhaps tens of amps, peak - will flow down through
the cathode. In such extreme cases, no bias device is likely to
survive... at least, not without some additional protection.

That's why, whatever bias device you use, an 80-cent varistor connected
from cathode to chassis is a very good investment.


A big, fat
zener on the other hand, is relatively immune to such abuse.

"Relatively" in this context is hard to judge. In most cases, the bias
device survives because it wasn't actually under threat - see above. If
a device does fail, it's only guesswork to say how a different bias
device would have fared in the same amplifier.

A more reliable picture emerges from a larger number of amplifiers.
There are more than 350 Triode Boards out in the field, with tubes
ranging from the 2C39 to the 3CXP5000, and they all use basically the
same TL431/TIP147 circuit protected by a varistor. Between them, these
boards have seen a lot of arcs, and there's no indication that the bias
circuit is notably fragile. Certainly there are occasional failures -
but in those cases it seems likely that a big, fat zener wouldn't have
survived either.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

John Walton wrote:
The question wasn't so much ZENER so much as it was BIAS -- the
LM317HV or TL783 will go much higher than an LM317.

The original question was specifically about cathode bias for a
grounded-grid power amplifier. That application requires a two-terminal
shunt regulator like a zener diode.

If there is a way of configuring a 317-family series regulator and a
power transistor as an adjustable two-terminal shunt regulator, I'd be
genuinely very interested to learn about it.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

John Walton wrote:
The question wasn't so much ZENER so much as it was BIAS -- the
LM317HV or TL783 will go much higher than an LM317.

The original question was specifically about cathode bias for a
grounded-grid power amplifier. That application requires a two-terminal
shunt regulator like a zener diode.

If there is a way of configuring a 317-family series regulator and a
power transistor as an adjustable two-terminal shunt regulator, I'd be
genuinely very interested to learn about it.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

On Sun, 07 Dec 2003 18:01:57 +1300 Skip wrote:

I am looking for a source for a Zener Diode. I am building an HF
Amplifier using a GS-35b and the schematic I'm using to build the amp
calls for a 36v 50w zoner diode for bias. Here in New Zealand there
does not seem to be anyone with anythings close to what I need.

50 W stud mount Zeners used to be a standard product, but most
manufacturers dropped them about 20 years ago. I understand that they
were still available from someone for a long time, but they might be
completely unavailable by now.

Also, would it be possible to string a few lower volt watt zeners
together to come up with a close value to what is needed?

Sure, string 10 3.6V/5 W Zeners, that would work fine if you can get
3.6V, which seems unlikely.

A more reasonable solution would be to make your own regulator using a
low power Zener and a power transistor that can handle the power. You
can make either a shunt or series regulator this way.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

On Sun, 07 Dec 2003 18:01:57 +1300 Skip wrote:

I am looking for a source for a Zener Diode. I am building an HF
Amplifier using a GS-35b and the schematic I'm using to build the amp
calls for a 36v 50w zoner diode for bias. Here in New Zealand there
does not seem to be anyone with anythings close to what I need.

50 W stud mount Zeners used to be a standard product, but most
manufacturers dropped them about 20 years ago. I understand that they
were still available from someone for a long time, but they might be
completely unavailable by now.

Also, would it be possible to string a few lower volt watt zeners
together to come up with a close value to what is needed?

Sure, string 10 3.6V/5 W Zeners, that would work fine if you can get
3.6V, which seems unlikely.

A more reasonable solution would be to make your own regulator using a
low power Zener and a power transistor that can handle the power. You
can make either a shunt or series regulator this way.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

Skip wrote:

Hello List,

I am looking for a source for a Zener Diode. I am building an HF
Amplifier using a GS-35b and the schematic I'm using to build the amp
calls for a 36v 50w zoner diode for bias. Here in New Zealand there
does not seem to be anyone with anythings close to what I need.

Also, would it be possible to string a few lower volt watt zeners
together to come up with a close value to what is needed?

Thanks for the help

Skip..

36V Zeners are available from www.bgmicro.com. I'm not sure about the
power rating though. You might have to use it with a transistor to
handle the current.

Rick

Skip wrote:

Hello List,

I am looking for a source for a Zener Diode. I am building an HF
Amplifier using a GS-35b and the schematic I'm using to build the amp
calls for a 36v 50w zoner diode for bias. Here in New Zealand there
does not seem to be anyone with anythings close to what I need.

Also, would it be possible to string a few lower volt watt zeners
together to come up with a close value to what is needed?

Thanks for the help

Skip..

36V Zeners are available from www.bgmicro.com. I'm not sure about the
power rating though. You might have to use it with a transistor to
handle the current.

Rick

I think I remember seeing something about this at AG6K
web site. I built a 3-1000Z amp once and strung 1N5400 series diodes
together. Of course the bias was lower.


Doug N4IJ

wrote in message
...
Skip wrote:

Hello List,

I am looking for a source for a Zener Diode. I am building an HF
Amplifier using a GS-35b and the schematic I'm using to build the amp
calls for a 36v 50w zoner diode for bias. Here in New Zealand there
does not seem to be anyone with anythings close to what I need.

Also, would it be possible to string a few lower volt watt zeners
together to come up with a close value to what is needed?

Thanks for the help

Skip..

36V Zeners are available from www.bgmicro.com. I'm not sure about the
power rating though. You might have to use it with a transistor to
handle the current.

Rick