"raypsi" wrote in message
...
On May 30, 7:20 pm, "coffelt2" wrote:
"JIMMIE" wrote in message

...

I want to replace the vr 0A3 tube used in the bias circuit of a 6146B
amp with a 75V zener string. Can I just directly replace the tube with
the diodes or will I need to make other modifications to the circuit?

Jimmie

Don't know if I read all the responses, but I hope someone explained the
need to provide a series current limiting resistor in the zener string.
Something that would keep the no load current up near the top of the
zener's
operating range would be good.

Old Chief Lynn, W7LTQ

Hey OM:


That resistor is part of the OA3 circuit. All you need is noisy zeners
that can handle the current. You would need 3 watts worth of zeners.
Three 25 volt 1 watt zeners, or five 15 volt 1 watt would give you
more head room. Or 15, 5 volt 1/2 watt zeners, or 1/4 watt. Put 15 SMD
5volt 1/4 watt zeners between the 2 pins on an octal plug, pin 2
connect the anode of the zener string and pin 5 connect the cathode of
the zener string. If you are lucky you can find a 5 watt 75 volt zener

73 OM

de n8zu

GE raypsi,
Wow, what happens when one is asleep! "programmable zener TL431?" One
of the hundreds
of devices that have slipped under my radar!
Is it as noisy in operation as just a few zeners?
My experiences as are no doubt yours too, are that solid state devices
inserted into
vacuum tube perfected circuits just add to the noise generated by computers,
cell phones,
switching power supplies etc.
Do you then need two of those little stinkers (TL431) to come up with
drive for series
FET? (shooting for 75 or so volts, I take it) (or am I too old to understand
solid state?)

TU RAYPSI

W7LTQ

coffelt2 wrote:
"raypsi" wrote in message
...
On May 30, 7:20 pm, "coffelt2" wrote:
"JIMMIE" wrote in message

...

I want to replace the vr 0A3 tube used in the bias circuit of a
6146B amp with a 75V zener string. Can I just directly replace the
tube with the diodes or will I need to make other modifications to
the circuit?

Jimmie

Don't know if I read all the responses, but I hope someone explained
the need to provide a series current limiting resistor in the zener
string. Something that would keep the no load current up near the
top of the zener's
operating range would be good.

Old Chief Lynn, W7LTQ

Hey OM:


That resistor is part of the OA3 circuit. All you need is noisy zeners
that can handle the current. You would need 3 watts worth of zeners.
Three 25 volt 1 watt zeners, or five 15 volt 1 watt would give you
more head room. Or 15, 5 volt 1/2 watt zeners, or 1/4 watt. Put 15 SMD
5volt 1/4 watt zeners between the 2 pins on an octal plug, pin 2
connect the anode of the zener string and pin 5 connect the cathode of
the zener string. If you are lucky you can find a 5 watt 75 volt zener

73 OM

de n8zu

GE raypsi,
Wow, what happens when one is asleep! "programmable zener TL431?"
One of the hundreds
of devices that have slipped under my radar!
Is it as noisy in operation as just a few zeners?
My experiences as are no doubt yours too, are that solid state
devices inserted into
vacuum tube perfected circuits just add to the noise generated by
computers, cell phones,
switching power supplies etc.
Do you then need two of those little stinkers (TL431) to come up
with drive for series
FET? (shooting for 75 or so volts, I take it) (or am I too old to
understand solid state?)

TU RAYPSI

W7LTQ


Mouser stocks a 75V, 5W zener
(http://www.mouser.com/ProductDetail/...miO99TtxueA%3d)
that should do the job. Cheap at $0.36 each.
http://www.mouser.com

You can also download the datasheet from that page.

--
David
dgminala at mediacombb dot net

On May 31, 1:10*am, "coffelt2" wrote:
"raypsi" wrote in message

...
On May 30, 7:20 pm, "coffelt2" wrote:



"JIMMIE" wrote in message

....

I want to replace the vr 0A3 tube used in the bias circuit of a 6146B
amp with a 75V zener string. Can I just directly replace the tube with
the diodes or will I need to make other modifications to the circuit?

Jimmie

Don't know if I read all the responses, but I hope someone explained the
need to provide a series current limiting resistor in the zener string.
Something that would keep the no load current up near the top of the
zener's
operating range would be good.

Old Chief Lynn, W7LTQ

Hey OM:

That resistor is part of the OA3 circuit. All you need is noisy zeners
that can handle the current. You would need 3 watts worth of zeners.
Three 25 volt 1 watt zeners, or five 15 volt 1 watt would give you
more head room. Or 15, 5 volt 1/2 watt zeners, or 1/4 watt. Put 15 SMD
5volt 1/4 watt zeners between the 2 pins on an octal plug, pin 2
connect the anode of the zener string and pin 5 connect the cathode of
the zener string. If you are lucky you can find a 5 watt 75 volt zener

73 OM

de n8zu

GE raypsi,
* * *Wow, what happens when one is asleep! "programmable zener TL431?" One
of the hundreds
of devices that have slipped under my radar!
* * *Is it as noisy in operation as just a few zeners?
* * *My experiences as are no doubt yours too, are that solid state devices
inserted into
vacuum tube perfected circuits just add to the noise generated by computers,
cell phones,
switching power supplies etc.
* * *Do you then need two of those little stinkers (TL431) to come up with
drive for series
FET? (shooting for 75 or so volts, I take it) (or am I too old to understand
solid state?)

TU RAYPSI

W7LTQ

Hey OT:

I used a programmable zener TL431 in an SMD package because in SMD
they were 1%. Then I just use the math to figure the resistors to
program it to the voltage for an OA2. I've replaced OA2's in 3 of my
EICO 753's, along with the Zeners they use in the VFO, with TL431's.
To wit all the TL431 is, is an 60db op amp with an internal temp
stabilized zener. Without temp stabilization a straight zener will
drift all over the place, with temp changes, I've seen 15 volt zeners
go down to 12 volts when heated up. So that string of 5 zeners would
get down to 60 volts when things get hot.
And as far as noise goes you can't use negative feedback in a zener.
The noise goes back into the input of a TL431 and gets canceled out.
You could let it go and just like a zener it would make a great noise
bridge source.


The main issue is temperature compensation, a zener will go down in
voltage when heated up, so if you use a zener you best put it in a
temp controlled oven. And you can use a TL431 to control the temp. of
that oven.

73 OT
de n8zu

On May 29, 2:15*pm, JIMMIE wrote:
On May 29, 10:59*am, wrote:









On May 29, 8:23*am, JIMMIE wrote:

I want to replace the vr 0A3 tube used in the bias circuit of a 6146B
amp with a 75V zener string. Can I just directly replace the tube with
the diodes or *will I need to make other modifications to the circuit?

Jimmie

Typical OA3 is idling with between 5 and 40mA of current flowing
through it, so make sure the zener string can dissipate the power.
(75V * 0.040A = 3 watts worst case, but I'd expect less idle current
in a grid bias regulator.).

If this is a major redesign you might want to use a solid state series
pass regulator (e.g. TL783 or the like) and probably get rid of that
old wirewound pot in the bias circuit.

0A3's are not unobtanium, rare, or expensive. I kinda like seeing VR
tubes around the PA, nice purple/pink glow dims and brightens as I
key, and if they're not glowing then something's wrong.

It draws about 30ma calculated by measuring the voltage drop across
the series dropping resistor. Once in a while it arcs, I think. I
tried another tube and that seemed to fix it for a while but it
started up again. There is a posssibility this is not the problem but
for now it seems most likely. If I can easily replace it with a zener
string I could be more sure that it is or isnt the cause. Plan for now
is to put 5 15V 1 watt zeners on a tube base I can plug into the
existing socket and see what happens. If that is the cause I will
readily invest in another tube because I think they look cool too.

During transmission (modulation peaks?) the VR tube glow intensity
will change and even shift pattern from one part of the tube to others
with some hysteresis. This is normal. Arcing is not normal. But note
that the glow and shadows might show some structure (kinda shadows and
shadow edges) shifting as the current through the VR tube changes and
maybe you suspect that is arcing?

Tim.

On Jun 1, 2:54*pm, Tim Shoppa wrote:
On May 29, 2:15*pm, JIMMIE wrote:





On May 29, 10:59*am, wrote:

On May 29, 8:23*am, JIMMIE wrote:

I want to replace the vr 0A3 tube used in the bias circuit of a 6146B
amp with a 75V zener string. Can I just directly replace the tube with
the diodes or *will I need to make other modifications to the circuit?

Jimmie

Typical OA3 is idling with between 5 and 40mA of current flowing
through it, so make sure the zener string can dissipate the power.
(75V * 0.040A = 3 watts worst case, but I'd expect less idle current
in a grid bias regulator.).

If this is a major redesign you might want to use a solid state series
pass regulator (e.g. TL783 or the like) and probably get rid of that
old wirewound pot in the bias circuit.

0A3's are not unobtanium, rare, or expensive. I kinda like seeing VR
tubes around the PA, nice purple/pink glow dims and brightens as I
key, and if they're not glowing then something's wrong.

It draws about 30ma calculated by measuring the voltage drop across
the series dropping resistor. Once in a while it arcs, I think. I
tried another tube and that seemed to fix it for a while but it
started up again. There is a posssibility this is not the problem but
for now it seems most likely. If I can easily replace it with a zener
string I could be more sure that it is or isnt the cause. Plan for now
is to put 5 15V 1 watt zeners on a tube base I can plug into the
existing socket and see what happens. If that is the cause I will
readily invest in another tube because I think they look cool too.

During transmission (modulation peaks?) the VR tube glow intensity
will change and even shift pattern from one part of the tube to others
with some hysteresis. This is normal. Arcing is not normal. But note
that the glow and shadows might show some structure (kinda shadows and
shadow edges) shifting as the current through the VR tube changes and
maybe you suspect that is arcing?

Tim.- Hide quoted text -

- Show quoted text -

I put a neon bulb across the dropping resistor, well across a piece of
the dropping resistor. I used a photo transistor to couple to my
counter. Over a 12 hour peiod I got 10 to 15 hits. With the Zeners in
place I havent had any.. First noticed the problem when the plate
current meter on my amp went from 0 to full scale just setting on the
bench. Bias supply is set up so unless the amp is keyed it has the
full -75V on the grids cutting the amp off when keyed it drops to
about -45. When I caught it on my storage scope it was about -18v. I
like the VR tubes too, now that I am more certain thats whats causing
the problem I dont mind ordering a couple more VR tubes. Before I
order new ones I am thinking of trying the same monitoring setup again
with the tube in place but this time changing the bias adjust pot.
That would be the only thing I havent checked. I hate intermittent
problems like this.

Jimmie

On May 29, 8:23*am, JIMMIE wrote:
I want to replace the vr 0A3 tube used in the bias circuit of a 6146B
amp with a 75V zener string. Can I just directly replace the tube with
the diodes or *will I need to make other modifications to the circuit?

Jimmie

Thanks all, problem turned out not to be the tube. still not
absolutely sure but it was either the bias pot or a cap. I'm fairly
certain I changed out the cap about a year ago so best guess is the
pot. What I am thinking was happening is the pot was opening up
sending its share of the current through the tube overloading it with
about 90 ma of current causing the tube to arc. Should have just
shotgunned the circuit from the beginning.

Jimmie

On Jun 4, 4:11*pm, JIMMIE wrote:
On May 29, 8:23*am, JIMMIE wrote:

I want to replace the vr 0A3 tube used in the bias circuit of a 6146B
amp with a 75V zener string. Can I just directly replace the tube with
the diodes or *will I need to make other modifications to the circuit?

Jimmie

Thanks all, problem turned out not to be the tube. still not
absolutely sure but *it was either the bias pot or a cap. I'm fairly
certain I changed out the cap about a year ago so best guess is the
pot. What I am thinking was happening is the pot was opening up
sending its share of the current through the tube overloading it with
about 90 ma of current causing the tube to arc. Should have just
shotgunned the circuit from the beginning.

Jimmie


hey OM

The dead give away should have been the series resistor to that OA3
heating up big time since P=I^2R the current doubled, the power
quadrupled that resistor would have could have let its' smoke out.

73 OT
de n8zu

On 06/04/2011 10:07 PM, raypsi wrote:

hey OM

The dead give away should have been the series resistor to that OA3
heating up big time since P=I^2R the current doubled, the power
quadrupled that resistor would have could have let its' smoke out.

73 OT
de n8zu
Unless the original designer was paranoid enough to use an over sized
resistor in the first place. Back when I first started out in ham radio
building with tubes I was guilty of using a 50W resistor in a 5-10 watt
circuit. (half because I got the 50W part cheap and half because I knew
that I didn't quite know what I was doing and wanted to be safe).

On Jun 5, 12:16*pm, Kenneth Scharf wrote:
On 06/04/2011 10:07 PM, raypsi wrote:

hey OM

The dead give away should have been *the series resistor to that OA3
heating up big time since P=I^2R the current doubled, the power
quadrupled that resistor would have could have let its' smoke *out.

73 OT
de n8zu

Unless the original designer was paranoid enough to use an over sized
resistor in the first place. *Back when I first started out in ham radio
building with tubes I was guilty of using a 50W resistor in a 5-10 watt
circuit. *(half because I got the 50W part cheap and half because I knew
that I didn't quite know what I was doing and wanted to be safe).

In my case problem was only momentary, maybe a second sometimes going
days without doing it sometimes 3 or 4 times in a couple of minutes.
For a long time I was convinced it was in the amp instead of the power
supply because I once disconnected the power supply from the amp and
it ran continuously for a few weeks without failing. This turned out
to be just bad luck. Fortunately I am on the air about as sporadically
as the amp was failing. There is no telling how many times it failed
while I was using it and I just didn't notice.


Jimmie