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