On Jul 21, 2004, at 10:44 AM, Will Matney wrote:
Yes, true the regulation is not near as good as some newer developed
methods but, that was the way it was done for years at least shown
from 1980 and back. Using 1/3 the rating on a zener is correct as they
tend to fail easily. The potentiometer was mentioned because most
customers I've seen wanted a linear variable voltage control
(especially bias circuits), but a stepped control with a rotary switch
will do a similar function, if using a long zener string or multiple
regulator tubes. The potentiometer method keeps down the number of
parts and the cost of the circuit.
Hello, Will -- The potentiometer method works well AB1 grid bias
circuits because there is virtually zero grid current. However, since
there can easily be over 100mA of screen current in AB1, the
potentiometer method causes screen unregulation which =s IMD. One
work-around is to use different value of zeners in the zener-string so
that a finer adjustment of screen V results. For example, the 4cx250R
requires c. 400V for the screen 12, seriesed 30V zeners would produce
360V. By putting 6, switch selected 10V zeners in series with the 12,
30V zeners, the screen-V could be set to 360, 370, 380, 390, 400, 410,
and 420V.
end
Actually the potentiometer can be placed on either side of the tube or
zener with its wiper connected to the screen lead. This way if the
wiper fails, the screen voltage most likely will be lower instead of
the full voltage being applied. Then if it does go high, the varistor
will take over if sized correctly. Another zener could be used for
this as a protective device. We used to use them across the C-E
junction on 12 volt HV power supply transistors to snub any spike from
the oscillating transformer primary. They were set to 80% of the VCE.
No matter what a quick acting fuse should be placed in the screen line
too. The rule being, the less amount of components the better as one
will sure fail. Heck, I've even seen some that got by on using a
string of neon bulbs, but I don't recommend it! For some home brewers
this is a must who have limited funds to achieve the same result, or
when rebuilding tube type equipment.
One last way to be looked at would be a series pass transistor with
zeners and pot on its base to ground, or simply a couple of zeners.
This was a basic circuit for control voltage but can be upped in size
for screen control also. The transistor should be able to handle the
unregulated voltage and at least twice the current. At 300-350 Vdc,
these transistors are available everywhere. Some televisions used
these as the horizontal output transistors, etc.. The screen circuit
still needs the protective devices between the supply and the screen
itself with any regulation circuit.
All in all, there are a multitude of ways to accomplish similar tasks
and all should be investigated. I'm a firm believer that in life, we
get to picky and complicated in our designs.
Will Matney
This is guaranteed to spoil dynamic regulation, both with zeners and
with gas regulators. The caveat with zeners is, except in Antartica,
don't run them at more than 1/3 of their dissipation rating. // The
easiest way to make a zener string adjustable is to make the tap
movable.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|