"A full-wave bridge rectifier configuration is the only way to go on a
quality amp"
- - For RTTY-broadcasting, yes. However, since the duty-cycle for SSB
is only c. 30%, a high-quality SSB linear amplifier can utilize a
full-wave voltage doubler / C-filter.
Also:
1. The diagram showing "equalizer" resistors and capacitors across
seriesed rectifiers is now widely recognized as not good engineering
practice.
2. The use of Delrin® in HV-RFCs is questionable since it has a medium
D-factor. ABS, G-10, or G-11 would be better.
On Jan 29, 2005, at 12:56 AM, Tomm Aldridge wrote:
This guy is my god now, mainly because he agrees with me on Plate
Chokes: http://www.qsl.net/kk5dr/ampbuilders.htm check out his
treatise on plate chokes.
Tomm Aldridge wrote:
More on the plate choke question. I just took a t200-3 core with
pretty high RF losses (good) and an Al of 42.5 nH/T^2 and wound 154
turns on it to get about 1mH. Because it is a multi layer
construction, am I to assume that distributed capacitance in the
winding will resonate with the inductance in a manner that would come
back to bite me? Maybe a T200-26 core with a much higher Al and HF
losses would be better???
Tomm Aldridge wrote:
Thanks Will, that makes perfect and practical sense. Plate choke
valuse for 1.8 through 30MHz amplifiers need to be high enough to
present a large reactance at 1.8MHz with large defined as >> greater
than the plate impedance, correct? And they must not produce any
resonances withing the 1.8 to 30MHz band, correct? But, what about
the fact that the tubes have gain well above 30MHz and well below it
as well?
If I say that the plate Z is 2k ohms and therefore I want 20k ohms
at 1.8MHz to satisfy the >> larger condition above, I get an
inductance of 1.77mH. Looking at some plate chokes for QRO amps out
there, I see values in the range of 200uH (a bit greater than the
plate Z) to 500uH, much lower than I would consider to be an
effective choke. A 300uH choke is approx 1" x 6" with 278 turns of
26AWG. Seems to be a reasonable DCR to be putting in a plate
circuit. I calculate about 2.9 DC ohms with a large surface to
distribute the losses. But why such a small inductance???
Tomm
Will Matney wrote:
Tom,
Actually, a ferrite core can be used if it's of the correct type of
material. The material is determined by the frequency that the
coil will operate at. There a couple of ferrite and iron powder
types that would work. The reason most are air coils I would think
is they are cheaper to make. An insulated form is all that's
really needed. The air coil formula is then used to determine the
number of turns for the amount of inductance wanted. The higher
the frequency, the lesser amount of inductance is needed to block
the RF, so the choke needs to be designed around the lesser
frequency that will be encountered. Then you need to make sure the
choke is not self-resonant at any frequency you wish to operate it
on. This is done by using a grid dip meter and shorting the coils
leads together. Any dip at any desired frequency means that the
inductance will have to be changed slightly to move the resonance
point to where it wont be encountered. Most of the time this is
done by simply adding or shortening a few turns of wire. Those
staggered windings on some chokes are done to stop self-resonance
at a particular operating frequency, and are really several
inductors being connected in series where Ltotal = L1 + L2 + L3,
etc.. Hope this helps as an explanation.
Will
On Fri, 28 Jan 2005 23:42:10 +0000, Tomm Aldridge <KD7QAE@ARRL.NET>
wrote:
Why are plate chokes seemingly black magic? Don't you just want a
good decoupling of the PS from the Plate; i.e. lots of impedance
from DC to Light and no resonances? How I get that should not be
an issue but all teh plate chokes I see are long skinny and
sometimes segmented single layer solenoids of questionable wire
size. Why wouldn't a really lossy powdered metal toroid with a
few fat turns on it work, assuming the inductance was high
enough?
KD7QAE
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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