In a message dated 1/23/2006 12:53:00 A.M. Central Standard Time,
vk4tzl@bigpond.net.au writes:
Hi Guys,
I'm just in the final stages of tuning up a GS35B on 144 MHx. The
original design was down by DJ5RE. It is featured on the N2DX QRO Russia
Site as a project by DG2KBC.
It has all gone together quite well, till I got to running a bit of
power into it. The Input side of things has tuned up well, with a low
SWR. However, I'm am having trouble with the Anode Tuning. As I am a
Ham, I changed things...instead of using a Flapper for Anode Tuning, I
tracked down some very nicely made brass Anode and RF Output threaded
Rods with Brass Disks on the end. I now have an 80mm Diameter Brass Disc
that approaches the side of the GS35B Anode cooler from the front panel
at right angles to the Cooler.
The threaded rod runs in a brass sleeve, that is firmly screwed to the
front panel, that can be locked with set scew when you get the tuning
right. In my mind, it achieves the same result as the brass flapper that
does exactly the same thing, but much more crudely.
My problem...on initial tune up using my h/held with about 2.5 watts
output, I could not get any hint of tuning...more a slight increase in
power as I wound the Anode Tuning Disc away from the Anode Cooler. I
figure that the plate line may to too long ( 240mm * 60 wide * 1.6mm
thick ) I shortened the length by around 12mm and found that I could now
get reasonable power out...around 60w with 2 w input, but the tuning
still is at max distance from the tube with no "peak". I made up
another Anode Line, shorter again at 214mm. Result....the same...Moving
the Disc away from the Anode till I run out of Thread just sees the
Power increase.
Getting brave, I hit it with 25W....now I see around 300W Output, about
300ma Anode I with 100ma Standing bias and 2800v on the Anode. It seems
stable, but its got me stumped why I cannot get the Anode to tune with
the Disc in nice and close ( which is where I would assume it would be,
as way out from the Tube it cannot be having much effect ) I can't
believe that the Anode Line is still too long...maybe it is. Anyone got
a simple way of proving this ?
At the moment, thinking about it, I could remove the Anode Tuning Disc
altogether and get max power out...doesn't sound right does it ?
This is my first Large Russian Triode, so I'm moving very carefully lest
I let loose a small mushroom shaped cloud and really scare myself :)
The Amp is protected by a G3SEK Board and all DC Conditions look OK. I
will be running more Anode Volts when I get then thing behaving, but at
present it is spread all over the bence with a different Anode Supply
running it.
Any thoughts ??
73
Glenn
VK4TZL
Hi Glenn, yes I have some thoughts about your amplifier:
When you look at the documentation from DG2KBC you see that he is reporting
only 36% efficiency while other builders report 60% or more.
This is a very strong indication that something is wrong with that
particular design! The plate tuning capacitor is connected in series with a
thin and
fairly narrow strip of metal to the floor of the plate cavity. The strip of
metal is almost exactly parallel with the axis of the tube and quite close;
allowing very heavy magnetic coupling into the rf circuit. The strip basically
will act as a link coupling loop that is connected in series with the plate
tuning capacitor to ground. In essence this will shunt an inductance across the
plate circuit and raise the resonant frequency of circuit. To resonate the
circuit, the plate resonator has to be lengthened and in this case to about 10
inches. Whatever losses may be present in the above described "loop" will
load the plate circuit and will degrade efficiency.
Your method of plate tuning avoids all of the problems mentioned above but
due to the missing inductive loading of the original design your resonant
frequency will be much lower.
To compensate for this, you have to shorten the plate resonator or you could
increase the width of the resonator or do both.
The fact that your output power increases with a shorter resonator lends
credibility to scenario presented here.
I encourage you to continue your mods to the plate resonator until you have
passed the desired frequency of resonance and require addition of tune C to
peak the output power. I think you are exactly on the right track already.
You can also do the resonance tests with amplifier off by feeding a low
power signal through an swr meter into the rf output connector. As you vary the
frequency of the low power signal you can see a definite dip in the swr as you
cross resonance.
Hope this makes sense and is helpful.
73/K5GW
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