[Amps] More HV feed through capacitor
mike at tubby.org
Sun Feb 12 03:11:39 PST 2012
Sounds like you have far too much RF reaching the 'cold' end of the
inductor and bypass cap. This might be because the inductor is
mis-placed on the anode line or because your inductor is too high or too
low in value, or self-resonant, or something...?
Seems that KC5GTT has built one or more of these amps with good success,
there's a video on Youtube, here:
much of the video is showing operation but if you fast-forward to the
end there's a tour around the inside showing how he built his - you
might find this a useful reference to compare yours against...
73, Mike G8TIC
On 11/02/2012 20:52, Larry wrote:
> FWIW here is an "update on my feed through capacitor problem:
> The amp in question is a GS-31B based on one of the YU1AW designs shown
> on the ND2X web site. This design specifies a anode choke bypass
> capacitor constructed from FR4 PCB material. It is 80x150 mm and is
> "microstrip" type. I.E. one side is ground and the other (hot) side is
> open. It is mounted on one side of the anode enclosure near the anode
> stripline resonator. Since I have had poor luck with PCB material and
> high voltage, I made sure that the edges were etched back and lots of
> clearance around the "hot" foil. It hi-potted with no leakage at 6kv DC.
> Even so the thing jumped more than 1/4 inch and melted a large part of
> the copper foil.
> After insuring I had no suitable feed through caps in the junque box, my
> next thought was a 1/4 wave coax stub. Several people suggested coax
> stubs of one sort or another but this is one of those concepts that is
> great in theory but in reality you can't quite get there from here.
> Making a 1/4 wave stub from RG-302 coax was easy. But when hi-potted, it
> went arcy-sparky at the open end at less than 3 kv. By the time I got
> the open end prepped and dressed well enough to handle 6kv, the other
> end wasn't much of a RF short. It is really hard to build a true "open"
> at these frequencies!
> Several people suggested the RF Parts feed through cap. It is a bit
> large physically for this project but I could probably make it work. But
> before I mess with ordering it and dealing with the minimum order
> requirements and the resulting wait for delivery, i though I'd explore
> other options.
> A single disc ceramic cap at these frequencies is worthless due to built
> in stray inductance. However, there are things a guy can do to mitigate
> these effects. With a combination of several 270 pF/6kv disc ceramic
> caps, some copper foil and a near 1/4 wave piece of RG-302, I was able
> to come up with a network that easily hi-potted to 6kv and still
> provided a bit more than 40 dB isolation between the tank on the HV lead
> to the power supply. This will put a little less than +20 dBm on the HV
> line, not great but at least it won't hurt anything. Still probably
> better than most HF amps with a small choke and a single bypass cap.
> The amp was reassembled and tested this morning. No arcy-sparky but
> power out was lower than before, efficiency down to 38% and eventually
> the telltale odor of Allen-Bradly farts caused me to cease testing. Odd
> because there are no carbon comp resistors at all in the amp.
> Dis-assembly showed the anode choke to have burned off the enamel
> insulation exactly the same as a 12 meter resonance in old HF amp
> chokes! Now this is the same choke that has gone through previous
> testing with no problem.
> This choke is supposedly connected to the half wave anode resonator at a
> current node so there really shouldn't be much RF to deal with. Given
> that the PCB cap arced 1/4 inch when it shouldn't have and this choke
> burned up with the new bypass assembly, my guess is that there is so
> much RF running around the anode enclosure that the bypass/choke
> assembly is "grounded" at a point on the side wall that is at a RF
> voltage node. If anyone has a better explanation, I'd sure like to hear
> about it.
> What next? I will fiddle with the choke but don't hold out much hope for
> success. My present thinking is that even a feed through cap at that
> point will have problems. It might be necessary to run the cold end of
> the choke out of the box with a ceramic feed through insulator and put a
> small "doghouse" on the outside with the necessary decoupling inside.
> Another option would be to make a "stripline" style capacitor from
> teflon sheets and aluminum so that the "hot" plate of the cap is not
> exposed to the RF field. I've used this method in the past with success.
> Even so, the choke would probably need to be tweaked.
> 73, Larry W7IUV
> Amps mailing list
> Amps at contesting.com
More information about the Amps