[Amps] Hard bus bar copper and inductor Q

[John Lyles]

PA5Y,
In my opinion, either material would work fine for your amplifier. 
Keeping interturn capacitance low isn't a big issue with so few turns 
for a 50 Mhz PA. With the copper tubing, you can always flatten the end 
and then torch solder a flat bar at the end to hold parallel capacitors. 
The flat material might take more space to wind a coil edge to edge. In 
the 1980s, I used to work with a device called a Tennsmith metal roller, 
which even put a crimp in flat stock to make a more rigid coil of metal.

For RF choke, you should measure its impedance if you have a device to 
do that, and try to design so that it has a parallel self-resonance near 
and above your operating frequency. You will work it on the low side of 
its resonance, which is the safest. As it gets mounted in the circuit, 
this may change, so measure it in the box, but with the hot end 
disconnected from the tubes. You can also use a grid dip meter and the 
first sharp resonance will be the one representing parallel resonance.

73
John
K5PRO


> Message: 1
> Date: Thu, 31 Aug 2017 22:48:37 +0200
> From: Conrad PA5Y <g0ruz at g0ruz.com>
> To: amps at contesting.com

> Hello again, I have gathered almost everything that I need for my
> 3CX800A7 conversion from 2m to 6m.
>
> I have choice of materials for L1.
>
> I have some 6mm soft copper pipe, some 4mm soft copper pipe and some 3mm
> silver plated copper wire. All the tubing is C102 copper and forms very
> nicely. VE7RF suggested that I use some flat strap and make L1 a flat
> wound coil to reduce capacitance between turns, it is also very easy to
> drill for blocking caps.? This makes sense to me.
>
> The strap that I have is 17mm wide and 1.5mm thick but I have some
> concerns. It was used as bus bar material and is much harder than the
> C102 copper tube that I have. It is also considerably darker in colour.
> It forms into coils quite easily and is not particularly springy so I am
> fairly sure that it is not Beryllium copper. I suspect that it has been
> annealed. I am worried that the resistivity will be high but surely as
> it was used for busbars it won't be lossy, will it?
>
> Soon I will have a DER EE DE-5000 LCR meter which I am told are very
> good value for money and I can measure component Q so I should see the
> difference between the flat bar and the copper tube, if there is a
> problem. I know for sure that the copper tube is C102 soft copper. I
> would be interested in hearing from anyone who has this instrument and
> what they think of it.
>
> Any advice? In the past I have made a number of QRO VHF amps for 6m and
> without exception the efficiency has been good. This is the first one
> that I have actually designed myself and I expect the loaded Q to be
> 10-12, previous amplifiers had big tubes, lots of stray C and had a
> higher Q and were mainly copies or adaptations of published designs.
> This time I want to design and truly understand what I am doing.
>
> I am still somewhat mystified by Anode RFCs for VHF amplifiers, I can
> plug numbers into the GM3SEK spread sheet but I do not know exactly how
> to design a good choke. Most of the stuff that I found on google was
> concerned with multi-band HF amplifiers and the perils of series
> resonance. What are the rules of thumb. I can do the calculations but
> what are my goals?
>
> Hopefully this will give the reflector a kick and start some interesting
> discussion.
>
> Regards
>
> Conrad PA5Y