[Amps] 200-ohm tank and "un-un" in HF amp design?

Larry Benko xxw0qe at comcast.net
Sun Oct 20 18:16:02 EDT 2013


The way I see it the 200:50 ohm bifilar xfmr reduces the loading cap 
value but doubles the voltage requirement and eliminates the need for a 
choke to ground in case the plate blocking cap(s) fail.  It does not 
need to be a super well designed xfmr since a little leakage inductance 
can be compensated by the PI network.  I have one in an amp (pair of 
3CX800s) of mine that is  30 bifilar turns on a T200-2 if I remember 
correctly.

73,
Larry, W0QE

On 10/20/2013 11:03 AM, Bill Turner wrote:
> ORIGINAL MESSAGE:          (may be snipped)
>
> On Sun, 20 Oct 2013 00:51:44 -0700, N7CXI wrote:
>
>> Assuming adequate core/wire sizing and airflow, can anyone think of a
>> solid reason *not* to design a legal-limit HF amp output tank for 200
>> ohms, then use a 4:1 "un-un" transformer to step it down to 50?
> REPLY:
>
> If the only reason to do this is to use a smaller load capacitor, I would
> suggest instead to switch in some doorknob type padder caps instead. IMO, a
> pi-net is a better impedance matcher than a transformer. Lower loss, less
> cost, more reliable. Designing a broadband transformer is tricky business, a
> pi-network is relatively easy.
>
> 73, Bill W6WRT
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