Date: Sun, 20 Oct 2013 00:51:44 -0700
From: Jim Barber <audioguy@q.com>
To: "Amps@contesting.com" <Amps@contesting.com>
Subject: [Amps] 200-ohm tank and "un-un" in HF amp design?
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?
I'm curious because doing so would bring the loading cap requirement for
a 4000-ohm plate impedance down to under 1000pF at 1.8 mHz, which can be
done with a large air variable alone.
Note: Yes, I know it *has* been done already, but I'm asking for
opinions as to whether you think it should be.
Thanks and 73,
Jim N7CXI
## years ago, QST tested a whole bunch of linear amps in one issue of the
magazine.
It may have been the QRO or Commander brand amp that had the 4:1 un-un in it.
Guess what, it overheated
badly and cracked in half. The idea of course was with a PI net based on a
200 ohm output
instead of a 50 ohm output, the load cap value is way less. Then the
broadband 4:1 un-un
takes it from 200 ohms...down to 50 ohms.
## I think it was beefed up after that, and is still used. With a sky high
4000 ohm plate load Z
your tune + load cap requirements will be a LOT lower vs say a 1800 ohm plate
load Z.
## If it was me, I would use a conventional PI net...and just pad the load
cap. Padding a load
cap is easy, piece of cake really. IF a NON shorting bandswitch is used,
simply use the 160m
contact to switch in the 160M padder. The 160m contact isnt used anyway...
since the exact amount
of inductance is used for 160m. IE: add up all the various coils and you have
the correct amount for 160m.
The band switch is only used to tap the 80-10m bands. We used this scheme on
the 3CX-6000A7 amp.
## IF a shorting switch is used, the 160m contact cant be used to switch in a
160m padder.... cuz of the
multiple pole rotor used on a shorting switch. Its kinda a waste to add an
additional wafer just so you
can utilize one contact on 160m for padding. Plan B is to simply use a SPST
relay or a DPST relay....
with all contacts in parallel...and use the relays contacts to switch in 160m
NPO padder capS. We
used the relay scheme on the 3CX-3000A7 amp. SPST- 40A contactor was used.
A standard
30A relay, like a spst or dpst has also been used in the past. You can also
use a small, shorting
style bandswitch to progressively add 160m padderS. IE: 3 x 170pf padders.
170- 240-310 pf.
## as noted, the 200 ohm un-un scheme means the peak V across the 200 ohm
loading cap will be
exactly DOUBLEe that of a load cap that’s at a 50 ohm point. The plate
spacing will have to be at least DOUBLE,
and the capacitance will now drop to just ONE QUARTER. You just lost it right
there. Cuz now you have to
Double or triple the size of the cap to get the C back up.
## BTW, the op said the plate load Z was 4000 ohms. He didn’t say it was 4
kv. With a 4k ohm load, vs
a 2 k ohm load, the tune + load C will be one half anyway.....and the required
uh in the tank coil will be double...
this is for a normal PI net..with a 50 ohm output...and no un-un used.
## another method that uses less load C is to use a PI-L.... but I don’t
recommend it. Any PI –L will typ
put the load cap at somewhere between 200-400 ohms. The main tank coil ends up
being even more uh than a PI,
then you also end up with the hassle of bandswitching the 2nd coil in the
PI-L....pita. The touted higher harmonic suppression
of a PI-L is mainly a myth.....a plane jane PI is more than ample for harmonic
suppression.
## Building a 200:50 ohm un-un that will handle 1.5 kw CCS on all bands
160-10M..with SWR thrown in on top of that
is doable though. Array solutions has several 200:50 ohm un-uns that will do
the job..and rated for 3-5-10-20 kw if you
need it.
### Another possible solution is to use a PI-L on 160m only...so a smaller
load cap can be used. Ameritron does that on
some of their amps.
## IMO... stick with a conventional PI-NET.....and just pad the load cap with
a bunch of surplus NPO 5 kv ceramic doorknob caps,
then use a relay to switch em in. Allen bond at max gain systems and also
fair radio in lima ohio have them. You cant blow em up,
and they don’t drift either. N470 caps ( N470 has a slightly worse TC than
a NPO..and will also work as padders..and still
wont drift..with the provisio that you use 2-4 of em in parallel.)
## I would avoid torroids in the main tank coil too...and ditto with the use
of torroids in the L portion of any PI-L.
later.... Jim VE7RF
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