[Amps] 200-ohm tank and "un-un" in HF amp design?
Jim W7RY
w7ry at centurytel.net
Mon Oct 21 01:22:51 EDT 2013
See below.
73
Jim W7RY
On 10/20/2013 10:01 PM, Jim Thomson wrote:
> Date: Sun, 20 Oct 2013 00:51:44 -0700
> From: Jim Barber <audioguy at q.com>
> To: "Amps at contesting.com" <Amps at 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.
XXXXXXXXXX HE most certanlly did! Please see the email where I asked him
what tube, plate voltage and current.
>
> ## 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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