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

[Jim W7RY]

Jim...

To figure the plate load impedance:

RP = EP / 1.8 X IP

1.8 = K factor for class AB1 (Changes with ZSAC)
RP = Plate Resistance
EP = Plate Voltage
IP = Plate Current


1.8 X .65 Amps = 1.17

1.17/ 4000 = 3418 Ohms plate load impudence.

Your actual voltage will more likely be 3800 with a .65amp load which brings the load down to 3250 Ohms.


Better to just switch in some loading padders with your bandswitch.

See the last 8 years of the ARRL handbook for the K8RA amplifier using the 3CX1500D7 tube. There are some great examples there.


73
Jim W7RY


On 10/20/2013 8:28 AM, Jim Barber wrote:
> 1500 watts
> 4kV, 625mA
> Single 8877 at around 4000 ohms plate impedance in AB2.
>
> Tnx,
> Jim N7CXI
>
>
> Sent from mobile wireless
>
>
>
>
> -------- Original message --------
> From: Jim W7RY <w7ry at centurytel.net>
> Date: 10/20/2013 6:31 AM (GMT-08:00)
> To: amps at contesting.com
> Subject: Re: [Amps] 200-ohm tank and "un-un" in HF amp design?
>
>
> What kind of power?  What plate voltage and current on the tube?
>
> 73
> Jim W7RY
>
>
> On 10/20/2013 12:51 AM, Jim Barber 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?
> >
> > 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
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>
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