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@centurytel.net>
Date: 10/20/2013 6:31 AM (GMT-08:00)
To: amps@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
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
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