EMRFD Message Archive 4027

Message Date From Subject
4027 2010-01-19 06:13:16 Nick Kennedy Class C tube amp load resistance
I'm working on my first-ever tube TX design. Getting to the output
network (pi-net), I need to know the resistance it should transform my
50 ohm load to. From ancient history I recall load lines and
published values of rp, but I don't think they're applicable for class
C. Right?

Does the simplified formula often used for transistor amps work, that
being Z = Vcc ^2 / 2*Po ? In my case, figuring 250 volts B+ and 6
watts out, I get 5,200 ohms.

Taking another approach, my 1969 ARRL Handbook (pg 158) gives charts
for looking up pi-net reactances of C1, C2 and L using the ratio of
plate voltage to plate milliamps and a chosen value of Q (10, 15, or
20). I figured plate current by assuming 70% efficiency and using the
6 watt RF output value, giving me 34 mA. Now, taking the long way
around, there's a formula in QST Technical Correspondence January 1984
for computing Rin based on C1, C2 and Rout. (Whew!) With this I
reverse-engineered the values from the Handbook's charts to give a
plate resistance value of 3,360 ohms.

The two numbers are in the same ballpark I guess, but I wonder if
there's a more direct way. I realize that a pi-net with variable C1
and C2 will allow me to adjust loading, so my question is more
academic than anything.

73-Nick, WA5BDU
4034 2010-01-19 12:46:07 Lou Burke Re: Class C tube amp load resistance
With 250 VDC and 34 Ma I make the plate load resistance at 7352.
Lou, W7JI

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4043 2010-01-20 05:38:04 Pat Bunn Re: Class C tube amp load resistance
I believe that you are off by a factor of two.

The plate load resistance for a pi - net is the plate voltage divided by the pate current x 2

That would be 250 /(.034 x 2 ) = 3676 ohms

The modulation resistance would be 250/.034 = 7352

The old handbooks use the formula for plate load resistance = Plate volts x 500/plate mA

That gives the same results as mine 250 x 500 /34 = 3676 ohms


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4052 2010-01-20 10:53:12 Lou Burke Re: Class C tube amp load resistance
You are correct......guess it's been tooooo long since I built anything with a tube hi hi.


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4053 2010-01-20 11:19:20 Tayloe Dan-P26412 Re: Class C tube amp load resistance
I would think this would not be any different than designing with
transistors such as MOSFET (which is very tube like).

Once you have determined the desired output power, Power = V*V/2R.

If you want 6 watts, 6 = 250*250/2*R, or R = 5200 ohms.

As far as the DC power, if the amplifier is 70% efficient, the DC power
will be 6/0.7 or 8.6w Power = V*I, 8.6 = 250* I, I = 0.032A or 32 mA.

- Dan, N7VE


4055 2010-01-20 12:29:20 Pat Bunn Re: Class C tube amp load resistance
It is very easy to get mixed up with Modulation impedance and Plate impedance - I just finished doing it for a dual 1625 am transmitter - so it was fresh in my mind.


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