[AMPS] Class C 4-400A

[Tom Rauch]

>     And I was going to drive it from a modern transceiver capable of 100
> watts into 50 ohms (70 volts peak).

70 volts peak is 50 volts RMS. That's 50 watts.

100 volts peak is 70.7 volts RMS, and that's 100 watts. 
 
>     Then connect a 1:4 toroidal step-up transformer between the
> transceiver and the grid of the tube to create the correct driving
> voltage,  swamped with an 800 ohm resistor...

800 ohms divided by 4 is 200 ohms. Are you sure you want to load 
the rig with 200 ohms? Or are you saying a 1:4 TURNS ratio, which 
is 1:16 transformer?

If you are going to wind a 1:16 transformer at 800 ohms, and 
expect a low SWR over the entire HF spectrum, you might not get 
it. The input C of the tube will kill it.

With 100 watts drive, 220 volts grid bias would require 242 ohms for 
the edge of grid current.  With 800 ohms you'd start to draw grid 
current at about 30 watts of drive.

The unneutralized amplifier might become unstable at upper HF, 
depending on layout and load impedance presented to the tube. 
You might take the time to calculate the reactance from grid to 
anode and estimate the power applied back to the grid from the 
anode and see if it gets anywhere close to the gain. 

You probably do not have enough grid swamping. Try around 400 
ohms or so for class C, depending on how far into class C you go.

>    Would that work o.k.?    Un-neutralized??   Or is that arrangement not
> "swampy" enough??? 
 
Probably needs more swamping, for many reasons.
 
>    Are there any problems when the grid is in the positive region where,
> if memory serves correctly, the grid impedance is on the order of 1000
> ohms?

Yes, if you use a broadband input. The exciter will be bombed with 
harmonic energy. That makes exciters do all sorts of weird things, 
like shut off.

Unless you use a combination of self-bias and fixed bias and don't 
go deep into class C, everyone will enjoy the sound of keyclicks. 
I'd run class AB2, the efficiency is hardly different than shallow 
class C  and the signal will be cleaner.
73, Tom W8JI
w8ji@contesting.com

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