James F. Hartley W1DIS
Raymondf Maine 04071
An Extremely Stable Configuration
This article will be of interest to the ham who wants a good linear amplifier which is easy to build and very simple to operate. I built mine into the pedestal oi a Johnson Viking Kilowatt which will be of interest to owners of this piece of equipment The type ol tetrodes and the amount of power can be almost limitless with this type of circuit he pair of parallel 4-400 tubes I used gives me a lull 2000 with no problems, SO through 10 meters.
What are the reasons for building a low-impedance, or terminated grid type of amplifier?
1. The input impedance has a resistance of low value. A non-inductive (Globar) resistor oi 50 to 100 ohms resistance provides an exciter load suitable for use up to 30 MHz— probably higher, It is non-reactive, except for stray capacity.
3. With proper shielding of the input circuit components, this low input impedance materially reduces problems of feedback from output to input Neutralization is seldom, if ever, required.
4. Good designs are almost invariably basically simple, and this certainly is simp e.
5. Bias may be brought up through an rf grid choke, the bias being isolated from the input terminal and the 50 ohm resistor bv a low voltage mica condenser of .001 to .01.
Alternatively, the 50 ohm resistor may serve as an input resistor, bias being brought up through it, in which case the low side of the resistor should he adequately by-passed,
6. An additional refinement is that a 100 K pot. iapprox) could be connected into the bias lead to provide ale voltage for the exciter. This value of resistance would provide about 10 volts peak when grid current reached M*» of 1 mA, The exac t values would have to be determined for the amount of ale voltage desired. Moreover, the moving arm of this pot. would have to go to a silicon rectifier diode connected to provide a negative ale voltage. Since any grid current flowing would already represent one rectification, the voltage taken from the pot would be audio; the second rectifier thus would provide an envelope voltage tot application to an appropriate stage in the exciter.
Fig, 1 shows the linear amplifier using a pair of Eimac 4-400s in parallel class AB-X as it is now installed in the pedestal of my Viking Kilowatt. In my rig the grid circuit reflects a 52 ohm load to the exciter at all times, This makes my Drake T-4X operate as if it were working into a 52 ohm dummy
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