H.F. LINEAR AMP


 

DANGER HIGH VOLTAGE !!!

Take care when working on high voltage equipment

The 50R input of the amp. is matched to the 200R grid resistor by a 1:4 impedance ratio (1:2 Voltage) transformer. Screen grid (G2) voltage is generated by voltage doubling some of the grid drive RF. This method of providing the screen voltage was used in the G2DAF linear. I used 5 x 1K 2W resistors for the 200R grid resistor. 5 x 2W = 20W! not quite, but it works fine. The diodes used in the voltage doubler were taken from an old computer monitor. Any high voltage (300v PIV) signal diodes should work. If you don't have any HV signal diodes, try using two chains of four 1N4148's in series, don't forget to put a resistor of about 100K across each diode. The diodes in the cathode circuit, are ordinary Silicon rectifier diodes. I used 1N5408's. I tried three different anode chokes, a 'proper' multi-section Litz wire choke, a hand wound multi-section choke and a single layer, close wound solenoid winding of about 150 turns of enamelled Copper wire on a 30mm former. All three gave similar results. The most critical component in the amp. is the 1n 6KV, DC blocking capacitor. Don't be temped to use a cheap 1 or 2 KV cap. here. For 80Mtrs, the output Pi-network inductor should be about 11 microhenries. On 10Mtrs it should be about 1.4 microhenries. I started with about 26 turns of silver plated wire on a 1.5 Inch (38mm) ceramic former. The valve (tube) is a Philips QE08/200H.

I have been using this amp since Feb. 99. Signal reports have been very good. Output is 210W for 16W of drive on 80M. Max output is 300W. Since building the amp, I have modified the output network so that it will work on 160M. See the table below for component values for all nine H.F. bands. You will not need nine positions on the band switch. 30M/20M, 17M/15M, 12M/10M, can share common switch contacts.



Band

C1 picoFarads

C2 picoFarads

L1 microHenries

160

355

2360

21.7

80

182

1212

11.1

40

96

636

5.8

30

66

443

4

20

48

317

2.9

17

37

247

2.27

15

32

211

1.9

12

27

180

1.6

10

23

155

1.4

TABLE OF VALUES FOR OUTPUT PI NETWORK

INPUT 3300 OHMS, OUTPUT 50 OHMS, Q 14.



LOW PASS FILTER HF Low Pass Filter, suitable for use with this amplifier


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