The VK4YE 1 Kilowatt Linear Amplifier.

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Assuming you have thoroughly done preliminary wiring checks, a step by step testing procedure can be implemented.

  • The high voltage supply can be tested first. Other transformer primaries must not be wired to the soft start circuit yet. Initially, connect a 100W light globe in series with the primary of the high tension transformer, which are then wired to the soft start circuit. This is to give a reduced primary voltage just in case there is a serious wiring error in the high tension area. The tubes should be out of their sockets. Switch on the 240V and observe that the soft start circuit works. A delay of approximately 0.5 seconds will occur before relays RX and RY operate. Note the reading on the high voltage meter. It should be less than the design value due to the presence of the light globe.

  • If all seems well, remove the 100W globe from the input and connect the high tension supply transformers directly to the soft start circuit. Switch on the 240V and note the reading of the high tension voltage. It ought to be around the peak value expected for your particular transformer/filter/bleeder combination.

  • Disconnect the primary of the high tension transformers from the soft start circuit.

  • Now connect the ancillary supply to the soft start circuit. Switch on and check that +12V is available. Check that relays RA and RB operate by inserting a shorting plug or a ground at the 'relay to rig' socket. Leave the ancillary supply permanently connected.

  • Connect an SWR /POWER meter between your rig and the input of the amplifier. The output of the amplifier should be terminated in a suitably rated 50 ohm dummy load. This is simply a "straight through" test. Apply low level carrier and observe that the SWR reading is very low. If all is well increase the drive to maximum. The amplifier should have only a minimal effect on the SWR reading in the 'straight through' position.

  • Connect the filament supply to the soft start circuit. Fit the two tubes into their sockets, fit plate connectors and switch on. Measure the filament voltage at the filament transformer and at the tube sockets. Previously you ensured that the filament voltage at the transformer [ no load ] was between 10.0 and 10.5V. The minimum voltage at the filaments must be 9.5V. The filament supply is to remain permanently connected.

  • Now connect the amplifier's relay socket to the rig's relay socket. With minimum drive, check that the input/output relays RA and RB operate when you hit the PTT switch.

  • Apply initially no more than 5W of drive. Observe the input SWR reading and adjust the slug in the input tuning unit for the band you are using. You should be able to get the SWR down to 1:1 for 10m, 15m and 20m [ if installed ]. Below 20m the SWR will remain quite low.

  • You can check that screen voltage is being developed with input power - perhaps 80V to 100V at 5W of drive.

  • Reconnect the B+ supply to the soft start circuit and switch on. If all seems well you are ready for the smoke test.

  • Connect your SWR/POWER meter between the amplifier's output and dummy load. If you have a second SWR/POWER meter connect it between your rig and the amplifier input. Leave the ALC disconnected for the time being. Set the band switch to the highest frequency band, say 10m, and turn the amplifier on. The plate current [ Ip ] should be zero. If all is well, set the standby switch to the operate position. The B+ voltage should fall a little and Ip should rise to around 50mA.

  • Monitoring Ig, rock the plate and tune capacitors through their range. There should be absolutely no hint of meter movement. If there is, you have either a VHF parasitic oscillation or stray feedback from the plate tank circuit to the grid circuit. Parasitic oscillations will have a frequency between 100MHz and 200MHz, while plate to grid feedback will occur in the HF range. Parasitic oscillations are cured by altering the dimensions of the plate chokes - more/less turns and by the placing of ferrite beads over the control grid and screen grid connections at the tube sockets. HF feedback requires careful shielding so that the plate components do not "see" the input components. Repeat this test for all other bands you have installed.

  • Assuming the amplifier is behaving nicely so far, select a mid-range band such as 40m, and un-mesh the load capacitor. Set the tune capacitor to half mesh and apply a couple of watts of drive. Ip should rise a little. Apply enough drive to bring Ip up to 100mA. Adjust the load capacitor until a peak in output power is indicated. Now alternatively adjust the tune and plate capacitors for maximum power output - it will be perhaps 50W or so.

  • Check that some Ig is indicated and then monitor Isg. At resonance, Isg is at a maximum value. Check this by rocking the tune capacitor and observing the change in Isg and power output. The dip in Ip at resonance may not be particularly noticeable.

  • Increase drive until 250mA of Ip shows. Readjust tune and load capacitors for maximum power output. Again monitor both control grid and screen grid currents. Increase drive further until around 400W rms is indicated at the output. Keep Ip under 500mA. Ig should be around 50mA and Isg about 40mA. Note that this power level of 400W rms is grossly in excess of the allowed 120W CW signal we are permitted to put to air.

  • Now repeat the procedure for the other bands. Do not be too concerned if 10m does not show 400W rms with 25W - 30W of drive. Losses will be higher on this band and key down carrier [ single tone ] testing is really quite severe on the dynamic regulation of the high tension supply. Proper two tone testing will show that the legal limit is easily being achieved on all bands. Furthermore, linearity can also be checked by this method.

Single tone test data at 14.2MHz indicates the DC power input is 650W and the efficiency works out to be 62%.






Power out





400W rms

Mounting and cooling of tubes. | Two tone testing.
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