Test set up and procedure for plate choke resonance.

Testing a plate choke.

I developed this test procedure to help the builder find out if the choke has series resonance in the range of operation of the amplifier it is intended to be used in. Feel free to use it to test yours.

The schematic drawing above shows the set up for testing a plate choke "in-circuit" , the set up for an "out-of-circuit" test is simple, the choke is tested out of the plate circuit with only the signal generator and oscilloscope at each end of the choke. A 50 ohm load should be placed across the sig gen output to show it a proper load during tests. The sign gen should cover at least 1-30MHz, more would be better. The O-scope should cover to at least 30MHz, more would be better too.

The 2.5K ohm carbon resistor simulates the plate impedance of the 8877 during actual operation. The 50 ohm resistor at the tank circuit output provides a load to the tank, simulating the operating circuit. The 50 ohm load across the signal generator provides a load to the sign gen to keep is from being swamped by the plate circuit.

The test procedure described here should work for other chokes both in and out of circuits. For in-circuit tests the plate impedance should be known and simulated with a carbon resistor.

Use the minimum levels needed to get an indication on the scope. It may be necessary to change levels to get the needed detection in the scope.

Sweep the sign gen up frequency from the bottom to the top end, watching the scope of a sharp rise in the RF voltage envelope, this would indicate a resonance on a given frequency.

I found that my choke design had a series resonance "out of circuit" of 1.6MHz. There were no other resonance frequencies found. For the "in circuit" test, I found a major resonance at about 450kHz, which was predicted. I found very small resonance at 6.0MHz, 12.3MHz, 17.4MHz, & 20.2MHz, no others were found above this into the VHF region. Since all of these resonance points are well outside the ham bands there will be little likelihood that there would ever be a problem in this area of the tank circuit.

I did a further test of the circuit, by testing the parasitic choke (PC-1 on the drawing). This test was to see how much VHF/UHF energy would be passed through the choke. I did this by moving the scope probe to the opposite side of PC-1 from the sign gen. I then swept the sig gen from 28MHz to about 250MHz. The only point I found to have a small rise in signal was at about 89MHz which is as predicted. There were no other points that gave more than a baseline signal level. So, the parasitic choke is doing the job it was designed to do, attenuate UHF energy and prevent a UHF parasitic oscillation. At VHF it should be fine as well. I did notice that there is a very sharp drop in signal throughput starting at 32MHz. This is good!

If your building an amp from scratch, you would do well to test your circuit before moving too far along, a serious flaw in a choke can cause serious instability in the amp. It could be difficult to repair at a later time in the build.