The same failure happened twice more even though all parameters seemed to be quite normal and I was not abusing the amplifier. I could not find any cause which I could confirm so I put it down to the fact that the amplifier was possibly unstable for this purpose. It was also quite evident that it was not suitable due to the very low output measured during testing. At this point I decided to abandon this particular amplifier project.
27th November 2010 - 9kHz Transmitter . The first choice of transmitter was not fulfilled due to the kit not being suitable for this purpose and became unreliable. I thought about a replacement and because I had not yet received a "Special Research Permit" I did not wish to purchase anything further for the time being. I remembered that I had retrieved a Laney 50 watt guitar amplifier from a nephew who was scrapping it some while ago. I got that onto my workshop bench and carried out the necessary work to put it back into service.
I used the same matching transformer connected to the loudspeaker output and terminated with a 50 ohm dummy load. Drive was applied at 8.9kHz from my DDS VFO to one of the guitar inputs, I immediately had output. I varied the ratio of the transformer and found it did not require any changes. I measured the maximum output at the dummy load to be 36 watts. The amplifier was then subjected to various tests to determine the reliability by subjecting it to several types of data signals at long transmit periods which it passed with flying colours. 36 watts is not particularly high but it will allow me to experiment with antennas and other criteria for 9kHz operation.
Laney 50 watt guitar amplifier.
29th November 2010 - 9kHz Receive Antenna. I have replaced the VLF pre-amp (shown in an earlier post) attached to the loop antenna with the M0BMU device which I could not get to work. I did a strip down and found one faulty component and "shame upon shame" a missed out coupling capacitor which had somehow been substituted by a wire link causing the bias to be incorrect on the input to the op-amp. The pre-amp operates fine now. I will soon complete the device by adding the switched 7.8 to 10.5kHz band pass filter seen in the circuit. Click picture for full size or full MS Word document of circuit details.