GK-500-C Strong harminic on 160M

[wa2ujx]

I am wondering if anyone else with a GK-500C has been confronted with strong harmonics when transmitting on the 160M band. Although my "stereo" transmissions are 5/9 into Pa., I really need to produce a good single signal asap. I have strong 3770khz sig on 1885khz into a D load as well as the antenna. Pulling the RF deck and verifying component values yealded one screen dropping resistor for the OSC tube (6AG7) which shifted to half its original value. PA coils and chokes are OK. OSC, Buffer and Driver (6146A) loading settings are near factory sugested positions. Driver tank coil is a little discolored but nothing leads me to believe that the turns are shorted (although maybe a small amount of resistive leakage between turns could alter the Q of the coil?). For technical interest only, my RF deck (GK-SR # 141X) was build without the 12AU7 bias tube.  I have yet to run the deck with a crystal instead of the built in VFO - but will do so soon. Since my antenna is 50 OHMs and the KG-500C only loads into 300-2000OHMs load, I use three types of circuits for impedance transformation. A low pass "L" network, a High pass "T" (Dentron super/super tuner) or a 4:1 balun. All produce second harminic output to a lesser or greater degree. I believe the answer will untimately be simple - but, like all things simple, they start with complexity. I am just starting on this diagnostic journey but I would appreciate comments from GK-500C owners as well as others. Thanks, Carl J. Verderber,  WA2UJX 

[WQ9E]

Carl,

How strong is your 2'nd harmonic (I am assuming you don't mean both it and your 160 meter fundamental are both 5/9 into Philly?).  I know this is difficult to answer unless you have a spectrum analyzer with a proper sampler or a frequency selective voltmeter but if you don't have this equipment to measure exactly how about comparing another vintage rig while listening on an outboard receiver?  Without an outboard tuner you are typically looking at perhaps 30 db down for the 2'nd harmonic with most vintage gear so locally you will still have a very strong second harmonic.  For example, Johnson talks about the "excellent harmonic suppression" from the Pi network in the Valiant and states with careful tuning the 2'nd harmonic may be as much as 30 db down.  They further state that this is better than the performance from the older link coupled output circuits.

Your L network should really knock the harmonics down, this is the major reason I chose an L network for my tuner with vintage gear.

If you really are putting out an excessively strong harmonic is there any chance you are driving the final too hard (perhaps the grid current meter shunt is no longer accurate).  Excessive drive will cause even greater harmonic generation within the final.  Excessive static bias could also have the same impact.

Again, make sure you aren't chasing a non-problem.  I recall that some of the amplifier makers had difficulty meeting the FCC requirement of harmonics being 40 db down when it was first instituted.  And this was for a linear amplifier being driven by a clean exciter as opposed to a class C stage typically being driven by an exciter chain intended to create harmonics.

Rodger WQ9E

[wa2ujx]

Rodger, Thanks for your comments. I have not instrumented the deck as yet. Setting up to take measurements "in situ" is awkward for this rig but I will achieve this soon. Overdriving the final (grid drive and bias level) sounds like something I need to explore. I have no way to measure the harmonic but I can assume it is about 5 to 10 watts while the power watt meter is indicating 300 Watts out on 1885Khz. BTW, I have no way of knowing if this condition has existed for many years with out my knowledge. I did a cursory harmonic check in 2002 when I put this rig on the air and found no problems. Since then, I have not deliberately checked for this condition. 73 Carl WA2UJX 

[W1VD]

Carl

If I remember correctly you're using a 'fan' dipole with both 160 and 80 meter elements tied to a single coax feed. This would enhance 2nd harmonic radiation over a conventional single band only dipole. Assuming the transmitter is operating correctly - 2nd harmonic 40 dB down (best verified with a spectrum analyzer after a  high power attenuator)  one could add a low pass filter cut just above the 160 meter band.

Jay