The AM Forum

I noticed when I peak my grid current on my Johnson Valiant with the oscillator set to ZERO on 40M that it almost doubles when I transmit. When on 20M, it cuts in half. On 80M it's spot on. If I peak it in CW on all bands, it's right on as well. What could cause the disparity when peaking with it set to ZERO and then transmitting? Thanks for the help.

When spotting with plate power off normally the grid drive will drop off when the final is active and fully loaded. It is unusual to see it go up with the final active but operating conditions are changed from standby to transmit so it is certainly possible for the drive level to change.  An improperly neutralized final will also cause unusual grid current readings.

For proper Valiant tune-up you should be in CW mode, peak grid drive in spot position and set for around 6 to 8 mils, then with plate voltage on resonate the final and load to rated input for either CW or AM if you plan to operate AM.  Ajust grid drive as needed to achieve around 8 mils.  Be careful not to use excessive drive since the 6146 control grids are easily and permanently damaged by too much drive.  3.5 mils per tube (10.5 mils total) is the maximum safe amount but that is more than needed so stick to 8 mils total.  It isn't a good idea to tune up in AM mode since the modulator section is active and a loud noise in your operating room will create high peak voltage issues with an unloaded final.

The spot position isn't useful for setting total grid drive because it will vary with amplifier loading.  Spot is intended to provide an easy method of setting the VFO to the receive frequency and it is fine for adjusting the grid tuning but not for setting final drive level.

Thanks Rodger. I will start using the CW position to peak the grid instead of the spotting.

Hi John,

The spot position is fine for peaking but do expect the drive requirement to change once loaded up.  The major caution is not to tune up in AM mode just in case you have the audio turned up with the final not yet loaded.  This could cause something to flash over which is never a good thing :)    A little touch up to the plate tune or drive level is fine in AM but it is best to do any major tune-up (as when changing bands) in the CW mode.