RF output Power drop off

Mal G3PDH

Running 6500 + Expert 1.3k, Windows 10 SSDR and DDutil: I notice that the maximum RF power generated from my 6500 is on 10m at 115w and that it gradually decreases as I go down the bands to around 95w on 80m. All measurements/observations taken in CW mode. I also notice that after initially showing such power levels after only less than a minute the power gradually drops back by as much as 5 or 6 watts and will then stabilize a bit. This also seems to happen at low power levels which can be a nuisance when driving the amp as its power then starts to drop back substantially.  Does anyone else see similar or can offer any thoughts?  I have set the DC PSU voltage to 14v as read on DDutil.  PSU is GSV3000.

James Del Principe

Mal, does the power supply remain at 14 volts under load?       73, Jim

George KF2T

Are you sure your wattmeter is accurate within better than 5% over the entire range? Are you transmitting into a non-reactive dummy load with a very short connecting cable? There may not be anything going on with the rig at all, as measurement error might account for almost all the observed variances. 

Mal G3PDH

Thanks for responses, the psu voltage drops from 14 to 13.8 when transmitting at 100w. Measurements taken into a Toyo 100w dummy load, model T100 which shows zero reflected. It is difficult to monitor small changes on the external power meter and cannot guarantee its absolute accuracy. My main concern is the dropping off of power. Regardless of testing into a dummy load or connected to any of my antennas via different feeders the power at 'cold' start is max for that band but gradually drops back several watts which I can also see happening on the flex power meter on ssdr as well as the digital power reading on Expert 1.3k amplifier (in standby mode). The drop then stabilises and the level seems to correspond in concert with the PA temp shown on DDUtil. After it has cooled the power is up again and drops off as it increases during transmissions. I will obviously try to carry out more detailed tests with better calibrated equipment but I just wanted to know if this is a characteristic of the rig and if others see similar. I notice that as the 6500 PA temp increases there is no change of cooling fan noise, is the fan supposed to increase speed as the PA gets warmer? Have never noticed this effect on my other rigs working with same antennae.

Tim - W4TME

PA efficiency is temperature dependent so there will be some drop in power as the transistors get warmer.  Since you lowest wattage is 95W, which is within the nominal specifications, that is not something to be too concerned about

What is a more concerning is the 115W when initially transmitting.  This sounds like the PA may need to be recalibrated by an authorized service center, but to know for certain, I am converting this topic to a HelpDesk support ticket so we can get a set of complete power curve data points in the initial and warmed up state.