Joihnson Ranger drop in plate current when 100% modulated.

[Knightt150]

 I have noticed that when I reach 100% modulation the plate current drops about 5 to 10 mils. When I over modulate the actual power gose up. I know that when 100 % is reached the final plate current is affectively cut off. But I dont think that the meter will show a drop if every thing is good it should show a increase in plate current.

I plan to put a new 6146 in and a new HV rectifier but dont feel this will help.

Another thing the drive to the final remains solid up to 100% mod but over 100% mod the drive begins to drop off.

Has anyone had this problem before and fixed it, or is it a problem.

John W9BFO

[K6IC]

Hi Joh,

Because I am no expert,  thought I'd respond.

Think thet Carrier Shift is a term used to describe this effect.  Some smallish amount is normal.  Probably one of the largest effects can be caused my the HV Power Supply voltage sagging under high modulation levels.  On large transmitters,  sagging Line Voltage can be an additional effect.   At 100 % MODULATION,  the plate current has doubled,  and the modulator tubes are usually drawing additional current,  and often from a common power supply,  and so on.

I would not worry too much about  5 % or so  shift.  Nor would I worry about the results from overmodulating.  Do not do that !

Good Luck

[N2DTS]

Since I am mostly clueless, I will also chime in!

Ham gear was not designed to be great, just work, so you likely have a small filter cap on the high voltage, and that will get a lot worse if you modify the rig to pass more low frequencies.
Under heavy modulation, the voltage can sag a lot, more so with vacuum rectifiers in the power supply.

On my homebrew 813 rig I use a lot of capacitance for energy storage, and the modulator and rf decks have their own supplies. No voltage sag.

On the 3x4d32 rig, I have one supply with a lot of capacitance and the voltage sags somewhat, its not a big deal.

Brett

[KB2WIG]

J,
The Ranger has 10 uF of capacity in the HV section and 30 uF in the LV.....  not a lot, not nowdays... I replaced my 10 uF with (2) 220 uF in series. The 30 uF got a 100 uF replacement. Also, the power supply was solid 'stated'.

 There are a lot of modifications discussed here on the forum..... many try to cut down on the heat generated, protect the drive pot, and to improve the audio..... then there is the dreaded chernobyl resistor ( Google this).


klc

[ke7trp]

While your at it, Put a 30 to 50 UF cap on the B+ to the VFO to stop the Carrier shift.  As you modulate, The B+ sags causing the VFO B+ to Drop.  This shifts the radio Freq. 

Its pretty normal for the plate to Drop a bit.  You can experment and load the Rig lower to find that sweet spot where the Rig does not show a downward motion to the plate meter. 

C