Plate current change with modulation

[W8ACR]

It is my understanding that final amplifier plate current, as measured by an analog meter in the final amp circuit, should not change during standard high level plate modulation. Sooooo.........if the measured plate current DOES change with modulation, what is the most common cause?

I have a friend whose BC-610E is showing an increase in plate current during modulation. FWIW, his rig sounds good on the air, but I told him I thought his plate current reading should be steady, modulation or no. IDK, maybe I'm wrong. Wat's the consensus?

Thanks, Ron W8ACR

[kb2vxa]

Correct, plate current should not change under modulation. I'm not familiar with the BC-610 series themselves, but knowing a smidge about the various AM systems the first thing I would look into is the control grid. A change in grid current will affect grid bias that in turn affects plate current. If there is a clamp tube the grid bias is fed to the grid of that tube cutting it off, in the absence of bias it conducts clamping the screen voltage protecting the FPA. For what it's worthless, feeding rectified audio into the grid of the clamp tube produces controlled carrier modulation I used to play with just to confuse the Cee Be-ers.

That's my nickel's worth, I'd start by looking into the grid circuit while the transmitter is operating into a dummy load, and mind the high voltage! If it has door interlocks don't defeat them, extend the test points out of the cabinet where they can be metered. I know we have some BC-610 egg spurts around here... WAKE UP!

[WD5JKO]

A few points to mention:

Neutralization if not optimal will cause modulation non-linearity (carrier shift), and therefore a shift in plate current during modulation. Could be up or down.

Improper PA bias can cause issues with carrier shift.

Asymmetrical audio (male human voice) will cause carrier shift (good thing when phased properly to modulate up beyond 100%).

Power supply dragging down with modulation will cause downward carrier shift. This is especially true with a common power supply between the RF and Modulator stages.

In all cases, setting up for a scope trapezoid pattern will be most telling.

Jim
Wd5JKO

[Steve - K4HX]

How much is it changing? A flicker of 50 mA? Or something in between.

[DMOD]

How much is it changing? A flicker of 50 mA? Or something in between.

That was going to be my next question as well.

Just the variable loading on the power supply during modulation can cause a needle flicker of up to 50 mA.

I ran a simulation on a dual 6146 final stage with modulation from 0% to 120% (load impedance = 1222 ohms, Vp == 750V) and the total plate current change over this modulation range (Delta) was 140 mA.

Now the ballistics of most meters are such that they cannot follow audio amplitude bursts from 0 to 100% or fast high frequency changes.  

And BTW, the simulation showed AC RF voltages at the plate node with a range of from 980Vpeak@0% modulation to 1.55kVpeak@120% modulation. Ever wonder why they had 2kv to 3.5kV caps in that area?  

Phil - AC0OB

[W8ACR]

OK, thank you to those who replied. It is apparently just a small flicker DOWNWARD, barely noticeable. I guess I misunderstood when he first told me about it. So I guess he probably doesn't really have a problem, but I'm glad I asked, because I learned something new

Thanks again,
Ron W8ACR

[DMOD]

Both my Apache TX-1 and Johnson Viking II-CDC show a small drop and variation in plate current during modulation.

Phil - AC0OB

[AG5UM]

I Realize your question refers to : during modulation, but also interesting, the Heathkit DX-100 manual says:
Phone operation, " The plate current reading should be slightly lower than for CW" (240 ma).

[kb2vxa]

Better late than never I suppose. That's a common occurrence when the FPA and modulator are operated from the same power supply, a quiver in voltage that follows modulation peaks. This is common in E.F. Johnson and Heathkit transmitters, the plate transformer lacks oomph. Collins being the Cadillac of Amateur equipment is a notable exception. I was given an on air demonstration of the one that really bites, the Johnson Desk Kilowatt driven by the matching Ranger. At half power it sounded fine, when switched to full power it sounded horribly muddy with severe downward carrier shift as the plate supply flagged. The operator told me on an o scope the positive peaks were severely flattened causing distortion to the point what I heard was barely intelligible. Obviously every transmitter needs a hard power supply on a hard mains supply for clean, solid operation.

[Opcom]

Obviously every transmitter needs a hard power supply on a hard mains supply for clean, solid operation.

I understand home-builders who skimp in the power supply and then spend much time chasing the meter wiggling problem, but never go near the power supply. I am guilty of it in the past. We sinners are not so much to blame because all we did was follow supposedly good examples like in the handbooks and manuals of reputable existing gear. Lots of people gave up on chokes when electrolytics became cheap.