The AM Forum

I have attached a photo showing my QRP transmitter in the "tuned" (ie maximum output) condition into one of my 75 meter dipoles.

I am curious about two issues, first being that in the pi network, the tuning cap at maximum output (3-4 watts) does not engage the stator plates very much.  That is not my main concern but in addition, the "loading" capacitor wants to be 100% unengaged with its stator plates for maximum output to the antenna.

I measured the antenna parameters with my homebrew analyzer to show what the antenna looks like to the transmitter.

I am interested in what, if any, change I might make to better utilize the capacitors, or, is everyone happy with the results seen?

The photo of the transmitter shows the capacitor engagement at max output into the antenna.

Am I just expecting too much adjustability and should leave it as is or possibly there is a problem.

I have tried adding turns to the coil with the result of only lower output with similar capacitor engagements. 

Thoughts?

What is the tube type, Plate voltage, and Plate current?


Phil - AC0OB

Also, what is the inductance of the coil?

If you cannot measure it, then how long are the close-wound turns?  From your picture I estimated the length to be about 2" and calculated the inductance to be about 21 uH.  But that was just an estimate.

And one more thing - can you determine the sign of the 53 ohm reactance?

Short out 1 turn on the coil at a time and retune until satisfied.
 

I would try removing one or 2 sections of the load capacitor from the circuit. That should be a clean and simple experiment. Then try scraping and shorting coils.
The plate looks OK to me

OK, here is some data today from testing.

The output tube is the 6AQ5

Plate voltage 350 VDC
Plate current at max output into antenna: 40 MA

Coil inductance: 30 uH

The reactance seems to be "capacitive" but that is not certain.

I have removed up to two of the load capacitor sections (ie down to 300 mmfd from the 1000) with no appreciable effect.  It surprised me but that is why I am inquiring.

I did add a 100 pF in series with the tune cap to ground and it did make some difference in the capacitor's engagement in that it engages a little more at max rf output. (maybe 10% or so)

Tonight I will load the set up into the 50 ohm dummy to see what the engagement does.... should have done this before these emails... I have done it before but my memory does not serve me well as to the engagement of the caps.  One thing I do remember was that into the dummy, I get more plate current (over 45 ma) and upwards of 7-8 watts indicated on the wattmeter....  but MFJ  may not be very accurate in that regard.

The set seems to be particularly sensitive to load impedance.

Try the following:

Plate 300 VDC.  Load it to 50 mA.  Bias the beast class C.  That would probably be around -35 to -45 on the signal grid, ballpark.

Assuming no sheath current on your coax, and nominal Z load around 70 ohms, you want:

C1 about 82 pF C2 about 500 pF, and L about 21 to 22 UH.

Try these parameters, and see if she will dip and load.  Remember, these are ballpark parameters.  Good luck!

You have way too much inductance.  30uh is too much.  Your plate load is about 4300 ohms.  For 80m something like 15-18uh is better, haft that for 40m.  The length of the plate coil should be about 1-1.5 times its diameter to get the right Q.

You'll need about 150-200pfd for the tuning cap and use all the sections of the loading cap.  1000pfd may not even be enough.

Fred

Well, changing the inductor is doable, will try it first as I have some coils that may be more appropriate.  Will report results.

Btw the set is series modulated with a pair of 6AQ5's in parallel...  gets great local audio reports and hits about 125% positive on peaks with 80-90 percent negative. 

Thanks, 73, Tom

Right on Fred!   

BTW: A friend of mine built a 6AQ5 rig with one in the final and another as a plate modulator and I can squeeze about 7W out of it on 80M.  I am also amazed at how much abuse this tube can handle and still be good.

Joe-W3GMS   

Dear Tom:

I assume you are on 75; my numbers would change on 40 meters!

Yes, too much inductance. 

I would also like to know about that dipole.  Coax feed or ladder line?  Length of line?

If coax, balun or no balun?

If the sheath current is high on a coaxial fed dipole, the impedance seen by that tank will be FAR AWAY form the 50 to 75 ohm range.


Matt

OK, some results....

Found a coil in the treasure chest that measured 24 microH.  

It did make some positive difference, the "tune" cap now engages about 50% at max indicated output and the "load" cap when using the 50 ohm dummy finds a peak at about mid range as well.

Got about another 1/2 watt out too.

Now the antenna...  it is a coax fed OCF "all bander"...  my analyzer indicates it is only really close on 80 and 40 meters.... other bands are too far off to work.

It really is a poor substitute but is what is up in the air.  

One indicator that the antenna is a problem is that into the dummy the set will achieve nearly twice the power output, i.e. 8-9 watts while into the antenna 3-4 watts only.

Tom