Final Tuning How-To

Tuning Your Hybrid

  This is the part that most hams, who are new to tube finals, worry about.  Several customers have asked me about it, and since there are so many descriptions of how to do it out there, I decided to tell you how I do it in the shop.

  I am very careful with final tuning here, as you can imagine, cause I never know what is going to happen.  A radio sent in for repair could have anything wrong with it, and I certainly don’t want to make whatever is wrong… worse.  One of the usual problems is “low power out” that we are trying to solve.  So I am probably more careful about it, and slower than even the most wide eyed, tube final newbie would ever be.  If the slightest bit of the process doesn’t seem quite right, I stop and try to figure out what is happening.   A good example was an 820S that just went through our shop that had no power out in Tune mode.  I stopped the whole process and it led me to some completely open resistors in the Tune screen grid circuit.  If something is wrong and I mess up, it could mean added expense and more time fixing stuff.  I never rush through this part.  This is how I do it.

1.  Meter on ALC.  Mode sw to Tune.  Check the drive plate and load controls.  Drive in the middle, Plate on its appropriate band, and Load at 10-11 o’clock.  Carrier control set to about 10 o’clock.  I check that the Antenna is hooked to my dummy load and HP test set, and the test set is in TX Test mode.

2.  Flip on the Send and peak the ALC reading with the Drive control.  Then, adjust the meter reading with the Carrier control until it is approximately 1/8″-1/4″ past the ALC box on the meter.  Then I will re-peak the drive control, which usually does not change.

  If all this worked correctly, it is telling me that the pre-final stages of the transmitter section are most likely working properly.  The Carrier control should probably be around 9-11 oclock, setting wise.  If I have to push it way further than that, it is telling me that something is weak or wrong with the drive circuitry, maybe a weak driver tube. 10 meters will usually need the highest setting of the Carrier control.

3.  I switch the meter to RF and then flip the Send again, and peak the reading with Plate and then Load controls.  I set the Load control on the right side (clockwise) side of its peak, not enough to cause the meter to go down appreciably, just to the point that it might start it’s downward movement.

  While all this is going on, I am using a dummy load that is being monitored by my HP test set.  It is looking at power output and reads it to about 5 decimal places.  At this point I like to see 8-15 watts out.  If I’m close to 15, or even more, very good.  If it is at 8 or below, I know I am going to have to tune some more.  A watt meter will tell you the same thing, probably not to that ridiculous level of accuracy, but certainly good enough.

  As long as you are in Tune mode, you can pretty much take your time as the final tubes can almost endure anything in this reduced power mode.  You can think of it as the Safe Tuning Mode.  While in Tune mode, you can switch the meter to IP and then dip the plate and load controls, it will give you an idea of what that looks and feels like.

  An added note.  Do not switch anything while you are in Send.  Ever.  You can adjust things like we describe here, but any function that has a switch, don’t do it while in Send.  You’re probably ok switching the meter, but nothing else.

4.  I then set the meter to IP, and the mode switch to CW.  With my right hand on the Plate control, I flip to send and immediately adjust the Plate for a minimum reading.  It will require very little movement of the Plate control to do so.  I quickly note the power reading on the HP, then Send off.  If you are using a wattmeter, peaking it at this point would work well also.  My HP test set is not analog, it reads out in numbers which makes it hard to peak just looking at that.  An analog meter is easy to peak.  I then flip to Send again and dip the load control, noting the power out.

  Three to five seconds always between these operations.  You want more time with the send off than you had it on.  Just being safe here, and protecting those 6146’s.

  If you are using a wattmeter, you should at some point look at the meter in IP and make sure that it has dipped its reading.  You want no more than 225 ma. on that dip.  If it is more, rotate the Load a little to the left and try again.  Note that the way I am doing this, I am not staying in send for more than about 2 seconds at any one time.  If the Plate and Load had to be significantly moved from their peak Tune position, I’m thinking there might be something wrong.

5.  Most likely I am not seeing the power out I want to see.  I advance the Load control to the right a little (like the pointer a 1/16″), and then re-dip the Plate control.  I look quickly at the power out and then Send to off.  I do this several times until I get the dipped plate current (IP) up to 225 ma.  The power readings must meet our standards at this point for the output section to be good.  The most important thing is that the Plate current, when dipped, should not be over 225 ma.

  Now, for the beginner, this seems incredibly detailed and complicated.  Trust me, and everybody else who is telling you how to do this, that it is very easy and simple.  I always suggest to a newbie that they take their time and practice a whole bunch with a dummy load, doing it very slowly as I have detailed here.  Change the frequency/band and do it again.  As you do it, it will become easier and easier, and you will eventually end up doing the whole thing in a few seconds. A lot of hams skip the Mode Sw in Tune part. Also, each radio/antenna combination is different, and you will learn where the settings are on your radio and that will make it even easier.  Most of us work on one or two bands, a lot of the time, and you will almost know exactly where the controls should be.

  I hope this helps.  I sympathize with beginners, because when I go to tune up a radio in unknown condition, or after I have just made a bunch of repairs, it stresses me out.  It’s the most anxiety ridden part of my job.  I would imagine I am feeling something like what a newbie does when they do it for the first time.

Good luck!!

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