How Tune A Linear Real Quick

[flintstone mop]

Hello All,
I am trying to get my linear tuning honed in so there is not too much QRM and get that peak power goal quickly.
I tune into a dummy, but sometimes the other antennas I have are not going to be perfect 1.2:1 that I'll have on the OWL dipole. What is fast easy way to get the peak power tuned in without a lot of tuning.
And the AM mode is another tuning procedure of adjusting the loading for some extra oomph? Or better linearity??

I usually tune off about 5 khz from the intended target, then go to the desired freq. and watch the munky schwang.

Thanks
Fred

[K1JJ]

Fred,

To tune the linear at maximum power with minimum abuse, build a simple "Belcher." It is an audio pulse generator. Simple to build using just one 555 chip.  I use it for tuning all my amps and works great to generate a pulse of 10% duty cycle and still accurately simulate full power tuning.  It requires an ssb driver rig to use properly.

http://amfone.net/Amforum/index.php?topic=22453.0


Otherwise, using a dead carrier, tune your amplifier plate and loading controls for maximum output and then increase loading (less loading capacitance)  until the power drops off about 7%. Then re-peak the tune cap.  This is for best linearity.

T

[ke7trp]

Depends on the radio but on my Icom, I tune up in RTTY mode using 10 watts or so to start,  With the RTTY mode, you can have 1 to 100 watts output variable.  I finish the tune in RTTY quickly setting the Plate dip and max output power while watching Grid current to make sure I do not over Grid the amp.  More Drive, back the load off to lower Grid current.

You can use CW mode if you have the key wired in.  Otherwise, Just use RTTY.


C

[flintstone mop]

Ok Thanks Guys
Tom.......the loading adjustment was what I was looking for. There was that last tweak that was thrown in there that did some sort of magic.


Fred

[KM1H]

The Last tweak is to do a final touchup of the Tune control if needed to peak the output. Otherwise the anode to load is not seing a resistive (resonant non reactive) match. This is usually not needed if the Q is reasonable and the anode R is low and is more noticable on the high bands.

The linearity (IMD) improvement with the heavier loading is about 5dB.

[flintstone mop]

This is good info to know, when using 100% duty cycle and wanting big pos peaks.
Also the ceramic tubes need a little more attention not to go wild on the grid current.
I learnt that heavy loading is with LESS meshing of the capacitor on the LOAD control. I was always going in that direction any way for less or no grid current and max power out in a CW mode or full drive. I can usually get the amp max'd out and tuned within a few seconds now. I had a Heath (single 3-500Z) that it took forever to tune properly.
I miss not seeing a glowing blushing pube, but I know my ceramic AL1500 amp is more than capable for the task at hand.

Fred..............Thanks,..............Happy New Year

[ka1tdq]

This is great to know!  I've just been tuning my homebrew linear for maximum power output...  I'll do the loading tweak now. 

Jon
KA1TDQ

[K5UJ]

I learnt that heavy loading is with LESS meshing of the capacitor on the LOAD control.

This is a point of confusion for some.  I had trouble with it also once.  The problem (part of the problem at least) is the way front panel amp controls are labeled.  You read the correct tune-up advice to increase loading.  Then, without knowing much one assumes that means to advance the load cap control to a higher front panel value on the knob scale.  But with some amps, that increases the capacitance setting of the load cap and decreases the loading.  With others it decreases the cap value (increasing the loading).   My amp was made so that turning the load cap clockwise moves the load cap to a higher value.   So when I was new, I was unwittingly decreasing the loading when I thought I was increasing it by turning the knob to a higher numerical setting on the scale. 

Any noob appliance op, reads "increase the loading" and looks at his amp and understandably thinks that means advancing the load control.   I eventually learned that I had to pull the cover off and look inside at what the load cap was doing when I turned the f.p. control ccw and cw in order to increase the loading properly.  That is still something I advise hams do when they get a new amp that they are unfamiliar with. 
 

[K1JJ]

Good points.  Some more info:


Yes, less loading capacitance (heavier loading) generally moves the tube's operating point more into the higher current, more linear (straight) part of its transfer curve.  As a result, we draw more plate current and generate more heat, thus less efficient.  But this is the way to get better positive peaks on AM.   (or ssb)

A linear that is lightly loaded may have better efficiency, but taken to an extreme, the peaks may get a more shunted, limiting look. Wheareas, with heavier loading the peaks will fly up effortlessly and stay rounded as they should when using a sine wave or voice peaks.

Years ago, the "heavier loading" fooled me too, thinking the capacitor needed more C. These days I have an arrow on each amplifer indicating the direction of less capacitance. For many vacuum variable capacitors, less capacitance means turning the shaft clockwise. (heavier loading.)    For an air cap, you just need to loook and see how it is mechanically set up.

As Clark already said, the grid current is another indication that should be monitored. Without viewing grid current, we can sometimes get ourselves into a tuning situation with too much drive, poor IMD and grid damage.  

Monitoring grid current in a GG amp with the grids directly grounded to chassis takes a little extra effort. It requires floating the negative lead of the HV supply thru a low value resistor to ground to segregate grid current from plate current. But, a good idea.  Most commercial amps are already set up this way.  This metering circuit is used in many of the linear amps as shown in the handbooks over the years.

T

[Steve - K4HX]

Writing or marking presets for each band or often used freqs can get you close (maybe even right on if you have verniers or turns counters) without even applying RF. Then it's just a quick tweak and you're ready to roll. My amp has separate tune and load caps for each band, so often I don't even need to tune when switching bands. I still manage to mess it up sometime by forgetting to switch the antenna to to the correct one!

[Todd, KA1KAQ]

Writing or marking presets for each band or often used freqs can get you close (maybe even right on if you have verniers or turns counters) without even applying RF. Then it's just a quick tweak and you're ready to roll.

I finally smartened up and adopted this approach for the big rig. Spent some time making up a chart for frequently used spots as well as some less-frequently used but strategic areas, to get me in the ballpark. It was extremely handy last night when moving up and down the band for the HMR. Easily touched up on the first transmission.

Works slick, except when I forget to change frequency. Yep, change the loading and plate tuning per the chart, then forget to crank the VFO and spot frequency....ooops. Good ol' CRS!

[K1JJ]

Works slick, except when I forget to change frequency. Yep, change the loading and plate tuning per the chart, then forget to crank the VFO and spot frequency....ooops. Good ol' CRS!

heh-heh...

Then the humiliation when we find out after 10 minutes of calling CQ on the "new" freq, we were absolutely cratering the original QSO we signed out from earlier.... yikes!

[I've been there]

T

[Opcom]

Writing or marking presets for each band or often used freqs can get you close (maybe even right on if you have verniers or turns counters) without even applying RF.

Just don't do it on the front panel like the previous owner of my NCL-2000!! AARGH! !@#$% it will not come off.

[W1AEX]

I eventually learned that I had to pull the cover off and look inside at what the load cap was doing when I turned the f.p. control ccw and cw in order to increase the loading properly.  That is still something I advise hams do when they get a new amp that they are unfamiliar with.  

That's really good advice Rob. Ameritron amps typically decrease capacitance (increase loading) as the loading control is advanced (turned clockwise or advanced to higher numbers). However, a run of amps escaped the factory set with the loading caps fully unmeshed, or 180 degrees out of synch, when the loading control was set at "0". Obviously, the loading would decrease as the Load control was increased in value and it caused all kinds of issues when new owners set their front panel settings "by the book".

My AL-82 that I purchased from eBay had (among other things) a loose coupler between the front panel vernier and the loading cap. It engaged nicely until I turned it CW and went beyond the 12 o'clock setting, where it would start to slip. Turning CCW it would pick up again and engage at the same point. It drove me crazy the first couple of days I used it because I could not get it loaded correctly on 160 meters. When I had the cover off to check the arc gap setting in the tank circuit I noticed the "slippage" and was glad I was able to end that mystery!

Like you said, it's a real good idea to actually look at what it's doing!

Rob W1AEX

[KM1H]

I finally smartened up and adopted this approach for the big rig.

I learned that after my first contest in 64 with the NCL-2000....and yes I marked the panel....but in pencil. That amp has been retired to shelf duty but still has the last markings as of the mid 80's

[Todd, KA1KAQ]

heh-heh...

Then the humiliation when we find out after 10 minutes of calling CQ on the "new" freq, we were absolutely cratering the original QSO we signed out from earlier.... yikes!

The times it really suck are when I moved to a frequency not too far from the one I was on. Then it just makes sense when I break in that the tuning doesn't need any tough up - it's close by (no, you dimwit - you're still on the same frequency, that's why!).

Other times I readjust the tuning and loading, only to key up and see the plates glow bright orange. The shorter key up duration causes minimal cratering, at least.

Just don't do it on the front panel like the previous owner of my NCL-2000!! AARGH! !@#$% it will not come off.

If it's indelible Sharpie pen, you can usually get it off with alcohol and some elbow grease, Pat. Just test it on the paint first, and don't leave a lot on the panel for any length of time. A Q-Tip works well for confining it.

If they carved it in with a screwdriver, all bets are off.

[flintstone mop]

I'll P-touch a note on top of my amp. I just checked and the higher loading numbers IS less capacitance in my Ameritron.
I start talking myself through QSY's like a pilot to co-pilot to avoid the bad stuff.

It's nice to see that peak RF watts schwangin towards 2kw............no grid current whilst talking in AM or SSB.........love ceramic pubes
Fred

[ke7trp]

Good work Fred. Do you use the ALC?   Alot of guys do not use ALC as they see this as just a way to limit power.  I like to hook up the ALC and then adjust it to where I want the max output to be on the amp.  The reason, I like the ALC is that my amp will throttle the Drive back on the radio.  Its one less thing to worry about while tuning up and makes the tune up that much faster. When the amp comes in tune, the ALC lets it fly to my limit. This way, i cant overdrive these little 800s and damage them. 

C

[The Slab Bacon]

heh-heh...
Then the humiliation when we find out after 10 minutes of calling CQ on the "new" freq, we were absolutely cratering the original QSO we signed out from earlier.... yikes!
[I've been there]
T

I did that myself one night way into the wee hours. Someone asked me If I could QSY to another frequency for a 1 on 1 QSO. I said sure, and retuned the transmitter and VFO (but forgot to move the receiver)

I went ballistic and totally lost it at the guy who had "cozyied up to our QSO again". I went off on a total flake-out as the operator that I lost it at was known for that. Only to realize that I hadn't moved the receiver. OOOOOOpppppsssss.................... I hate when that happens!! Needles to say, then came the humiliation of the appology.............