Linear Pi-Network Question

[ka1tdq]

I changed out the inductor I was using in my two 3-500 homebrew linear.  Both were about 6uH on 40 meters.  Before, I would get 200 watts carrier and 1500 watts peak.  Now, I'm getting close to 400 watts carrier and just over 1000 watts peak.  However, the exciter is capable of >100% modulation.

I want to get back to the original numbers.

What has changed now is that I'm using less plate capacitance than before.  

Jon
KA1TDQ

[KA2DZT]

Q of the coil is less than before if you need less plate capacitance to resonant.  Although, your measurements seem to be wrong somewhere.  Try remeasuring everything to see if you get the same results.

Fred

[W1ITT]

There are many L-C combinations that will appear to "work" in the pi-net of a linear amplifier.  A proper setup will be more linear than others.
First, figure out the plate load impedance that you anticipate at desired operating conditions.  Obtain a resistor, non inductive as best you can get, not high wttage, and connect it from the plate blocking capacitor to ground.  Leave the tube(s) in place and don't turn on any voltages in the rig.  Then, using an impedance analyzer looking back into the amplifier (looking INto the output port) adjust the pi-net  until things match.  I prefer a swept network analyzer, but one of the common MFJ or similar analyzers will work.  It'll just take a bit more time.
I used this technique on a few homebrew amplifiers, as well as a few overseas short wave broadcast transmitters, up to the 300 KW level, where we had to find tuning conditions for many broadcast frequencies without stressing the transmitter doofing around with it.
In the case of one linear amplifier that I measured, I took the 3rd order IMD from the high teens to over 30 db down.  When the pi-net is set properly, the tube is properly loaded over most of the operating cycle, rather than just a part of it.  And that's one of the things that makes a linear amplifier linear.

[KD6VXI]

I think the easiest way is:  put the old tank coil back in.

--Shane
KD6VXI

[ka1tdq]

I would, but it used PVC pipe for a form, and the new one uses just air and is beefier.

I'm going to remove one turn off this coil and see if plate capacitance goes up. It should. I don't have any test equipment besides a multimeter and a wattmeter. If I screw up with this coil, I have another one new in the box (hamfester special). New coils, unopened in hermetically sealed packages since the 1960's).

I've got a warm fuzzy on this one. I think this will get me back to where I need to be.

Jon

[KD6VXI]

Well,  that could pinpoint it.   You said both where about 6 Uh.  Then later mention the black of good test equip.

Yeah,  I'd say your on the right track.

Remember,  max q generally coincides when the inductor h and w are about the same.


--Shane
KD6VXI

[W3GMS]

Make sure your tuning the amplifier for 1500W carrier output power.  Then reduce the drive until you have 375W of carrier output power but do not touch the tuning and loading from your 1500W settings.  Now with the applied power of 4 times the carrier power for 100% modulation you should be able to hit the 1500 peak power providing you exciter is capable of it. 

Joe GMS 

[N2DTS]

You can short a turn as a quick test.
What are you driving it with that gives 200 watts carrier and 1500 watts pep?
That must sound screwed up in a normal receiver, almost 200% positive modulation at 90% negative?

I can get some high peak power out of some screen modulation setups, but nothing like that.
Loading is the thing, if my stuff is loaded very high, carrier power goes down, plate current goes up, and peak power goes way up.
Its likely better for me to run 300 watts carrier and 1200 watts pep then 200 watts carrier and 1500 watts pep.
With 1200 watts of plate dissipation I can run up to about 350 watts carrier and almost 1500 watts pep.
I could also run 400 watts carrier and 1200 watts pep or almost any combo of carrier and pep power.

I use plug in coils and can swap them out quick, and it does make a BIG difference in how things tune up and work.

Maybe make some jumpers and experiment.
  

[ka1tdq]

I guess it's all a question of loading.  I did remove a coil turn and it helped a tiny bit.  Then I messed with the antenna loading capacitor setting.

At first, I tuned for maximum output.  My antenna loading variable capacitor is the same as the plate (360pf max), but I paralleled a 470pf doorknob with it as well.  Maximum power output occurred with the mesh about half way.

Then I backed off on that and put the mesh to minimum (minimum capacitance) so that I'm dealing with about 500pf of loading.  The transmitter liked that a lot better and my peak power went up to around 1400 watts. 

Now that it's 1am and happened to be up, I put a 330pf doorknob capacitor on instead and the mesh is about half.  I'm back to about where I was.  Carrier power is 250 watts and voice peaks are at 1400-1500 depending on how hard I hit a "P" (and without screaming into the mic).

Jon

[WD5JKO]

   Jon,

   Keep in mind that a lot of those door knob capacitors around 500 pf are meant more for bypassing instead of being used as part of a high Q tuned circuit. The dialectric will heat from the RF current, and the capacitance will change as the capacitor heats up. The more stable types are labeled NPO or COG. It is sometimes better to use several smaller values in parallel so that they will divide the RF current by the number of capacitors used.

  If you have a way to measure the linear RF input power, then you can get an idea of the stage gain. For example 20 watts input and 200 watts output is 10 db  ->  10 * Log (Po/Pi). Also if you can modulate with a triangle wave, adjust the loading for a straight ramp. If you adjust the loading for maximum carrier, the ramp will curve as audio drive is increased. The trick is to find where you can get 100-125% modulation peaks as the carrier level is brought up. There will be a point where any increase in carrier is met with decreased modulation percentage, and of course a non linear ramp. With a scope, and two watt meters you can juggle the variables: Loading, RF Drive, and Audio Drive. Get energetic and add some more bias to the RF tubes...something called Class BC.

   I recall you don't have a scope at home....you need to work on that Jon

Jim
Wd5JKO

[ka1tdq]

I know, I know... I'll get a scope eventually. It's especially necessary with solid state transmitters. Tubes, you can still guess a little.

I'm using a Ruskie doorknob rated for 4kv on the output. The output tuning capacitor sees less voltage anyway. For the plate blocking cap, I'm using a known good American rated for 7.5kv at 500pf.

And point taken on tuning up for 1.5kw. I can just pump in 100 watts from my ricebox and then switch over to my AM rig.

Jon

[K1JJ]

I know, I know... I'll get a scope eventually. It's especially necessary with solid state transmitters. Tubes, you can still guess a little.

I'm using a Ruskie doorknob rated for 4kv on the output. The output tuning capacitor sees less voltage anyway. For the plate blocking cap, I'm using a known good American rated for 7.5kv at 500pf.

And point taken on tuning up for 1.5kw. I can just pump in 100 watts from my ricebox and then switch over to my AM rig.

Jon

Jim is correct.

It's circulating current in the tank circuit that kills. In this case that cap is part of the tank circuit. If it's rated at 4KV, it is probably designed for low current like what a coupling cap would see. (1A?)

Anyway, the formula for tank current is Q* plate current. So if the Q is 12 and your plate current peaks are 1A, that's 12A of peak RF circulating current. (On 10M that's quite a strain)

You might be OK depending on the current sharing split between the variable loading cap and this fixed cap... though I have personally blown up a few of the higher quality doorknob caps myself in tank service in the early years.


T

[Opcom]

Had a finger burned by a 500pF 30KV doorknob. Good thing I was testing and intentionally checked it or it might have failed while in use and made real trouble. Its why I use five of them on a 4-1000 at 1-2KW. Those have not failed.

[K1JJ]

I should add that those grossly overheated doorknob caps can go off like a grenade. A friend of mine once used a totem pole of them selected by a rotary switch as the plate tuning cap. YIKES!  One of them exploded inside of his 4-1000A amplifier and made a huge mess. He was surprised that the glass chimney survived. Those 4X1 chimneys are tough birds.

We don't get off with a pansy-hiss like when a PO'ed electrolytic cap goes south.

T