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Plate Blocking Cap and Pi Network Padding Caps




 
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Author Topic: Plate Blocking Cap and Pi Network Padding Caps  (Read 332 times)
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NW2K
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« on: March 22, 2020, 11:53:00 AM »

I'm experimenting with small CW amps for 160/80/40.  Power supply is 1500v/350ma.  Attached (I hope) is a picture of the caps I have.  One is 400pf/2500vdc that I thought I'd use for a plate blocking cap.  The other is 240pf/2500vdc that I thought I'd use across the variable tuning cap in the pi network.  I have similar looking caps that I could use as padding across the variable loading cap in the pi network.  I leak tested all to 500v.

There is no current rating on these caps.  They measure 1.75"x1.35"x.44".  To date, the amplifiers have had a plate load of somewhere between 1500-3000 ohms, depending the design and the antennas are 50 ohms.  I never cared about current rating and nothing has blown up yet, but I thought I would try to learn something.

How much current will these caps see and what is the ballpark rating on these caps?  Thanks, 73 Dean NW2K





* IMG_2766.JPG (78.68 KB, 648x268 - viewed 29 times.)
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K1JJ
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« Reply #1 on: March 22, 2020, 12:57:05 PM »

Hi Dean,

Sounds like a cool CW project.

At that power level, you will get away using those 2500VDC caps for a CW rig as a plate coupling (blocking)  cap.   Though, I would use a 500/1000 pF 5KV ceramic cap (looks like a TV doorknob) but better quality and sometimes a little bigger.  The Russians sell good ones on the web.

But in the pi network where there is circulating RF currents that multiply in value based on Q, it may explode. The currents can get tremendous there.  Use just air variables and vacuum caps in the pi-network loop.  (for C1 plate tuning and C2 loading caps)  (It may be more the RF voltage that kills them in the C1 position)

That said, I have used them as C2 160M and 75M LOADING padding caps in the pi-network with success. Higher frequency use can sometimes make them become unglued.  Much will depend on your final Q, frequency and the power run at.


I have a friend who used a stack of TV doorknobs to pad his C1 plate tuning cap in his 4-1000A linear amp. They exploded and smashed his chimney and tube.  Just want to make you aware.

T
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W4AMV
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« Reply #2 on: March 22, 2020, 05:24:49 PM »

I was curious about this as well. Taking a similar approach to a pi net for several 807 tubes. I set this up for 200 W at 1 k tube Z. So about 885 V peak. The pi match is at 7.1 MHz and the Qo is 12. The Z transfer is 50 ohm up to 1 k. You can run your conditions as well check my assumptions. The pi net parts are loss free, so that's worse case! The schematic and currents displayed in color. Useful to look at the un tuned off resonance case as well! Peak current through the block is under 1.5 A. Need to take rms value to get the equivalent heating. The other parts of the PI... more severe.

Ooops, that load cap is wrong. Try 1120 pF. The current thru the block a little less.


Alan


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* Tx_PI_NET_currents2.jpg (40.64 KB, 697x275 - viewed 20 times.)
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W4AMV
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« Reply #3 on: March 23, 2020, 04:04:18 AM »

Here is the block current vs. the load real SWR. No doubt this is different for reactive cases. I varied the real load from 10 to 100 ohms. An attempt to check what happens as we tune the antenna. Or re tune the match.


* Block_current_vs_SWR.jpg (48 KB, 503x548 - viewed 29 times.)
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NW2K
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« Reply #4 on: March 23, 2020, 09:07:55 AM »

Alan, Tom, thank you.  I did not realize that the currents would be that high.  I need to hunt down some bigger variables and proper fixed caps.

That caused me to learn a little about the RF choke across the output.  Most of the time, I see little 2.5mH/100ma chokes there and that now seems like a very bad idea.  I've also seen a power resistor across the output.  I'll make a 200uH RFC from #22 or #24 magnet wire for that position so it should survive at these power levels.

I also looked at glitch resistors and fusing B+ and found some notes/pictures on using thin wire fuses.  If I use something like a few inches of #36 wire, can it be placed either before or after the glitch resistor or does its relative location matter?  Thank you, 73 Dean NW2K
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W4AMV
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« Reply #5 on: March 23, 2020, 10:07:33 AM »

Hi Dean,

I used one strand of wire from the shield of RG58 coax cable to make a glitch fuse.

I took single sided pc board and stripped away the copper and formed a channel to lay the single strand of wire down. I left just a pair of solder pads topside on either side of the card to solder the wire down as well bring HV to the card and connect the RF plate choke to the other pad side.

So the glitch fuse is between the HV and in series with the plate RFC on the COLD or RF decoupled side.  For my case HV was 1300 V. There may be a better technique at higher HV values. I used... added (overkill) a set of low value HV resistors in series with the glitch fuse, about 10 or 15 ohms to assist in limiting current from flash overs.  I can try to take a pix if needed.

Alan
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W4AMV
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« Reply #6 on: March 23, 2020, 10:18:42 AM »

I need to hunt down some bigger variables and proper fixed caps.

Others can chime in on this. Although the simulation peak current seems high, the rms value is lower. And the simple simulation assumes sine wave source voltage. A class C tube envelope current is more pulse like so this is another approximation. I think what is key here is the Q factor or the dissipation factor of those molded caps. I have measured them at .001-.002, a Q of 500. So the series R is quite small! And hence the dissipation heat is just several watts. Now this was only an input of a few hundred watts. Clearly at 1 or 2 kW, a whole different story. I think for your application they would be fine.

One other key point. The extension or padder caps will be LESS than the variable and no larger. So, the current divider that is now present reduces the current in each subsequent padder cap. If you need say 1000 pF total and have just 300 pF max variable, then 7-100 pF pad C's with a successive shorting bar switch would play out nicely.
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KD6VXI
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« Reply #7 on: March 23, 2020, 11:00:42 AM »

I had a '10 meter' amp on the bench that used 1x3x6 6lf6 tubes at 1.2kv come across my bench long ago.  Used a pair of those caps in parallel as Cblock.

That amp was 30 years old at that time.  And they worked fine.

That amp was capable of over 1.5kw pep. 

I think you'll be fine as a padder or block.

--Shane
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