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Author Topic: Plate choke exploded  (Read 21947 times)
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K1JJ
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« Reply #25 on: November 20, 2013, 11:21:46 AM »

Actually, that's a duplicate ground for the grid. Im still going to put copper flashing on though.

The filament leads are under the chassis and steel ground. Should be fine.

No antenna relays. Beverage antenna for rx and dipole for tx.

Jon - on cell with small screen. Im getting a huge Samsung Mega tonight! The wife decided to treat me.


Good on the fil leads being already shielded.

Yes, that LONG, thin grid lead to ground could be a reason for the TPTG oscillation.  When you later ground it with wide copper strap to ground, make the connection as short as possible. (1" long) Do it separately for each tube. You can run a grid strap between the tubes too, if you wish.   Under the right conditions, even a few inches is enough to cause a VHF parasitic to come to life.

I don't remember, but if there is more than one common grid pin internal tube connection in a 3-500Z, strap them all to ground.  Since there is no screen to bother with, the grid is the critical element to worry about.

I see you are using an aluminium plate for the chassis. Look at all RF paths and make sure they ALL get grounded directly to that plate with short copper flashing connections.  All fil caps, grid leads, RF plate bypass cap, C1, C2, input tuning caps, safety choke, swamping resistor/cap, *everything* related to RF needs to go to that common plate groundplane.  Connect your exciter to it thru a thick braid too, as well as the shack common ground for audio, HV power supply, etc..



You should have much better luck next time... Wink

T
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« Reply #26 on: November 20, 2013, 05:34:14 PM »

Yes, safety aspects noted.  I'm building the plexiglass housing now and everything will be totally safe.  I've got a baby boy on the way and a Persian cat that I want to keep around (not to mention my wife and I). 

The sooner the better!  In an instant, your wife can be a widow and your child without a father, my friend. Do you have that guard around the PA yet?

When I was working near the pi network on my homebrew 833C amplifier earlier this year, I found myself waaaay too close to live 3500 VDC. I decided it was time to stop right then and install some hardware cloth screen around it.

And you need a chicken stick, if you don't already have one. Voltmeters and their dropping resistors can fail without warning.

Don't let us see your name in the Silent Keys section of QST!  Smiley
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73 Mike 
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ka1tdq
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« Reply #27 on: November 20, 2013, 05:40:10 PM »

I'm selling an old Admiral 399-6M (working!) tabletop radio on eBay right now, and when it sells I'm and going straight to Home Depot to get my HV section plexiglass.  When that plate choke exploded, it reminded me instantly about how much power is sitting there.  That should be an exercise for newbies... blow up a plate choke.

Jon
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« Reply #28 on: November 20, 2013, 06:39:41 PM »

Sounds like a plan, Jon.

The following experience should be required reading for every tube amp builder. :-)
https://www.eham.net/ehamforum/smf/index.php?topic=72496.msg490120#msg490120
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Patrick J. / KD5OEI
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« Reply #29 on: November 20, 2013, 07:33:07 PM »

That's a great looking amp. I like how everything is accessible and you can work it hot if you have to. Mine, like an iron maiden I have to clamp an interlock to work it live and you can't see anything.. but once the bugs were out that has not been necessary to have it open.

It would be nice with Lexan covers so its beauty could still be seen and the dust would stay out better. Another member here built an amp with clear panels, although it was not so interestingly illuminated. You could also engrave the covers/panels with the text or ornate patterns of your choosing and light those, as were the old ruled oscilloscope graticules lit.

A suggestion if you do want to cover it for example with Lexan or Plexiglass to show it off while preventing dust getting on it, might be to put magnets on the frame and epoxy little steel discs to the lexan and 'stick' the covers on to enclose each side as you like.Easy on/off.. not pesky.
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« Reply #30 on: November 22, 2013, 09:20:19 PM »

Regarding electrical safety, everyone working around high voltage should view this video...

http://www.youtube.com/watch?v=hfnEuRA7-vo
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« Reply #31 on: November 22, 2013, 11:52:26 PM »

Regarding electrical safety, everyone working around high voltage should view this video...

http://www.youtube.com/watch?v=hfnEuRA7-vo


Most radio people working around high voltage should not be. 99% of the work can be done with the power off.
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KA0HCP
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« Reply #32 on: November 23, 2013, 12:28:37 PM »

Thanks for the safety reminder.  I just pulled a power supply from the shelf last night to begin restoring it.  A piddly 800v compared to  2300v, but I promise not to take short cuts.  b.
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« Reply #33 on: November 23, 2013, 10:34:17 PM »

Jon,

From where I sit you are far too cavalier about the HV and the danger.

Imo, if you are not scared out of your wits each and every time you are near that stuff, then you're
a prime candidate for a pine box. You should be scared silly, especially at 4kV.

That's serious HV and hyper dangerous.
It can and will reach out and grab you.

Plexi/lexan is ok up to a point.
In my view at minimum the tubes + local parts should be in a faraday cage.
It could be as simple as wire cloth/window screen (it comes in different weaves), held up
by a frame.

There is a REASON that amps and other HV gear uses interlocks and double shielding.

The POWER SUPPLY MUST BE GUARDED and be unavailable to be touched by anything or anyone at any time, regardless of what unexpected events are taking place that hurtles them into the amp!!

I'd buy the magnet hold down ONLY if there were also pins that hold the plexi in place, so they can not be removed by simply sliding them off.

Ok... think about this more carefully, please.
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The Slab Bacon
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« Reply #34 on: November 26, 2013, 12:04:51 AM »

When that plate choke exploded, it reminded me instantly about how much power is sitting there.  That should be an exercise for newbies... blow up a plate choke.
Jon

Nah........... When I first built my 4X1 rig, I was testing it into a dummy load. I was slowly bringing up the power and the audio. At about 500w of carrier, I socked an atomic yaaaaeeellooo into the mic and blew the plate blocking caps to bits! It went off like a blockbuster!

After I cleaned my drawers out, I looked to see what blew. When I saw the caps blown to bits, I sat back with a big shit-eatin grin and laughed. That crapout was a good thing! ! ! !    Grin  Grin

Frank
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K1JJ
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« Reply #35 on: November 26, 2013, 12:38:53 AM »

heheheh....

Speaking of blowing up caps, I think we all make this mistake in the beginning: We try to use a string of fragile 500 pF TV doorknob caps as a padder for C1 in the plate tank. They cannot handle large RF circulating currents as found in a KW resonant tank circuit.

A friend of mine built a 4-1000A linear and didn't have a plate tuning cap. He made a series stack of five 500 pF door knobs that were selected by a bandswitch - using a roller inductor as the tuning.  He fired it up and BAM!  I was gagging from laughter. There was tan doorknob material all over the inside of the amp. What a mess. He's lucky he didn't blow a hole thru his chimney and 4X1 tube glasswork.  The amp worked FB once he replaced the doorknob garbage with a conventional air breadslicer cap.

T
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« Reply #36 on: November 26, 2013, 10:54:23 AM »

I think we all make this mistake in the beginning: We try to use a fragile 500 pF TV doorknob cap as a padder for C1 in the plate tank. They cannot handle large RF circulating currents as found in a KW resonant tank circuit.

Funny you should mention this. About the time you posted this, a 90 pF 5 kV disc ceramic padder across C1 in my homebrew amp failed. It just shorted and smoked, nothing spectacular. I was loading the amp into a high SWR with ~3500v on the plates when it happened. I wonder what the best type of cap for that purpose is, other than one with a higher voltage rating?
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73 Mike 
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« Reply #37 on: November 26, 2013, 11:44:51 AM »

from the pics it looks like the plate blocking cap in this amp is one with a 5kV rating. I would wear safety glasses around that amp.
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« Reply #38 on: November 26, 2013, 11:53:34 AM »

One has to consider both the DC blocking capability of the capacitor, and (separately) the ability of the capacitor to handle AC (i.e. RF) current.

For example:

An RF bypass capacitor from the modulated B+ line to ground on the power supply side of the plate choke has to handle the peak value of the modulated B+ DC across it, There is essentially no DC current flowing through the capacitor, unless it is defective... and therefore no associated heating due to DC current passing through the series resistance of the capacitor. In this application, the RF voltage across the capacitor, and the associated RF current passing through it is very small. Therefore, there will be very little heating caused by the capacitor's RF dielectric losses or RF current passing through the capacitor's series resistance.

On the other hand, as Tom pointed out, a padding capacitor in parallel with the plate tuning capacitor will have a large amplitude (roughly equal to the value of the B+) RF voltage across it, and a correspondingly high RF current flowing through it. Since the padding capacitor is not ideal, the high RF voltage across it with cause high dielectric losses (large amounts of heating), and the high RF current flowing through the capacitor's series resistance will also cause large amounts of heating.

The situation regarding a plate blocking capacitor is in between the extremes of: a modulated B+ line bypass capacitor (full modulated B+ across it, very little RF voltage across it, and very little RF current flowing through it); and a padding capacitor placed in parallel with the output tuning capacitor (no modulated B+ across it, but very high RF voltage across it, equal in peak amplitude to the peak of the modulated B+; and an RF current passing through it that is some significant fraction of the total current circulating around the output tank circuit).

The plate blocking capacitor will have the modulated B+ (audio frequency modulated DC) across it; but it will have an RF voltage across it whose peak amplitude is much less than the peak value of the modulated B+; and the RF current passing through it will be approximately (1/Q) x the RF current circulating around the output tank circuit.

For the tuning capacitor padding application you need a padding capacitor specified to handle the peak RF voltage that will be across it (and the associated peak RF current that will be passing through it) at the highest frequency that it will be used. The RF peak voltage across the padding capacitor will be equal in amplitude to the peak value of the modulated B+; which is equal to: the B+ at carrier x (1 + the plate modulation index). The peak RF current passing through the padding capacitor will be a fraction of the total peak circulating RF tank current. The total peak circulating RF tank current will have an amplitude of (roughly) the average DC plate current (at carrier) x the Q of the output tank circuit x (1 + the plate modulation index). The fraction of this total current flowing through the padding capacitor will be: [(the padding capacitor's capacitance) / the total tuning capacitance, including the padding capacitor's capacitance, the "bread slicer" capacitor's capacitance, and any other padding capacitors' capacitances)]

Stu
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K1JJ
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« Reply #39 on: November 26, 2013, 11:58:09 AM »

I wonder what the best type of cap for that purpose is, other than one with a higher voltage rating?

Well, for C1, (plate tuning cap)  the very best method is to use a 500 pF @ 10 to 15 Kv  vacuum variable cap with counter, of course... Wink   In the end, this is sometimes the cheapest and best quality way to go. Worth the extra $.

As for fixed padding caps, the Russians sell a few lines of very good quality fixed mica caps for reasonable prices that are physically large and more apt to handle high currents. I've seen some 15 KV units the size of lemons or even oranges which should handle big RF currents.  They can be found on eBay as well as a few importing American dealers with websites.

The common 500 pF TV door knobs are rated at 30KV, but at currents I would guess to be less than an amp. I have used them for padding in loading circuits for antenna tuners, but even that is pushing it.  

For amplifiers, the Q * I  = total circulating current in a plate tank circuit is the problem. At a Q of 12, 1 amp of plate current becomes 12A of RF that is exchanged between C1, C2 and L1... yikes.

Once in a while big fixed caps show up at flea markets for cheap.  The common 1000 pf @ 5KV fixed tan mica caps should not be used in the plate tank or for plate coupling caps in rigs bigger than say 1000w pep.  I know it is commonly done, even in commercial 1500w pep linears, but it pays to stay conservative. For 1KW AM rigs, we need to go bigger in both voltage and current ratings. (size matters)

I'll bet the guys here can suggest some more high quality fixed RF caps that are available on the web.

T
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« Reply #40 on: November 26, 2013, 12:54:59 PM »

At freqs. Up to 50mhz or so, ceramic capacitors with leads are perfectly acceptable provided sufficient number are put in parallel and leads are short.   Alpha for example uses a gang assembly of five  75pf/18kv ceramic caps for the 160 meter pad in their PI-L output .
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« Reply #41 on: November 26, 2013, 01:24:40 PM »

from the pics it looks like the plate blocking cap in this amp is one with a 5kV rating. I would wear safety glasses around that amp.

The blocking cap in my dual 833C amp has two .001 uF/10 kV axial-lead disc ceramics in parallel. It was the 5 kV padder next to them that failed.

Thanks for the other helpful posts.  Smiley
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K1JJ
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« Reply #42 on: November 26, 2013, 01:30:45 PM »

At freqs. Up to 50mhz or so, ceramic capacitors with leads are perfectly acceptable provided sufficient number are put in parallel and leads are short.   Alpha for example uses a gang assembly of five  75pf/18kv ceramic caps for the 160 meter pad in their PI-L output .


Yep, the wisdom in this approach is that they are using only 75pF caps at 160M, which splits a relatively small RF current thru each individual padder cap. Very high voltage rating too.

T  
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« Reply #43 on: November 27, 2013, 01:01:23 PM »

In this case, the impedance of each of the parallel 75pF capacitors, at the highest frequency in the 160m band, is:

-j / [ 2,000,000 Hz x 2pi x .000000000075F] = -j1061 ohms

If (for example) the B+ is 3000V, and if we assume that the amplitude of the RF voltage across each of the parallel tuning capacitors is equal to the value of the B+...

Then the amplitude of the RF current flowing through each of the 75pF capacitors is: 3000V/1061 ohms = 2.8 amperes. The rms value of the RF current flowing through each 75pF capacitor is 2 amperes.

Spreading the RF current among 5 padding capacitors in parallel (as well as the bread slicer tuning capacitor) is a very good thing to do... but keep in mind that each of the 5 parallel padding capacitors still has to handle 3000V peak RF across it (in this example), and 2.8A peak flowing through it (in this example).

Stu


At freqs. Up to 50mhz or so, ceramic capacitors with leads are perfectly acceptable provided sufficient number are put in parallel and leads are short.   Alpha for example uses a gang assembly of five  75pf/18kv ceramic caps for the 160 meter pad in their PI-L output .


Yep, the wisdom in this approach is that they are using only 75pF caps at 160M, which splits a relatively small RF current thru each individual padder cap. Very high voltage rating too.

T  
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« Reply #44 on: November 27, 2013, 03:01:19 PM »

Wandering off topic a bit......

Any quick way to test padding caps in ckt? My VK2 has a bad cap somewhere, and I don't want to shotgun the whole thing by replacing all the caps. Nor do I really want to unsolder everything, and add caps to find the bad one. If I can help it. And all I have are some chinese scope probes.....


klc

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