Arcing in RF Deck

[K8RS]

I have developed a mysterious arc in my home brewed pair 4-400s AM amplifier. It runs at either 1350 vdc or 2700 vdc and is driven by a Johnson Ranger II. The amp has worked flawlessly for a few years, but has recently developed an arcing problem around the bottom of the HV choke whenever I try 2700 vdc. Everything works fine at 1350 vdc. The choke is the High Power Transmitter Choke (RFC-3) from RF Parts. It is bypassed above the chassis with a 500 mmf 20k vdc doorknob capacitor. The arcing was coming from the bottom of the HV choke where a 3/4” metal clamp terminated the choke’s windings. I found carbon tracks running down the ceramic at the base of the choke toward a 6/32 binderhead screwhead on the chassis close to the choke.

This arcing started when one of the 4-400s arced internally during a 2700 vdc CW session.
 
My initial attempt at solving the problem was to (1) remove the 3/4” metal clamp from the bottom of the choke and run the bottom winding directly to the top of the bypass capacitor, (2) change the 6/32 binderhead screw with a 6/32 flathead screw and countersink the screw and (3) to place a 2” x 2” piece of HV tape on the chassis centered under the choke.  These changes did not solve the arcing when I went key down with 2700 vdc on the plates of the 4-400s But I couldn't find where it was arcing.

I am at a loss to know what is going on.  The amp worked perfectly for the three years since I built it and has showed no sign of parasitic oscillation.

You can view the amp at K8RS on QRZ.com website.
                                         Roger K8RS

[N2DTS]

First you need to figure out where its arcing now.
If at the choke, it should show.

I always mount the chokes on a ceramic standoff.

[K1JJ]

Try electrically floating the choke on a piece of wood or Plexi. Then place a big mirror behind  it and darken the room. Stay clear and fire it up to try to see the arcing.

If there is no arcing, then ground whatever points you had before and try again.

Also, ceramic standoffs and posts with internal bolts can sometimes arc internally thru the ceramic.  I've lost a few like that and are very difficult to see unless they are pulled apart or explode first.

T

[w4bfs]

Also, ceramic standoffs and posts with internal bolts can sometimes arc internally thru the ceramic.  I've lost a few like that and are very difficult to see unless they are pulled apart or explode first.

steatite is only a good insulator when completely glazed .... anywhere it is not is hygroscopic and will not be a good insulator .... iffin you need or want to use one as an insulated feed thru then use teflon tubing(s) where it counts

[N8ETQ]

  It Happens...

   BTI LK-2000 w/3CX1200...  Beast!

/Dan

[W2VW]

  It Happens...

   BTI LK-2000 w/3CX1200...  Beast!

/Dan

Nice destructive yellow color like the sun.

[flintstone mop]

I had a T-368 that did that and would pop the breaker, when modulated.
Your problem is different in a linear amp.
A parasitic possible?
Maybe just a voltage breakdown point after 1350vdc?
3CX1200A7?? that is a real big TUBE!
Fred

[kb2vxa]

Assuming a full wave rectifier and not a doubler, put the choke in the negative, grounded lead.

[Steve W8TOW]

Parasitic....
I got the same problem in an amp....could a parasitic suppressor
have gone bad?
gl es 73
Steve 8tow

[Detroit47]

I wind all of my plate chokes on Teflon. I have seen to many ceramic chokes explode. Also once it arcs throw it, it will do it again. When I was doing Dielectric work we used exclusively Teflon. Don't want to shut down an assembly line because of a stinking choke. The customers tend to get irate. Just my 2 cents.
john 

[Opcom]

  It Happens...

   BTI LK-2000 w/3CX1200...  Beast!

/Dan

Nice destructive yellow color like the sun.

When it is green you know you are getting into the copper.

/\/\/\

My plate choke is on a 3" standoff and volts are <3500.
It just happened that way, but it's never arced there.

Hygroscopic? so, could old Steatite be baked at 300 deg for a day and the moisture be driven out? That seems interesting as a process.

[w4bfs]

Hygroscopic? so, could old Steatite be baked at 300 deg for a day and the moisture be driven out? That seems interesting as a process.

that is a good question, Pat .... it seems reasonable as long as there are adequate vents to allow the water vapor to disburse .... then reseal with perhaps urethane varnish ... the important areas then become the threaded mounts .... they will need sealing as well during assembly

[N2DTS]

I assume its the RF choke in the final, not a power supply choke.

You need more space, if something arc's, its too close.
Once a track forms, its toast, but mounting it on another ceramic insulator may work.


Peak voltages can be VERY high, at least 4X the plate voltage.

I used to blow out 14KV diodes in a NCL circuit running 2000 volts on the rf deck...

[W3GMS]

Roger,

I am not going to give you design changes, but will instead try and focus of what went wrong since your design did work well for 3 years. 

Not sure if you have checked your RF bypass capacitor or not.  If it fails or partially fails you will have some amount of additional RF present at the base of the choke.  You have inductance from the output of your P.S. to the base of your choke and then ultimately to the plate of your tubes.  I would in your case sub another known good RF bypass cap and see if the problem is still present. 

Over time, a certain amount of contaminants can get on the plate chokes form.   So as you saw, conductive paths can present themselves.  Sometimes its very difficult to clean it well enough so you may want to sub another RF choke. 

Again, your original spacing requirements did work at the 2700 VDC level for 3 years so focus on component changes and not immediate design changes.  After all is said and done and you have fixed the problem you may choose to do some of the design changes suggested and it will be easy to pick which one(s) when you know what changed that caused the problem. 

Let us know how this turns out.

Joe, W3GMS