HV Arcing

[nutcracker]

Hi, I have a couple tube boxes, one phantom, and a pdx-400 that I just got going, but a problem with the 400 has me stumped. The final loading capacitor continues to produce a blue arc across random plates in no order, but only on the high setting, not on the low. This is feeding it with a 2.5w DK swinging 14 pep into a 50 ohm input and output load. I can tune it up to 650 watts, but the swr on the upper portion of the band is unacceptable and blue arc still persist.. It new parts including caps with bleeder resisters added, and all new chokes.. I originally thought one of the chokes may be open but all the chokes are like new in the orange wrapping. This amp was suppose to be setup for 11 meters but im not sure at this point... I never seen a pdx setup for 11m in person, but the tank coil just dosent look right either. I didnt even think there was suppose to be high voltage on the capacitor plates, almost seems as its oscillating? I am starting to think it was not properly modified for 11m, but since everything is so highly modified I couldnt even begin to determine that by looking at the STOCK 11m mods for this amp. Any help would be appreciated, if not Ill have a nice paper weight

[Night Crawler]

This amp was suppose to be setup for 11 meters but im not sure at this point but the tank coil just dosent look right either. I didnt even think there was suppose to be high voltage on the capacitor plates I am starting to think it was not properly modified for 11m

I think your right on that assumption also there isn't any DC high voltage on the plate or loading caps or shouldn't be if the plate blocking cap is good. What would cause the loading cap to arc is that the tank coil has too much or not enough inductance also once the plate or loading caps arc you could try and file or sand down the burrs but sometimes after their damaged they'll keep on arcing even with right inductance.

[nutcracker]

Well the arc seemed to get bad... real bad.. i keyed once and it seemed to start "welding" best as I can describe it on the bottom of the cap. Actually sounded and looked like a welding arc... seems like something is seriously wrong here. The previous owner said he did not have these problems with this box.. Just dont get it, or what to look at.

[DixieEcho82]

I'm not gonna really tell you to do this but sometimes when you just get FED UP enough you will GO FOR IT. One of my amps, i don't really remember which one, started doing this. Filed polished, bent, etc. Finally I was sure which one it was. These things swell when they arc, sometimes the polishing works and sometimes it does'nt. What did work though was a longnose pair of duckbill pliers. I pulled that thing like a bad tooth. Problem solved!!!

[nutcracker]

Lol that may be a bad idea, since it arcs at random points, it will sometimes arc in the same location, other times at another, so i may be left with 2 plates when done hah. And night crawler that plate blocking cap on the tank circuit is fine, no shorting there. Checked all chokes, everything checks out.. I dont get it, never did this in the beginning but guess I will never know. What were the stock modifications required for the tank coil on this? As of now it is 5 turns, tapped with solder at the 5th turn on the bottom, at the bend.

['Doc]

The spacing between capacitor plates is selected to prevent arching because of the voltage being applied to that capacitor. The two most common causes for that arching is dirt and too high voltages in the associated circuit of that capacitor.
If that capacitor has arced, one of the 'cures' is to polish out the pits caused by that arching, which can mean disassembling that capacitor. Certainly possible with some, impossible with others.
Unless that capacitor 'lives' in a really dirty environment, the typical way to stop that arching is to lower the applied circuit voltage. Don't 'push' the 'fire' out of it (pun intended).
Happy thought, huh?
- 'Doc

[DixieEcho82]

Hey man my stuff came off the antiques roadshow LOL. old VINTAGE. Just messing with you!!

[nutcracker]

The capacitor plates are all clean, dosent matter if I repolish them to a chrome finish it still just finds a new place to arc again! Almost seems like it is under coupling, but if i increase the final load the arcing may decrease, but it will not put out any watts, maybe about 80 but that is it. At the sweet spot, around 1/4 of a turn on the dial it will produce a 400w dead key and swing 850 but there will be a lightning storm inside.. I have read every single piece of information I can find about caps arcing, none can point me in the right direction! Frustrating to say the least.

['Doc]

Try not driving it as hard, lower input power to the amplifier.
- 'Doc

[nutcracker]

1/2 a watt carrier, modulating to 7 watts, just as bad as it is with 2.5 watts input. Didnt arc with the carrier, but as soon as I spoke I watch the arc form on the bottom of the plate. It makes a sizzling sound, like I described before "welding". Got a lemon for sure..

[crazy_cooter]

I had the same problem with a 12 tube amp....here is what my problem was.....

The HV relay had a hairline crack in it, and it caused high voltage to jump from one leg to the other and made my amp do the same thing. Inspect your relays with a magnifying glass thoroughly, and make sure they aren't pitted or that the plastic isn't cracked. The center relay is suceptable to this, and it's very common due to the fact that ALL the amps power goes through that one relay in the center of the chassis. Be sure to discharge the caps before fooling around inside one of these, but i'm sure you already know this. I've been electricuted many times! Let me know what you find, i'm almost posotive that might be your problem.

['Doc]

Another part of that arching problem is that you really do have to 'clean up' the variable capacitor after an arc. That means taking the thing apart and buffing out the arc points (at least two of them). Or, just replacing the thing. That won't 'cure' the problem, only part of it, you should still find out the 'why' of it.
- 'Doc

[nemisis225]

Hello I have the same problem with my PDX 400 I polished plates and sanded them , used compressed air,sprayed with hair spray,stopped arcing at low power