Arcing between adjacent tube pins on Ceramic tube sockets

[nevetslab]

I’ve had three different Orange Amps…a Thunderverb 50 (pair of EL-34’s), an AD-200 (quad 6550’s) and a Rockerverb 50 (pair of EL-34’s) that failed due to arcing between the tubes’ Plate and Heater pins (3 & 2) on two of the amps (Thunderverb & AD-200) as well as on a Rockerverb 50 tubes’ Plate and Screen pins (3 & 4).

Ceramic octal tube sockets on each amp, and in each case, the arc-over burned into the ceramic base, leaving a carbonized conductive path. Also carbon deposits on the base of the tube’s header. It took grinding away the carbon deposit with a Foredom hand grinder with a carbide burr to get to clean virgin material. And, after cleaning up the tubes (only one tube had failed of the 3 amps), I swapped tube locations to eliminate any further re-occurrence at the same tube/socket. No other damage occurred, amps worked fine after surgery.

I’ve only seen this happen on the Orange Amps with the ceramic octal tube sockets they use. Any clues as to why this occurs? I don't have any details on what speaker cabinets were used with the amps or other specifics, like mismatching loads. All are within CenterStaging,LLC's backline gear rental inventory

I posted this prior to seeing there was a thread on Power Tube Arcing from 2008.

[xjbear]

You can scrape a tube pin across a ceramic socket and leave a trace of metal on it, a little gray streak, which might add to the problem. I would certainly be looking very hard at the cab hookup and be vigilant about impedance matching. Usually thats what causes the majority of these arcs between the pins 2-3 filament and plate or pins 7-8 filament and catchode. Nearly always from shoving a load into the OT that its not dissipating.

[xjbear]

This for example is a '65 Dual Shown ran into a 16ohm 412 cab used for multiple gigs at loud volumes. The arcing was consistant and subtle and eventually ate away the bakelite, lost enough connection to finally arc big.

https://m.facebook.com/pages/Bear-Am...471527052?_rdr

[loudthud]

A substance called "Corona Dope" used to be used to suppress arching in tube TV sets. I don't know if it's still made.

[Enzo]

Yes they do, still a current product from places like Mouser and Allied and MCM.

I used to work on video monitors for the arcade industry, we used it on them sometimes. it is something TV repairmen used, among others.

[g1]

The stuff I have is called "liquid tape", made by GC electronics.
Dielectric strength is rated at 1400 v/mils

[tedmich]

You idiots are all wrong! Arcing between tube socket pins means you have to go sacrifice a $10,000 custom made silver mica cap on the alter of the Tennis.com CEOs 1000,0,00 square foot mansion, and clean the circuit board with the blood of a virgin ermine! 110% of the time I have seen this in my 46 years doing absolutely everything this has been the outcome, and anyone who says different is a complete moron, inbred or both.


SGM channelling over...

[J M Fahey]

You forgot to add that you invented the silver mica cap.

[TimmyP1955]

Are the ceramic sockets in question the unglazed type? First time I saw one it seemed to be a bad idea owing to the porosity. Don't think I've had a problem because of them though (knock wood).

[Mick Bailey]

The appearance of porosity is just that - the body is vitrified and isn't actually porous. Most glazed sockets are only glazed on the upper surface to prevent the problem identified by xjbear.

I wonder if the failed sockets are down to the tube being rotated in contact with the socket to orient the key and leaving a trace of metal? Perhaps wouldn't be a problem with 9-pin sockets due to lower voltages.

[nevetslab]

Corona Dope. Or Liquid Tape. Cool....something to track down and add to the chemical stash. After grinding away all the visible carbon (I work with surgical loupes & headlight to get a good look at the work area), that sounds like the finishing touch.....to coat over the now-rough surface of ceramic after the grinding operation.
Fortunately the amps are still in our inventory, with my notes on the repairs having the asset numbers.

Thanks for the advice.

[nevetslab]

You can scrape a tube pin across a ceramic socket and leave a trace of metal on it, a little gray streak, which might add to the problem.

That is a possibility. I'm not sure whose ceramic sockets Orange uses....they are glazed on the surface, and I don't recall there being any marks like that elsewhere on any of the tube sockets. I'll never know if there was one where it flashed over.

[nevetslab]

The appearance of porosity is just that - the body is vitrified and isn't actually porous. Most glazed sockets are only glazed on the upper surface to prevent the problem identified by xjbear.

I wonder if the failed sockets are down to the tube being rotated in contact with the socket to orient the key and leaving a trace of metal? Perhaps wouldn't be a problem with 9-pin sockets due to lower voltages.

An interesting view. While the surface of the ceramic sockets are very smooth due to the glazing, I'd think they would be less prone to accumulating any metal deposits while orienting the power tube to fit into the socket. The pins are always in contact with the socket surface until you find 'home' and push them into place.

The subject came up while talking with a new chap hired on in our show department, who has a friend building boutique amps, and has had similar problems in production....also with ceramic sockets. I'm about to install ceramic sockets (from CE DIST) in the Traynor YBA-1A I'm restoring, though I haven't had any issues with those in prior use. An odd failure.

[Leo_Gnardo]

Corona Dope. Or Liquid Tape. Cool....something to track down and add to the chemical stash. After grinding away all the visible carbon (I work with surgical loupes & headlight to get a good look at the work area).

Heck in the college hi vac hi voltage lab I worked in, we used ordinary GE RTV successfully in gear that went up to 25,000 V. Despite all the "theoretical" reasons why not - contains acetic acid etc. Point is, the stuff just worked, it's cheap & available. Of course we didn't use it on equipment that was going to be inside the hi vac systems, it would be outgassing 'til doomsday.

So that IS you in your avatar? Hmm I'll have to shoot a selfie with my magna-visor on, plus shooters muffs. Sometimes silly looking headgear is just what's necessary to get the job done. Where did you get your surgeon's loupe specs? I've been hoping to find a pair at some retired dentist's garage sale. So far, no luck. I know good ones are awful expensive new. My DDS says @ $700, OW!

[nevetslab]

Yup...the Avatar is me, tethered to the bench (and the phone always rings when the headlight is in place/on & iron or desoldering iron in hand.

The loupes are 2.5X, and are adjusable in both pupilary distance as well as focusing range...which is rare. They're a little heavier than Luxtec or the like. Most surgical loupes are fixed focus. These I found several years ago on ebay, made by Task Vision Type PL-75. VisionUSA - Task-Vision Loupe I like these due to their focus feature for each eye...it allows you to set your working distance and compensate for how each eyeball sees the composite view....and the fiber optic light illuminates your detail work zone, without having a bloody illuminated viewing lamp in the way!

The headlight system is from Applied Fiberoptics, which has a fan-cooled base unit that supports two headlights (or a spare when one of the bulbs finally burns out. Uses standard DNF 150W/21V halogen lamps....seems to be the industry standard in headlight systems.

My vision has gotten worse over the years, so I have a system I travel with, as well as one at each bench where I work.

There's another adjustable focus 5X loupe that I've been wanting, but those are approaching $500.