The AM Forum

I was checking over my 4-1000A pulls and found one had a small glass dimple deformation in the glass. It's about the size of a bee bee and goes inward. It's near the lower 1/3 of the tube. There is no damage to the tube plate at all, so it was not made by a hole in the plate.

Years ago I had one like this and eventually after a year the tube imploded as the dimple failed. Unlike this one, it was caused by a hole in the plate creating a hot spot in the glass. This one seems to be a manufacturing defect. The name has long wore off.

I plan to put the tube into service soon, so I put a large drop of clear, slow setting epoxy to cover the dimple, hoping it would help to support and fill the glass in. But maybe now the epoxy will not let the heat radiation thru as well and make the surrounding edge even worse, dunno.

Any ideas on this?  Should I pour another small layer of epoxy to build it up more?

Tom, K1JJ

 Hi Tom, 

I'm dissappointed...   We've not met, but by your posts I can tell you are quite the technical guy.

Epoxy?  No... duct tape!

Seriously, I think your assessment is correct - mfg defect.  I've seen lots of sweep tubes suck in, but most of them are not (were not) made with high temp glass. 811s too, but like you say, usually right opposite one of those nifty swiss cheese plates.

I had one 4-1000A with a similar defect & used it during early testing on a 2X4-1000 amp I built - got them both very very hot on purpose to stress the power supply.  Not many hours for sure, but never a problem.  I swapped it out when the amp went 'into production' - but only because I had another pair that were very closely matched.

Now, all that said... that particular tube did fail about a year later during a routine swap out (I like to 'cook' the backups yearly). It literally went up in smoke upon application of filament voltage. Clearly an air leak had occurred.  But I think it was an electrode seal, nothing was visible at the site of the dimple.

So, take off the epoxy and add a little video camera to the RF deck.  Pour the coals to it!

73, Bill   N2BC

I would think putting anything on the glass would make the potential for disaster worse.  Read somewhere about even fingerprints on power toobs can cause thermal spots that can weaken the envelope.

Put a fuse in it and use the piss out of it till it craps out.

Tom,
My first 4-1000A an American Penta Labs had a bubble in the glass and never failed.
Don't touch it or it will be sure to fail. Too bad I sold all my spares to the Slab in a covert operation carried out by Mikey on the Main Street of Stafford in broad dayllight.
Then we checked out the custom bikes in a shop up the street.

don't bother Tom...expansion coeficient will cause the epoxy to flake off while creating a hot spot.

Brent

Thanks for the info. guys!

Well, based upon comments, it appears like it wasn't such a good idea to add the epoxy.  In fact, I looked at it this morning and the "slow setting" stuff flowed into a spot about the size of an oval nickel.... ::)

Not sure what to do. I could use a sharp razor to move it maybe, or leave it there.  I plan to run this one with plenty of air as one of the switch tubes in the PDM rig, so it will not see any plate color.  I've got FIVE 4X1's  going in service with the various new projects, so we're scraping the bottom here for tubes.

Maybe I'll let it run for a short time like this and see if the epoxy flakes off as you said, Franz, or it starts to do funny things to the glass.  It might "fix" itself one way or the other.

John, you might be right about the dimple  NOT being caused by a hole in the plate meaning it may still be structually strong. But it does have a hole inward on one side and does not seem to go outwards on the other side of the tube - the thickness is not uniform like the rest of the glass envelope.

Back in 1978 I used my old, now defunct 4X1 linear to heat my bedroom. It stayed on all night with HV on, keyed, at high idle. After a few months of this, while asleep, I was awoken by a big bang as the breaker blew. The tube had imploded with broken glass everywhere. The dimple had gotten progressively worse and just opened up.  But, this was the result of a hole in the plate beaming into the glass.

T

Tom,
Run the tube with fil on and heat up a little I bet it pops right off.
A soldering iron will sometimes break it down. Anything that blocks light will cause a hot spot on the glass. Heat SWR.

Tom.... go to   w7jhs.com    click on 2005 tube shoot....   oh yeah!..Steve

Wow , what a great target it would be!

Well, I sliced off the epoxy patch and cleaned it thoughly. Back to stock.

Under a magnifying glass I notice the dimple has long 1" lines coming to and away from it's center. Like a ripple in the glass that terminates at this dimple. Maybe it will be OK.

Speaking of glass breaking, Steve, I once had four 750TL's here back in 1972. Two used as finals. One by one the filaments broke due to the lack of tungston from WWII.

One sat on a shelf looking pretty. One day the cat knocked it over. I watched it as it teetered and slowly fell to the ground. What a smash.  Should have had it tied off.

The last one I gave to the The Derb who put it in some museum down in MD.

T

Once I saw a Gates BC1(something) in service at WPUV in Pulaski, VA, which had lost bias on the DC coupled 833 (from the 807 driver) and the tube ran white hot, sucked in the glass and failed when it went to air.

In my Continental 314R1 (1 kW PDM) the damper diode was shorted, so that the single 3-500Z switch tube kicked on (for carrier level) and glowed very bright plate spot-almost white, along with some purple funk around the glass - before the plate breaker kicked off. I just swapped that tube into the RF side (2 x 3-500Z) and it runs fine since.

Hi John,

Sounds like a nice efficient rig being PDM and all.  Interesting that they use only ONE switch tube for TWO finals of the same kind. I was told that I needed  2 X 2. Though, if you look at the 90% switch effciency and the ~80% final, it makes sense to use only one for the switch tube.  Does there seem to be any switch tube saturation and what color does it run compared to the finals if you load the rig up fulll tilt?

Could you describe the commercial PDM filter coils they used in that rig?  I'm going to be winding mine soon for the 4X1 PDM rig and wondered.

BTW, looks like that 3-500Z final needs some protective fixed bias... :o

T

John and others are right about not putting stuff in the hole where it doesn't belong. :o

You can add stress since the glass will expand and contract at a different rate than the epoxe.
May also create a hot spot since epoxe does not ship heat as well as nekikid glass.
Nice fight, you lose.

Now that it's scraped off there, really really clean the glass with the best non-residue solvent you've got and use cotton gloves like photographers use to handle negatives. Finger oil will bake on and cause the same problems as epoxe.

Just use the tube and keep air on it. Do some macro JPEGs before you start using it, and come back after a few hundred hours and check any changes, or NEW spots that may show up elsewhere in the glass.

Me ?
I'm hyellified about a tube I've got that seems to have crizzle.  Yes, crizzled glass. It's not a good thing.
Any experience out there? The problem has developed in a brand new, old stock tube that I've only put a few hours on.

From a glass site --
Crizzling is caused by some deficiency or instability in the chemical makeup of the glass, where either too much alkali or too little lime was used in the manufacturing. It takes a long time, but eventually the salt leaches away, chemical chain reactions occur and the glass deteriorates. Crizzled glass is said to be "sick", or "diseased", and the prognosis is nearly always terminal: broken glass.

Tom - I'd ignore the dimple and just make the 4X1 "sing for it's supper"!

4X1 with melted plate structure effes up the glass?

I have an 833A in my collection that has a huge dimple in the glass and a bright shiney spot on the plate.  Evidently the user assumed the tube was ruined by the overload and didn't even bother to try it out.  The glass is sucked in at  least a half inch and the diameter of the dimple is about 1 1/2".  The thing had been sitting out in the garage in a box full of other junk for several years.  After I got the BC1-T going, I decided, for the hell of it, to see just what that tube would do and stuck it in.  Turns out it be one of the best 833A's in my collection.  Performs exactly the same as any of my supposedly new ones.

Tom
The Continental 314R1 is same rig as Collins 828C1. Collins sold out to CEC all their B'dcast rigs. I can send you a copy of the brochure on it from 1980 via email if you want to see pix of it, PDF document is 3.26 Meg in size.

From the anode of the Switchtube 3-500Z, it has a 43 mH inductor first, then 1000 pf to ground, then 54 mH, in parallel with 160 pF, then 1000 pf to ground again, then 54 mH in parallel with a 56 ohm 2 watt R and 100 pf cap, then a 430 pf to ground. This goes to the cathode of the pair of 3-500Z RF stage. Built as a floating deck, coupled RF to their grids through a HV/RF transformer from the driver. The inductors in the PDM LP filter are layer wound, like you would see in old JW Miller inductors. Looks like litz wire. They are 8 inch fiberglass forms, 2.25 inch OD, with 7 sections of windings on each. Each section of windings is layer wound, about 5/16 inch wide and 1/2 inch tall. The capacitors are 10 kV epoxy mica cylinders, CDE brand. At the cathode of the switch tube, it has -8500 VDC main power supply. So it is basically a hot cathode on the RF tubes, and the plate is grounded for DC, only carrying the RF out to the network. It doesn't need bias for the RF tubes, as they are driven and self biased. However the switch tube gets bias from a driver card, with HV transistors on it. It is grid driven hard positive (saturated switch) or in cutoff at 70 KHz rate.

It was sold to be ready for AM stereo in the 1980s. Good fidelity and modulation capability, when you can keep one running.

73
John
K5PRO

Thanks for the info, John.

Sounds like a desirable rig.

Tom/KLR sent me some pictures of one that Bob/ZM owns. I was surprised to see the PDM filter coils looked like big 2.5MH RF chokes... ;D

That might be a tough thing to wind, so guess I'll stick with using several sleeved single layer coils - one inside the other.

Hope you keep that rig running. If I were ever to acquire a BC rig, it would definately be something like what you have.

73,
T