Odd Tube Behavior?

[Todd, KA1KAQ]

Was visiting my old pal Ray KC1BT last evening when he asked if I had any 811As kicking around. After reminding him that my last 4 NOS RCAs went into the 30L-1 I restored for him a few years back, I asked why?

Seems he blew a hole through the plate of one tube in his Ameritron amp (uses 4 Svetlana 811As). He said he'd been using the amp when he heard a *bang* and it went dead. He pulled it out of the cabinet, saw no obvious problems, stuck new fuses in and got the same result. This time there was a large, visible hole visible inside one tube. In his words, it didn't melt; it appeared with a bang.

Ray is a retired BC engineer among other things and has seen just about every RF-related phenomenon you can imagine from AM broadcast to RADAR. He said after the second bang he found a bad zener diode and 50 ohm grid resistor associated with that tube, nothing else.

What kind of failure could cause such a result, and why just one tube instead of all 4? Beyond a direct/hard short within the tube itself(which should dump the fuse before blowing a hole in the tube), I'm clueless.

[w4bfs]

you've reard of Russian Roulette ... sorry about that, I couldn't resist ... I have seen in power tubes a 'favorite' arc point as they get gassy ... maybe thats what happened ... unusual problem ..73...John

[W3SLK]

Todd, when I got my Warrior amp a few years ago, the 811's that were in it had holes in them. Since then I've found that is a common thing. Rich Measures would say it was caused by a parasitic. I'm going to go off the beaten path and say there must have been a mechanical failure of the grid. Which is unusual. I had 4 of those Svetlana 811 and they were built pretty darn rugged. If you can, look down through the top of the tube and see if there is anything out of place. Let us know what you find.

[Todd, KA1KAQ]

Thanks guys -

That's the same path I was following, Mikey - I asked him if he had a parasitic, suggested he check the suppressor for damage, he said all was well. That left the mechanical/hard fault. I'll pass on your suggestion to him to inspect the tube further visually. I was fascinated by the large hole and never gave it a thought.

Yeah Beefus, Russian Roulette or Chinese Chicken. I've never heard bad reports about the commie tubes from Russia like those about the commie tubes from China. Many a Collins 30L-1 transformer was lost due to sagging filament zorches. Svetlanas seemed to be stout, long-lasting tubes even compared to US examples. But they also stuck with the technology a lot longer than us weak imperial yankee dogs.

[WQ9E]

Todd,

Very likely a hard short developed grid to plate and that took out the grid resistor.  Since the tube is dead if I were him I would be curious enough to disassemble it and the insides will probably tell the tale.

811's will take some abuse and I saw a poor (in every sense of the word) home built 4 x 811A amp for sale at a hamfest that was a CB unit.  The little fins on the plates had all drooped downwards but the envelopes hadn't yet sucked in, this was probably due to the fact that although the construction was generally poor it had a large blower with pressurized chassis and some sort of homemade glass tube chimney around each of the 811 tubes.  It was offered along with a matching "International" receiver (better known to us as a National NC-173) and he wanted $400 each.

[WA1GFZ]

It may have been the tube with the highest emission and ran the hottest until it ran away. The good die young in this case.

[N3DRB The Derb]

because of my Gonset situation, I have been reading a lot about 811a's. It seems that the Svetlana tube has quite a bit more gain ( Mu) than your garden USA 811A, causing VHF parasitics when substituted where there were none before the tube swap.

From the RF Parts website:
 
811H Heavy Duty Substitute for 811A

Just drop this tube into 811A socket. No changes or adjustments. Ideal for Ameritron AL811 & AL811H Series Amplifiers, where the tubes are run hard. RF Parts makes special selection from our Svetlana 572B stock and has assigned the 811H part number to them. Rated maximum plate voltage 2000V.

It's got's a carbon plate like a 572B does.

if the amp made a audible noise/pop/bang, its got a parasitic somewhere.

Richard Measures is sending me a retrofit kit to lo Q parasitic the Gonsets after I emailed him asking for written advice without charge. standup guy. I didnt ask him to, he just did it.

[flintstone mop]

Yeah DERB
Richard seems like a good guy who knows some tricks about our tube friends. Although, after I made some useful mods to the TX and RX audio in my TS440, there was one more maintnenance step to replace the memory batt. One of those dime sized types, and I axed and axed where this dam battery was located and never received an answer. Time flies and the rig is 21yrs old and this mysterious battery never replaced. I guess it will be dead the next time I use the rig, now that I'm writing this.

My AL811A amp is still going strong. It's good for low power AM on 40 or 20.

Phred

[Ed-VA3ES]

Rich Measures is one of the good guys.  He doesn't talk much about it, but one of his jobs is servicing MRI machines.   His responsibility is to swap out  tubes after a set operation period; he replaces the tubes on a scheduled basis, and the pulls are typically 100%.     A few years ago, I bought an excellent set of 3-500Z's from him for half the price of new ones.  He's also very generous with advice.

[WBear2GCR]

Yeah DERB
Richard seems like a good guy who knows some tricks about our tube friends. Although, after I made some useful mods to the TX and RX audio in my TS440, there was one more maintnenance step to replace the memory batt. One of those dime sized types, and I axed and axed where this dam battery was located and never received an answer. Time flies and the rig is 21yrs old and this mysterious battery never replaced. I guess it will be dead the next time I use the rig, now that I'm writing this.
<snip>

Phred

Ya didn't ask me! 

It's behind the front panel near the uproc chip.
The front panel folds down once you remove the top and bottom covers, and the 4 screws in the side... fairly easy to spot after that, IF I recall properly... might be under the shield that sits behind the display and covers the uproc, would have to check the service manual to be certain, but it is there...

             _-_-WBear2GCR

[N3DRB The Derb]

its really hard for me to comprehend that a Kenwood TS 440 is a 21 year old radio.

seems like yesterday they were brand new.

[Opcom]

Can we have good pictures of the blown up tubes with holes in the anodes please? I'd like to compare it to a melted plate in a final from an HT-32.

[N3DRB The Derb]

I had a parasitic in my HB rig years ago. Burned a hole right through the plate of one of the 812's. happened in less than 2 seconds. I was looking right at the tube. Oddly enough it still worked perfectly with full power. No audible sound at all.

seemed to me to be doing it more aat reduced drive levels.I installed a pair of conventional plate suppressors but only 1 grid suppressor. The idea being to break up the push pull effect at vhf.  Never happened again.

that rig did have some rather long & unhealthy grid leads.

[N5RLR]

...Although, after I made some useful mods to the TX and RX audio in my TS440, there was one more maintenance step to replace the memory batt. One of those dime sized types, and I axed and axed where this dam battery was located and never received an answer....

...It's behind the front panel near the uproc chip...

QST had a blurb awhile back about replacing button cells for rig memory.  The trick is to attach a length of small-gauge wire to the original solder pads, thus "remoting" the cell to a more easily-accessible location.  At the opposite end, attach internal-tooth lockwasher-style solder lugs.

Slip the cell with these against [proper polarity, remember!], into a short length of heat-shrink tubing of suitable diameter.  Depending upon how tight it is, one may or may not need to shrink with heat.

This allows the use of a more "garden-variety" button cell, available at drug stores/Wal-Mart/RadioShack, rather than having to special-order a unit with welded-on solder tabs [NEVER try to solder to a lithium button cell...they've been known to explode; and if shrinking tubing on per the above, be very careful].