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8877 Mystery

stuart truba (ve2xx) on December 30, 2002
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I've been using the same tube on my 8877 2m amp for the past 16 yrs and after some 1100 hours of use, an internal short has occured. The filament pin 2 was shorted to other pins 1, 3, 6, 7. The hv suppy blew primary fuses and the supply came to a halt. It took a while to figure wht was going on before I removed the tube and decided to check for continuity. After finding that there was continuity between what I asume is the grid and the filament I applied 5v eratically untill there was no spark between pins 2 as a reference , and 1,3,6,7. After doing this I re applied voltage to the proper filament pins , 5 & 2 . Seems to draw the proper amount of current ect. Has anybody ever experienced anything like this before?? Is this tube still usable??

What happened internally?

Thankyou , any help would be appreciated.

ve2xx, stu

Member Comments: Add A Comment
8877 Mystery Reply
by AD8P on December 31, 2002 Mail this to a friend!
This also happened to me many years ago with a 3-500 which had a grid to filament short. I applied the voltage as you did and "burned" the short away. I know that the tube was usable for a while after that. It is possible it is still in my amp.
8877 Mystery Reply
by K8ZE on December 31, 2002 Mail this to a friend!
Not sure what happened to you exactly. There is a site you should visit and perhaps you could get a better idea.

You might also want to visit the amps reflector from this site see other archives on the left below.

RE: 8877 Mystery Reply
by N2MG on January 2, 2003 Mail this to a friend!
Definitely go to Rick Measures' site

then send him an email. He'll actually write back to you!

Mike N2MG
8877 Mystery Reply
by K1XT on January 2, 2003 Mail this to a friend!
Your 8877 obviously had a short and it's time for a new one. Despite what others may tell you, the 8877 is a great tube, but nothing lasts forever. Sixteen years of service is rather good. If your amp has been stable all these years, I wouldn't do any mods to it, just put in a new or used tube. You'll save some money if you can find a used pullout.

Bill k1xt
8877 Mystery Reply
by w8ji on January 6, 2003 Mail this to a friend!

You have a shorted tube. There is a lot of misinformation around, largely from one source, about the causes of tube failures.

Tubes, whether we like it or not, have always been a life problem in radio equipment. It doesn't take any "special events" to cause a failure, and the 8877 has a long history of being a difficult tube to manufacture.

The very same things that make the tube have high gain and power in a compact package also make the tube less reliable than other tube types. Unfortunatly, you need a compact tube with very close element spacing to work reliably on upper VHF or higher.

The most likely cause is flaking of cathode material (the oxide is deposited on the cathode in bands with gaps aligned with the hundred and fifty or so grid wires), or perhaps a failed weld on the grid allowing a wire to drop.

Some production periods of 8877's also had material problems with the cathode support structure, causing a high incidence of cathode to grid short failures.

Other than having positive action grid current protection through fast electronic interupt (electronic grid trip) and maintaining filament voltage CENTERED at Eimac's published specifications, there is very little you can do.

Beware of advice outside Eimac's published data! While some of it is harmless, much of it is actually harmful or foolish!!
8877 Mystery Reply
by k0tv on January 28, 2003 Mail this to a friend!

I've heard of problems similar to this with 8877s used in VHF amps.

If your amplifier doesn't have a filament choke (yes I know the 8877 is indirectly heated) and the filament is at RF ground, what can happen is that the difference in potential causes a short to be generated internally in the tube. The solution to the problem is to install a bifilar filament choke and remove the bypass capacitors holding the filament at RF ground. Let the filament float at RF. The effect causing the cathode to filament short is removed and the tubes will last longer.

Hope this helps... I might be wrong in your case, but it's my $.02

GL, Jerry, K0TV
8877 Mystery Reply
by VE2DC on February 26, 2003 Mail this to a friend!
They don't last forever... yes I've had power tubes short... and no, I would not use it any more. It has certainly sustained internal damage, probably to the grid structure, and would no longer meet it's performance specifications. And watch out for used tubes... they may have gone through the same experience. A new one every 15 years isn't bad!

By the way, I knew Jimmy, VE2XX, the previous owner of your call... now SK.
RE: 8877 Mystery Reply
by W8NF on March 12, 2003 Mail this to a friend!
If that tube is 16 years old, that means it was produced in 1987. I think that was right in the middle of a known bad production run from Eimac - the semi-famous "cathode dam" problem. The symptom was that over time, the tubes developed a cathode-to-grid short, because some metal structures inside the tube, after enough heating/cooling cycles, fracture and bend.

For reference, the broadcast industry used to use the 8877 as a driver for bigger tubes in many designs. Reported life of the tubes was 18,000 to 20,000 hours, although the warranty was more like 2,000. The typical end-of-life condition for a tube "treated right" was a deterioration in cathode emission. But in broadcast service, the heater is kept running 24 X 7.

In ham service we cycle the heaters far more often than in broadcasting. Therefore, we see failure mechanisms that the broadcasters don't. Whether it's an 8877 type oxide-cathode tube, or a 3-500Z directly-heated tube, we really need to avoid those heater/filament cycles. If I'm going to use an amp, I switch the power on, and won't switch off until I'm sure I've used it for the last time that day.

If this amp has seen regular use (at least weekly), then 16 years isn't too bad. It's always worth asking Eimac if they would cover it under the "cathode dam" warranty. The tube has a date code on it that they'll want.


Dave W8NF
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