I have a 3-500 question

[Jim, W5JO]

I just read in another forum that the 3-500 tubes have problems with the seal around the pins after long periods of storage, especially those made in China.

I have been fooling with tubes since 1955 and have never seen a problem with this type of thing.  I don't use 3-500s anywhere but have used 4-400s and 4-250 in the past with date codes into the 50s with no trouble.  In fact the tube in my Globe King 500A is from the 1950s.

So is this an urban myth or is it real?

[WQ9E]

Jim,

I have never run into this problem and I have owned a number of 3-500Z based amps since the mid 70's.  I recall there was a problem with some 3-500Z's being damaged by rough insertion into sockets which damaged the base seals.  Heathkit (and probably others) added a note in their manual about this problem, particularly the need to avoid rocking the tube as it was inserted or removed from the socket.

There is a ceramic reinforcing ring beneath the base seals on most (all?) 3-500z tubes now and I have seen one set of older ones that did not have this-I imagine they were the ones most prone to damage.  Of course all of the 4-125/250/400/1000 family members have the additional base support that is part of their cooling system and helps prevent damage from careless insertion/removal. I have a feeling the damage caused by rocking tubes into/out of their sockets is largely responsible for this reported problem.

It is certainly possible that quality control is lacking in some of the Chinese tubes.  I have seen photos of them where the anode structure is tilted and I remember seeing a display Chinese 3-500Z at a hamfest that appeared to have cigarette ash sealed inside.  I imagine that one didn't work so well!

Rodger WQ9E

[Jim, W5JO]

Well Roger, it appears the myth lives without substance.  I cannot understand the trouble except for my experience with 572Bs.  It seems the envelope is much thinner than the 811 series for I have broken a couple of them with very little force. 

But. I have inserted and removed many 4-400s, 4-250s, 3-400s and 3-500s without problems.   In fact the 4-250 in my Globe King is an early version Amperex which does not have the support at the base.  Just the wires exiting the glass with pins soldered on them.  Every time I work on this thing, I remove it and put a soft 4-400 in its place for testing.

There must be some real ham handed people out there and some who perpetuate myths.

[W2VW]

Just more recycled baloney from that forum.

Same misinformation gets posted on still another popular ham forum by the same guys. The game is repeat it enough times to make it into a fact.

Some of that myth's backers have financial interest in selling amplifiers. The one guy is never wrong and it can get interesting when he starts down the wrong road. Just Google his call followed by the word "wrong" for some entertainment. 

Others are monkey read, monkey post.

Look at a 4-1000. Same ingredients. I've lit many a 4-1000 that had been out of service for over a decade. I just tried out 4 in my BTI LK-2000. A few had low emission but they were not ruined by components fabled to rust inside a vacuum.

Same goes for 3-400.

I'm sitting on (6) 3-500Zs right now which have not been used for at least 5 years that I know of. Maybe I'll take pictures of my house burning down and the world ending while trying them.

Also lit numerous 4-400s which sat unused. Not one has a seal leak problem.



 

[WA1HZK]

I have seen this with 4-400's. In an amp without enough air the fil. pins will become too hot and the solder runs out of it. This happened to Brent's Drake amp. with 3-500's and I have had that problem here. Cure was to resolder it and make sure there is enough air to protect the tube.
Keith

[w3jn]

 Maybe I'll take pictures of my house burning down and the world ending while trying them.


 

No wonder some wives hate ham radio 

[WQ9E]

Keith,

Check the tension on the filament pin connections as that is the most common reason for these pins to overheat and lose their solder.  If those connections have been severely overheated then the connectors have probably lost their tension and you need to replace either that part of the socket or the entire socket assembly.  You can check this connection by using a AC voltmeter; put one probe on the socket and the other on the filament pin and with filament power applied gradually select lower voltage ranges on the voltmeter.  Any significant voltage on the meter indicates a poor connection.  You should check both filament pins and it is a good idea to do this at startup and after a few minutes of operation if there is any doubt about the health of the sockets.

Rodger WQ9E

I have seen this with 4-400's. In an amp without enough air the fil. pins will become too hot and the solder runs out of it. This happened to Brent's Drake amp. with 3-500's and I have had that problem here. Cure was to resolder it and make sure there is enough air to protect the tube.
Keith

[KM1H]

It certainly isnt a myth and as many who bought Eimac 3-500's over a 2-3 year period in the mid 80's can attest to.

The problem was they went gassy after sitting unused after a few years and this led to the pretty much standard of running them at least once a year. There were quite a few unhappy peoplle that wound up with junk and Eimac wouldnt extend the warranty.

This was only for those few years and AFIK all before and after were fine.....except for the time when the anodes fell off....

Carl
KM1H


.

[The Slab Bacon]

Just FWIW,
               In my quest for 4-1000s a few years back, I acquired 2 of them from 2 different people that had "lost vacuum" while sitting. Both looked like they were brand new till I lit the filaments.

Both had perfectly clear glass and no "burns" on the grid shields. As soon as I put power to the filament, the filament started smoking inside of the bottle and covered the inside of the bottle and the elements with yellow debris. Afterwards the yellow stuff covered everything inside the tube and they werent even good for paperweights or display tubes. I just assumed that they had gotten large amounts of air into them. However there was no obvious cracks or any signs of anything compromising the outer envelope. I just assumed that the leakage was around the bottom pins. I have also had this problem with 826s and a few other all glass tubes.

                                                                The Slab Bacon

[W2VW]

It certainly isnt a myth and as many who bought Eimac 3-500's over a 2-3 year period in the mid 80's can attest to.

The problem was they went gassy after sitting unused after a few years and this led to the pretty much standard of running them at least once a year. There were quite a few unhappy peoplle that wound up with junk and Eimac wouldnt extend the warranty.

This was only for those few years and AFIK all before and after were fine.....except for the time when the anodes fell off....

Carl
KM1H


.

Good information. Anyone keeping track of bad date codes?

If these are known it would make buying old tubes much safer.

[KB5MD]

I purchased (5) 4-400a's on epay.  The seller did a beautiful job of packing them except he let the pins touch the bottom of the box,  plenty of bubble wrap and peanuts around the tubes, but not the bottom.  They arrived with cracks in the base of three.  Oh well, atleast two were still good.

[Bill, KD0HG]

Filaments are brittle. Try bending a piece of tungsten from a light bulb, it will break, not bend, and the filaments in radio tubes are even thinner than that.

It's a good idea to slowly bring up the filament voltage on old tubes the first few times they are lit to relieve the stresses. They do try to squirm around when you give them full mojo voltage.

[KM1H]

I have the 3-500Z date codes somewhere and will post if I find them.

The quality problems were related to moving production from San Carlos to Salt Lake City and other tubes suffered as well from what I was told years ago.

Were all 4-1000A's built by Eimac and then marked for RCA, Westinghouse, etc? I used to get 2-4 at a time from local AM stations back when I was actively building 1-2 tube amps for customers. That was long before the Internet and my understanding of the EIA codes.

Carl
KM1H

[Todd, KA1KAQ]

Filaments in the WWII-era tubes like the 100 & 250TH types are a true problem due to lower qualities of metals allowed as the war progressed. Along with the problem of brittle filaments breaking in tubes stored horizontally instead of upright, there is also the issue of contamination to the vacuum from impurities in the metals used, resulting in a lot of these tubes being gassy. Always wondered how much of that got blamed on leaky pin seals. My understanding is that you had to have a serious heat problem or impact damage to get those seals to leak.