How Fragile Are 100TH and 250TH

[W1EUJ]

I am working on a BC-610. When I bought the set, and the spare tubes, I was told that the tubes are extremely fragile, and to always carry them upright.

I'm confused. How does a transmitter mounted in the back of a truck, that might be jostled by blasts, use tubes that are frigile. The question really is, can damage be done to the filament that would not be measurable as an open filament? Can I lay the tubes on their sides?

David Goncalves
W1EUJ

[WA1GFZ]

look at the date code on the tubes. During WW2 some of the trace elements were in short supply so some tubes had weak heaters. You stand them upright so the internal parts have the best support. I don't think the BC 610 operated on its side.

[k4kyv]

The story I have heard is that particular kind of filament became brittle with ageing, so tubes issued during the  war might have been more rugged when new.

I store all my transmitting tubes upright.  V-shaped wire filaments, like the ones 833A's, 811A's, 810's, etc. tend to sag and short out to the grid.  That's why those tubes can be used horizontally, but must be oriented so that the plane of the plate-grid-filament structure is vertical and not horizontal.  Sitting in storage for long periods of time might result in a grid-filament short, just like operating them in wrong position.

The Eimac type tubes like the 250TH must always be operated vertically.

[W8KHZ]

I run a homebrew transmitter with a pair of 250THs in the final as well as a BC-610 which has a single 250TH final and a pair of 100THs in the modulator. Here are my observations...

It appears that at some point during the production of these tubes, the filament structure changed. The filament in the earlier tubes were typically made up of perhaps 6 rather fine filament strands arranged in a cylindrical "cage" type of configuration with a spring at the bottom to provide tension and perhaps a bit of protection from mechanical shock.  Later versions used a single heavier filament that is formed in a helical pattern with no tension spring that I am aware of.  I assume the spring like shape of the filament itself makes a separate tension spring unnecessary.  It has always seemed to me that the later production 250THs are more mechanically durable.  I have bought a fair number of 250THs of the later variety and had them shipped to me.  So far, I have never had one arrive with broken filament. In fact, I had had an occasion where I was handling a 250TH and fumbled it, causing it do a couple of bounces off a table top with not apparent harm... so I have the opinion that at least the later production 250THs are actually quite robust. Larger tubes in the series with the "cage" filament such as the 450TH are extremely fragile... that much I know.  I'm not sure how fragile the the later production versions were..

Brian - W8KHZ

[John K5PRO]

It is a proven fact that thoriated tungsten (TT) wire is made more brittle by the process of carburization, which is the addition of a carbide layer over the TT base metal. This process is done with all TT filament power tubes. It enhances emission at a lower temperature, protects the filament better from ion bombardment, and lengthens life of the filament emission. However, the trade off is that the filament becomes brittle. This gets worse over the life of the tube, and very old filaments that have lots of hours on them, are more fragile than new unused filaments. NOS TT tubes, which have low hours on the filament, such as old military stores, would have less fragile filaments, for the same reason.

I removed a tube that had 40,000 hours on it, at work, and it was laid on its side for whatever reason (this is a big tube the size of a beer keg). The filament became shorted, broke.

I later opened a similar old tube, and the filament wires were very easy to snap between the fingers of one hand. Not so with virgin TT filaments before being installed into a tube.

So I would suggest that there are two things happening for these particular tubes. If are high hour tubes, then they are brittle, period. If they have a particular mechanical filament design, they may be more susceptable to shock damage, as some have suggested. In any case, be very careful when moving high hour tubes,I have read some manufacturers recommending not setting them down onto a floor or hard table top for risk of shocking the filament. They recommended a soft material to set them on.

[w5omr]

CWO John L. Mohn, W5MEU, worked in the Air Corps (before there was an air force)

His job was working on transmitters and receivers.  In particular, the BC-610 and R-390.

In the mid-80's, John's suggestion was to store any of the big transmitting tubes vertically, because of filament sag.  Someone pointed out that the 833 could be operated on it's side, as long as the grid and plate caps were vertical.  This way, the filament can't sag into the grid.

Not only are *TH's and *TL's fragile, filament-wise, they're also prone to 'gas-up' if they've not had any filament voltage put to them for some time.  Case in point, I had some spare 250's laying around here for my HB rig (in the picture) and wanted to see how good they were.  Stabbed 'em in, waited about 5 or 10 minutes for them to come up to operating temperature, and then hit the exciter.  There was a blue cloud that formed in the plate, and worked it's way down, literally eating the plating off of the plate.  Burned it up.

I believe it was Don/K4KYV who suggested that some of those tubes can have the gas burned out of them, if the filaments are left on for an extended period of time.  I may be misquoting him on that.  Over all, the ratio of rescuing gassy firebottles is -way- less than 25%.

[The Slab Bacon]

Geoff,
          IIRC, most of the current manufacturers of power tubes and some of the older ones as well, recomend that you let the filaments burn for an extended time before applying the plate voltage. The extended filament burn time will burn up some of the "gas". Most of the big power tubes do not have "getters" and rely on the filament and heating of the plate to burn up any gas that may be in the tube.  IIRC, they recomend this for new unused tubes as well as used ones that have been sitting dormant for long extended periods of time.

                                                                                     the Slab Bacon

[w5omr]

Geoff,
          IIRC, most of the current manufacturers of power tubes and some of the older ones as well, recomend that you let the filaments burn for an extended time before applying the plate voltage.

I've heard that too, and moreso since I burned up what could have potentially been a perfectly good 250TH.

I've also heard that running the filaments a bit on the 'high' side (250TH's are 5v @ 10.5A) say around 6v that this will also burn up some of the gas.  How long is 'extended'?  24 hours?  100 hours?

[The Slab Bacon]

I would guess that it is at least 24 hrs or possibly even more. I guess more cant hurt. I always burn power tubes that have been sitting dormant for a long period of time for a few hours just for good luck anyway. (all of mine are used) It cant possibly hurt.

I think RF Parts publishes a "brake in" procedure for their big tubes. It might be worth a cruise of their websight. I dont know it there is an Eimac web sight or not, but it might be worth searching for.
                                                                                    The Slab Bacon

[KL7OF]

A couple years ago I built a 100th push pull final and out of the 6 swap meet tubes I had .......two had good filaments.  They showed quite a bit of blue when under load for about the first 8 or 10 hrs of use...the blue gradually decreased until it became just a blue doughnut that walked up and down the top of the plate stem and then the doughnut disappeared.. The tubes have very good output and are working fine...One of the tubes is a spiral filament and the other is cage type.....I bought a pair of NOS (cage filament )100ths from Antique  Electronics  and they arrived with open fiaments...AES refunded my money.....and said they had a lot of the same trouble with that bunch of NOS 100ths ......

[W1EUJ]

I suppose that I could get a set of tube sockets and build a fixture to perform a stepped heating. It is the coating on the plates that absorbs the gas (from the H&K Gammatrons onward). I can see how heating the plate with the filament both activates the coating to absorb gas and tests the filaments.

Would one suggest that, after a good heating on filament, that ramping the plate and grid to slowly increase the heat dissipated in the plate to some value below max (50%), or until the plate glows, would be beneficial to further degas the tube and test it for arcing?

Thanks for all this information everybody. BTW, eBay has a fellow in Spain selling NOS 100THs for $57 each.


David Goncalves
W1EUJ

[N0BST]

One other recommendation in addition to leaving the filaments on for an extended period- bring up the plate voltage slowly.  Seems to me there was also a way to degas tubes using a neon sign transformer but I don't know the specifics.  Being they self limit the current it's harder to kill the tube.

Scott Todd

[WA3VJB]

In correspondence with a factory rep in the early 1980s, Amperex told me their recommended process to restore their glass transmitting tubes that had been in prolonged storage was to apply correct filament power for a matter of hours. More time is better.

Of course, slowly bringing up the filament voltage to the correct specification always provides less mechanical stress unrelated to driving out any gas.

The writer turned out to be our own Brown Beezer, W1NZR, who I had not yet met on AM in those days. We had a good laugh about my "discovered" cache of Amperex 7527A, all of which still provide full rated service in two different transmitters here.

[W8KHZ]

I had some sucess degassing a 100TH in a manner very similar to the way others have described...  After a lengthy filament burn, I simply ground the grid and apply plate voltage... starting at zero and slowly ramping it up until just a hint of blue haze begins to appear inside the envelope.... I then allow it to burn at that voltage until the blue haze goes away then ramp the voltage up until the blue haze appears again and let it burn some more.... I continue doing this until I work it up to typical operating voltages.  You will want to monitor plate current so as to not exceed ratings. At some point you may have to apply a negative bias voltage to the grid to keep plate current in check.  Ideally, I think tantalum plate triodes such as 100ths and 250THs like to have their plates run red since the tantalum plate acts as a getter to absorb gas and is most effective at doing this when they are good and hot.  As others have mentioned, this method may or may not work... if there is too much gas, there may be no way to bring it back... but its worth a try.

Brian - W8KHZ

[N3DRB The Derb]

then you use it in a slime o tron to refine heavy pissoline. Form your own pissing cartel and refuse to sell to the great satan.

[The Slab Bacon]

TH's are not type accepted for use in a slimatron!! You MUST use 833As to be type accepted!!

[Todd, KA1KAQ]

Not a 'big tube' approach, but there used to be a trick that some old radiomen swore by, using their Zippo lighters to apply heat to the shiny getter material on the side of the tube allowing it to absorb the particles released from years of being dormant.