------------------------------ From: rthomas@lambda.mfg.sgi.com (Robert E. Thomas) Subject: Re: tube woes! Date: 31 May 1993 00:29:48 GMT In article <1u6dqpINNs0s@uwm.edu>, Andy Moss writes: |> SFp I'd been running 4 NOS Tung Sol 6550s in a Sonic Frontiers |> SFp SFS-80 for a couple of months when I noticed that the 2 |> SFp left channel tubes seemed to be "aging" at a rapid rate. |> SFp The "getter" on those tubes was turning white around the |> SFp edges, whereas the right channel tubes were still completely |> SFp silvery. I was told by the fellow who sold me the tubes |> SFp that as long as the bias was correct (it was, at 50mA) |> SFp they should be ok. |> |> Can someone tell me what a "getter" is? Maybe I've been asleep too long! |> |> Cheer's, |> |> Andy |> --- |> ~ Magical Mail ~ "Mids were so harsh the pan-flute sounded like a train whistle" |> The getter is a small loop of material, located usually near the top of the envelope. It's made from, as I recall, a Beryllium compound. It serves to attract stray molecules of gas that may be present in the envelope, having boiled off of an electrode or seeped through the seal. It's not meant to save a leaky tube, but rather to "clean" the vacuum of a well- sealed envelope. If there is too much gas present, the getter will quickly be overwhelmed. In gassy tubes, the getter may (or may not) glow. -RT ------------------------------ From: msv@unl.edu (mark vanroojen) Subject: Getters Date: 1 Jun 1993 20:04:02 GMT A previous poste described getters as a "loop" of metal usually near the top of the tube. I'm pretty sure that that is incorrect (I haven't got my books here to give a reference). The getter is a deposit of some material usually on the envelope of the tube to absorb gasses in the tube. It is often silvery in color and tends to change color with use. At least, that is what I've been told. MvR ------------------------------ From: bilver!bill@peora.sdc.ccur.com (Bill Vermillion) Subject: Re: Getters Date: Thu, 3 Jun 1993 13:18:44 GMT In article <1ui7h2INNn1e@uwm.edu> msv@unl.edu (mark vanroojen) writes: >A previous poste described getters as a "loop" of metal usually near >the top of the tube. I'm pretty sure that that is incorrect (I >haven't got my books here to give a reference). The getter is a >deposit of some material usually on the envelope of the tube to absorb >gasses in the tube. It is often silvery in color and tends to change >color with use. At least, that is what I've been told. And in the days of the big tubes with 4 to 8 pins, often the 'getter' was in a cup near the edge of the tube. Used to absorb the oxygen to keep the tubes from burning up from what I remember - that's a long time ago :-( When a 'newbie' asked what that was for, they were told it was to 'catch the grid leak' :-) -- Bill Vermillion - bill@bilver.uucp ------------------------------ From: kevin@Novell.COM (Kevin White) Subject: Re: Getters Date: Fri, 4 Jun 1993 16:01:26 GMT In article <1ungbtINNo9u@uwm.edu> bilver!bill@peora.sdc.ccur.com (Bill Vermillion) writes: >In article <1ui7h2INNn1e@uwm.edu> msv@unl.edu (mark vanroojen) writes: >>A previous poste described getters as a "loop" of metal usually near >>the top of the tube. I'm pretty sure that that is incorrect (I >>haven't got my books here to give a reference). The getter is a >>deposit of some material usually on the envelope of the tube to absorb >>gasses in the tube. It is often silvery in color and tends to change >>color with use. At least, that is what I've been told. > The getter is a piece of metal on the loop. After the tube is evacuated the loop is heated by means of an RF current from a coil outside the tube. The loop heats up and the piece of metal (I am not sure what metal) vaporizes. As it condenses on the glass of the tube (forming the silver film) it traps any remaining gas left in the tube. The getter only does this once in its life, it doesn't protect against gas leaking into the tube. If the vacuum is lost and oxygen does get into the tube the film will oxidise and turn white. You can also tell if a tube is going "soft" if it glows blue inside while operating. Kevin White ------------------------------ From: billc@col.hp.com (Bill Claussen) Subject: Re: Getters Date: 4 Jun 1993 20:32:42 GMT Getters....Getters....Getters Now, let's see.....If I remember right... A getter is (in most cases) a small round loop of some sort of metal, usually near the base of the tube, that when heated (very hot) burns off any oxygen left in the tube after it is sealed. I think that the shiny stuff on the inside of the glass envelope are deposits that are formed when the getter is glowing. After this action, the getter does absolutely nothing for the rest of the life of the tube. Lets not make it something that it isn't. No magic here....Bill Claussen ------------------------------ From: msv@unl.edu (mark vanroojen) Subject: Re: Getters Date: 6 Jun 1993 18:14:08 GMT [Let's let this be the final word. -tjk] In reply to several postings in which we were discussing the function of getters, how long they worked for, and whether they are the loop in the tube, some material attached to the loop, or the material deposited on the envelope (It will turn out that all of these views are close enough to correct), Bill Claussen (billc@col.hp.com) writes: A getter is (in most cases) a small round loop of some sort of metal, usually neat the base of the tube, that when heated (very hot) burns off any oxygen left in the tube after it is sealed. I think that the shiny stuff on the inside of the glass envelope are deposits that are formed when the getter is glowing. After this action, the getter does absolutely nothing for the rest of the life of the tube. Lets not make it something that it isn't. No magic here.... Bill Claussen [end of quote] The sources I consulted seem to indicate that getters continue to function even after initial "flashing", when the getter is evaporated with a high voltage charge, combines with the gasses remaining in the tube, and some of it condenses on the envelope or elsewhere. For example, The Radiotron Designer's Handbook (Langford Smith ed.), 4th edition on page 4 states: Most thermonic valves are vacuum types and operate under a very high degree of vacuum. This is produced during manufacture by a combination of vacuum pumps and is made pemanent by the flashing of a small amount of "getter" which remains in the bulb ready to combine with any impurities which may be driven off during life. Robert Tomer, in Getting the Most Out of Vacuum Tubes (NY: Bobbs-Merril, 1960) p. 24 goes into more detail: These "getters" as they are called, are patches of evaporated metal which have adhered to the inside of the glass. Their purpose is to absorb gases which evolve during the life of the tube. They work best when warmed by the normal bulb temperature, but can be evaporated by excessive bulb tempera- ture. Thus, not only will they add their own vapors to the gas content of the tube, but in so doing, they will release any gasses which they may have entrapped. And in 70 Years of Radio Tubes and Valves by John Stokes (NY: Vestal Press, 1982), p. 17: Magnesium has the ability to readily absorb oxygen, so that as well as contributing to the process of evacuation when vaporized during firing it continues to absorb any subsequent released oxygen during the life of the tube. In the former case it is known as a `getter' and in the latter as a `keeper'. This important step in tube manufacturing, although improved by the later use of mixed getters, remained a basic feature thereafter. After looking at these sources, it seems to me that the loop is what remains behind when the getter is flashed, and that some of the getter is what winds up on the tube envelope. Whether we quibble about calling that a `getter' or a `keeper' doesn't seem to me to effect the basic point which is that it is supposed to keep absorbing gasses for the life of the tube. I don't particularly mind it when someone mistakenly tells another on the net that they are wrong. (After all, my original posting that the getter was the stuff on the envelope and not the loop, was only half right.) But I'd just as soon that we didn't immediately brand dissenters as believing in magic when we disagree. Those who claimed that getters continue to absorb gasses in the tube did not make claims about the "magical" powers of getters. Niether did those who disagreed with them. But the literature on the subject seems to bear out the former view. Peace, Mark ------------------------------ From: molly!carl@uunet.UU.NET (Carl Shapiro) Subject: Re: Getters Date: Mon, 7 Jun 1993 08:24:28 GMT >The getter is a piece of metal on the loop. After the tube is evacuated >the loop is heated by means of an RF current from a coil outside the tube. It can also be operated directly, with the wires going through the glass, by means of an intermediate material (ie. uranium doped glass) to mediate the different coefficients of expansion. >The loop heats up and the piece of metal (I am not sure what metal) vaporizes. I used to use barium when building lasers. It's eager to combine with many of the common contaminents. >... The getter only does this once in its >life ... Not necessarily. With practice and a good Variac, you can make the getter sputter for a while without completely burning it up. Thus, you can save some for later (or install multiple getters) to take care of stuff that might outgas over time. ------------------------------ Date: Mon, 7 Jun 93 11:20:25 EDT From: jas@proteon.com (John A. Shriver) Subject: Getters Tubes are pumped to a very high vacuum while heated quite hot, especially the metal elements, which are heated to a dull glow by a powerful induced RF field. There is a lot of occluded gas in the metal parts, especially the plates. (This is a major issue in RF power tubes.) After the tip is sealed off, the getter (barium titanate?) is fired. I'm not sure if this is done with a different RF field, or with a voltage pop to that element. Maybe both. Certainly firing the getter eats up the residual oxygen in the tube. However, the silver flashing does have a residual capacity to consume additional oxygen. This is most important in power tubes, since the plates do run hot enough to liberate more of the occluded oxygen in them. (Reference: old RCA Transmitting Tube Handbooks.) Very early tubes (brass base WD-11's), and a few exotic ones (WECo 420A, a very stable DC amplifier tube) do not use the "sivler" getter. They use white stuff which is not fired, I think it's a phosphorous compound. It's more expensive, but has a more reserve capacity. ---------------------------------------------------------------------- Date: 8 Jun 93 09:16:00 EST From: "SI03::PICHULOR" Subject: RE:Getters >>The loop heats up and the piece of metal (I am not sure what metal) vaporizes. Barium ------------------------------ From: xkkkha@ericsson.se (Karl Hansell) Subject: Re: VTL300 - Two tubes look "odd" Date: Mon, 5 Jul 1993 08:37:16 GMT Simon David writes: >I have a pair of VTL 300W amps and have just noticed that a couple of the >tubes seem to have lost half of the silvering at the top of the tube. They >are the original VTL stock 6550's, in the following layout: >Is there anything wrong here? I'm sure they weren't like this before and that >chanel does appear to be a few db down from the other. I have had the amps >for a couple of years. Probably the valves have always looked this way. The silvering has nothing to do with the function of the valve, infact it is only a bi-product. It forms when gases inside the tube is absorbed by a gas-absorber (I don't know the english name of this device, I only have tube books in Swedish). It doesn't matter how much of the top of the valve that are silvering. There might also be other coluors, depending on the gas that is absorbed, but if it has turned white the valve has started to leak, and the valves must be replaced. best regards -Karl Hansell ------------------------------ Date: Mon, 20 Sep 93 19:10:17 EDT From: jas@proteon.com (John A. Shriver) Subject: getters Given previous interest in getters, here's an interesting article on tube getters, from the February 1958 issue of Radio-Electronics. (Page 42.) "Notes on the Getter" by NORMAN V. BECKER. IN high-gain audio circuits, tube noises such as hiss and frying are some of the most troublesome things encountered. They can be eliminated only by selecting tubes which are inherently quieter or by reducing stage gain with negative feedback. In the latter instance gain might have to be reduced by such a factor as to defeat the orignal purpose. Hiss is created by dc resistance paths existing between the various elements inside the tube. These leakage paths may be as high as 1,000 megohms and would not upset normal tube operation _if they remained constant_. But, like a bad carbon resistor, they create noises of their own through random and erratic changes of resistance. Leakage paths of this sort are primarily located on the top mica support wafer, where the support rods are punched through. If the wafer is contaminated by impurities, it becomes a higly unstable conductor, connecting tube elements through very-high-resistance paths. Unfortunately, contamination of the wafer during tube manufacturing is almost unavoidable. Before sealing the tube envelope, as much air as possible is exhausted by vacuum pumps, but a small percentage of oxygen and other gases remain inside. This is where the getter comes into the picture. A small square loop of wire usually located at the top of the tube, part of it coated with an explosive substance similar to that used in photoflash bulbs. High-frequency radio waves penetrate the sealed envelope and heat the getter to a temperature high enough to fire this coating. This miniature explosion burns up the remaining atmospheric gases inside the tube and, at the same time, splatters a mirrorlike silver coating over a portion of the inner surface (a familiar sight in glass tubes). _Some of this splatter falls on the mica wafer, making it slightly conductive._ To reduce contamination of this sort, certain premium tubes are manufactured in which the space between getter and wafer is materially increased. In other types _two_ top wafers are used -- the upper one insulated from the lower -- and act as an umbrella to receive most of the splatter. Another method is to punch oblong slots in the wafer. These openings effective lengthen dc leakage paths and thereby reduce noise. In designing high-gain input stages for microphones and low-output pickups, it is desireable to use premium tubes whenever possible. Special manuals describing these types are published by tube manufacturers, giving electrical data, physical dimensions, recommended applications, etc. In many instances premium tubes are directly interchangeable with standard types which you might now be using. In addition to reducing hiss, premium tubes are less microphonic, have lower hum and are generally more dependable -- and are more expensive. END Some comments: 1. The 8223/E288C tube (a super-super-premium 6DJ8) is one example of a tube with a greater seperation of the getter. The tube has nearly an inch of empty glass between the top mica and the getter. 2. Somg 6SN7's had the getter on the side. That may well have reduced contamination of the micas. A lot of compactrons did as well, perhaps since they had so many tube elements in close proximity. (There were no compactrons in 1958!) 3. The RCA Super Red Tubes (5691, 5692) are examples of tubes have the double mica on the top to sheild from the getter spray. 4. The WECo 420A tube used a phosphorous getter, rather than the flashed getter. More expensive, but this was a tube for stable use in DC amplifiers, and the leakage paths would have been a problem. WECo was also obsessive in cleaning the micas on some of their most high performance tubes. 5. The manuals on premium tubes are now very hard to find, and if you looked at the price of a 5692 recently, those tubes are very expensive! ------------------------------ Date: Mon, 1 Aug 94 14:30:57 EDT From: jas@proteon.com (John A. Shriver) Subject: Re: tube - why clear top Two posters have asked why a tube might have the getters on the side, instead of on the top. The probable reason is that the getter mostly fires out towards the glass (when it is exploded), but some of the material goes any which way. Since it is a conductive material, it is not something that you want on the mica insulators. It will conduct there, sporadically, increasing the tube noise. Thus, it might be that putting the getters on the sides reduces contamination of the mica insulators. Of course, on power tubes, it may be that one can flash more getter at two sites on the sides rather than one site on the top. Since power tubes give off gas from the plates, they need a lot of getter to consume that gas over the life of the tube. (For instance, Tung-Sol 6550's have three getter sites, top and two sides, and very used ones have most of the getter gone or white.) I'm sure that there are numerous articles on the subject on old conference proceedings from the 1950's and 1960's that discussed tube design. (However, these are not the articles I choose to seek out and copy, since I'm not about to start making tubes...)