Discussion in 'Amateur Radio Amplifiers' started by KB4MNG, Mar 11, 2013.
The glow factor is nice.
Mention gas and the AM guys show up
But seriously folks, hollow state is far from extinct.
I agree but 4-400's do seem to lose some vacuum from long storage. Since I do some work with the Bauer 707's I get to see 4-400's x 4 in action and twice have gone on site for a "normal" tube failure and found the customers NIB set of spares from USAF surplus are dancing in the wind. Ive recommended to my customers that they have a rotation schedule or at least test whatever surplus or used ones they drag home.
I have never seen or heard of an 80's graphite Eimac....maybe they bought Amperex again during the pair of 80's Eimac fiascos. One was when moving to SLC and quality was out the window and tube seals leaked in a year or so. The other was a stunt to save money and the anode cap, which was fully welded to the anode, became spot welded in 3 places. Funny things happen when the weld breaks when in use. I have a couple of those as display pieces. I was an Eimac reseller in those years before Richardson completely ruined the tube industry. Im very glad that China got involved since Burle, who bought the Eimac glass tube line, couldnt produce good ones.....maybe that production equipment is what initially went to China.
As far as Eimac and the 3-500, the story I was told by an employee is that Amperex supplied the 3-500Z which was graphite since Eimac was having production problems with their tantalum plate. Makes sense since it was marketed before that under a European designator and actually built in the Philips plant which had been building graphite anodes for years such as the (5867) TB3/750 and (5868) TB4/1250.
Here is a 1961 examplehttp://www.ebay.com/itm/NOS-Amperex-5867-Tubes-PAIR-/110958750042?pt=US_Radio_Comm_Tubes&hash=item19d5a8295a
Eimac did make a 5767A in the mid 60's but its graphite anode looked nothing like anything else. When I compared a 1969 date code pair of Eimacs in a SB-220 against an Amperex they were identical.
Given no actual vacuum leak in the glass seals, I was told that Helium (He) diffusion through the glass walls was the cause of radio glass tubes becoming gassy. Does anyone know about this old wives tale? Actually, He is monatomic and has a small atomic radius and could work its way through the glass walls. The inside concentration of He over the years will eventually reach equilibrium with the outside atmospheric concentration. Running the tubes (hot) for long periods will force the He atoms back across the glass barrier due to increased pressure thus reducing the internal levels. So, it may be true...
He is a really small molecule and is routinely used for hermetic testing.
However, I don't know that tubes have ever been subjected to a "Helium leak test," since to do that would mean having to infuse the envelope with He to begin with, and then measure the leakage rate. I don't think they do that. And I have my doubts about vacuum tubes being able to pass that test.
But where would the helium come from? In ordinary air, the helium content is about 5.2 parts per million. It is so low that helium was first discovered on the sun before it was found on earth.
Back in the 1920s, Raytheon develope a series of cold-cathode tube rectifiers that used helium. The CK-1006 is an example of the technology, which goes all the way back to tubes like the BH. Many examples still work after many decades in storage. Of course in those examples the helium would be trying to get out, not in.
73 de Jim, N2EY
73 de Jim, N2EY
Helium does leak through glass when the glass is placed under low pressure.
Helium is used as a tracer gas to test the integrity of a hermetic seal.
Not to freak you out, but liquid He at that temperature acts like the entire fluid is in a single quantum state. This super-fluid state causes weird things like the liquid to creep up the sides of a vessel like it's alive (super-viscosity). Super conductivity in some special materials is thought to occur because electrons of opposite spins pair up to become zero-spin Bosons like He becomes at low temperatures. When quantum effects occur at the macro level, they are very observable but are quite odd indeed. Maybe this is why Einstein himself never could buy into some aspects of Quantum theory. A famous physicist once said something like: "If you don't think Quantum theory is crazy, then you don't completely understand it!"
And how much would it take to poison a tube? What is the gas content in that purple glow deep inside the tube; has anyone done a spectroanalysis?
Here are some numbers that might aid in putting all this into some prospective.
As I said above Helium is used as a trace gas to test the hermetic seal of electronic components and subassemblies. The parts in question are placed in a vessel and exposed to 60PSI of Helium for a period no less than an hour. Based on my experience working in the semiconductor industry the typical metal can transistor has a leak rate when exposed to high vacuum of about 1-4 X 10-10 atoms per cc3 That is really, really, really small. We commonly use a glass capillary filled with Helium and sealed at both ends the capillary is mounted into metal, usually brass housing with an opening at one end though a flange. The one we had to set up the mass spectrometer leak decetors with leaked at a rate of 4.7x(I don't remember) x10-8 atoms per cc3. This leak was expected to outlast any of us who used it because again that leak rate is very, very, very, small.
Could you poison a glass tube?
Not by spraying Helium on it. If you pressurized it long enough at 60PSI yah maybe...
Just as an aside;
Pulling the tubes out of my SB-220 does make me nervous simply because they need to be pulled straight out and inserted straight in. No rocking motion because as far as I can see that can damage the base seals.