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Loose rivets
26th Mar 2009, 05:40
On a radio restoration forum, I asked about what turned out to be an Acorn valve/tube I've owned for 50 years or so. It was made about the same time as me.;) Some of its characteristics were incredible, namely its high frequency handling capabilities.

The thread turned to the Russian aircraft supposedly fitted with valves rather than solid-state electronics. What was the real story behind this? Were they really concerned with their kit being zapped?

MD11Engineer
26th Mar 2009, 06:35
Yes. At least concerning military aircraft. A nuclear explosion causes a massive spike of RF energy, which induces huge voltages and currents in unprotected electronic systems (first seen during the Bikini atoll hydrogen bomb tests, when the electric power supplies in far away Hawaii started going bonkers). This pulse is enough to fry most semiconductor components in these circuits.
Vacuum valves are much more resistant to these pulses.
The western countries went a different way and hardened their electronic equipment by shielding it and using overvoltage protective elements, similar to those used for lightning strike protection today.
BTW, the Russians (and Chinese) are still manufacturing vacuum valves and use them extensively in e.g. in high power RF final stage amplifiers. For low band applications in the multi-kilowatt range vacuum valves are still the technology of choice.

Bushfiva
26th Mar 2009, 07:03
Ulyanov is still a big manufacturer for the Russian military. Ruggedized tubes in metal cases have also been used in artillery shells. Smerch-A airborne radar was way more powerful than its western counterparts. The technology was seen by the West when a MIG-25P pilot defected to Hokkaido in 1976.

Expopul owns the Mullard brand. What was Mullard is now Blackburn Microtech.

gas path
26th Mar 2009, 16:44
Plus if you want a decent high end 'HiFi' system, tubes is the way to go!:ok:

hetfield
26th Mar 2009, 16:52
Plus if you want a decent high end 'HiFi' system, tubes is the way to go!

Yes, musicians especially guitar players went back to tubes long ago.

(Love my Fender:))

goudie
26th Mar 2009, 16:53
My first Hi Fi amp.,(Sansui) in the '60's, was a hybrid of valves and solid state. Had it for 20 years with only one valve requiring replacement. I gave it to a friend for his son to dabble with.

rotornut
26th Mar 2009, 17:18
Correct me if I'm wrong but aren't all high power rf finals tubes?

Also, from my ham experience, tube finals can take more of a beating than transistors.

IRRenewal
26th Mar 2009, 21:30
Correct me if I'm wrong but aren't all high power rf finals tubes?

No they're not. Try a search for 'class D' amplifiers.


Or "class e", "class f", "class g" ................

kms901
26th Mar 2009, 21:46
Big, megawatt RF transmitters use valves. Valves in guitar/hi-fi amplfiers distort on different harmonics to transistors and sound better the human ear.

But all hi-end Hi-Fi is basically marketing hype.

ChristiaanJ
26th Mar 2009, 22:13
Big, megawatt RF transmitters use valves.Thanks, kms, it's what I thought. Unless anybody has different info?
Klystrons, TWTs, magnetrons for the microwaves, and more 'classic' valves for large broadcast transmitters. The exception is probably large planar arrays of smaller transmitter elements, which nowadays are solid-state.
Oh, and if you're looking for valves, look no further than your kitchen. Even today, the best way to generate a few hundred watts at 2.4GHz is the magnetron in your microwave :ok:

Valves in guitar/hi-fi amplfiers distort on different harmonics to transistors and sound better the human ear.With HiFi being defined as less than 0.1% distortion (or some similar figure), there is no way to distinguish a valve amplifier from a transistor amplifier..... until of course you drive them into saturation, upon which the distortion is a lot more than 0.1%.... and it's no longer HiFi.... and indeed valve amplifiers saturate a bit more "gracefully" that transistor amplifier, so sound less awful.

But all hi-end Hi-Fi is basically marketing hype.Amen.

CJ

Landroger
28th Mar 2009, 12:36
Correct me if I'm wrong but aren't all high power rf finals tubes?

Also, from my ham experience, tube finals can take more of a beating than transistors.


Not in my industry - any more. :) Until the early 2000's our RF amplifier output stage was a big, metal bottle vacuum tube. I am a Magnetic Resonance Imager (and CT Scanner) field engineer and the 20Kw amplifier - admitedly narrow band - worked in the 63Mhz frequency for 1.5Tesla. The latest models are solid sate, although due to 'black box technology' I am no longer required to know exactly how. Just change the FRU. :(

However, 3.0 Tesla magnets are becoming more popular for clinical use and those amps have gone back to thermionic valves - 35Kw at 127Mhz.

As for reliability, you may be right. I've only change a few 'tubes', but the solid state 'box' has failed far more.

Roger.

Graybeard
28th Mar 2009, 14:56
The Collins 618T-x HF transceiver, 400w PEP, built from about 1962-78, was universally equipped in overwater airliners and military. It had tube finals, 4CX250A(?), and remained in service for many more years. In about 1995, the shop at Continental airlines noticed a drastic drop in service life of new tubes. They then looked closely at a couple of the tubes: both said Eimac, with same model number and date code, but one was rough finish silver, and the other nickel. They called the FAA, who called the FBI...

Succeeding generations of airline HF are solid state.

My younger son is in the A-V business, and I laugh every time I see a 6L6 tube. Those were obsolete when I was a ham, half a hundred years ago.

GB

411A
28th Mar 2009, 18:00
and I laugh every time I see a 6L6 tube.
This type, however, was the first of the pentode beam power tubes, used in the very first (true) hi-fidelity amplifers for home entertainment units.
The first of these?
Magnavox AMP-101, circa 1948.
Together with its companion AM-FM-Shortwave tuner, these were priced at $1400...in 1948.
Magnavox was first in many areas...the hi-fi loudspeaker, for example.
In fact, loudspeakers in general, all used the early Magnavox patents.
Just as radio receivers of the superhetodyne variety, all used RCA patents.
FM receivers, Hazeltine and Armstrong early patents...likewise.
The Collins 618T-x HF transceiver, 400w PEP, built from about 1962-78, was universally equipped in overwater airliners and military. It had tube finals, 4CX250A(?), and remained in service for many more years.

Used one of these just last night to communicate with Khartoum.
Worked to perfection.:)

cwatters
28th Mar 2009, 18:37
The story I heard was that for years the Americans thought the Russians used valves for EMP reasons but after the cold war they discovered Russia was using them because they were so far behind technologically. The technology gap was also apparant when the Russians finally allowed access to the space craft they were originally planning to send to the moon.

Landroger
28th Mar 2009, 18:57
Going slightly off thread, but in the same vein, when NASA found our how and with what the Russians controlled their space craft, they were within an inch of cancelling the Apollo/Soyuz link up. In the end they came up with a plan that more or less had Soyuz launch first and establish an orbit - wherever - then Apollo would go and find them. All manoeuvering was done by the Apollo spacecraft.

Apparently all flight sequencing was done on the equivalent of a Player Piano roll. :eek:

Roger.

Loose rivets
29th Mar 2009, 01:37
On the radio restoration forum, a gentleman in Newcastle posted this as one of the answers to my thread. I was astonished.


there were also the wartime valves designed to be used in anti-aircraft shells that were assembled at ROF Birtley and of which I have a few examples. They were designed to withstand 40,000G (yes) laterally from an end of life gun barrel.
There was an earlier thread on these about 2 years ago and I believe the leeds valve museum has some samples.
There were about 3 types. a triode, a pentode and a thyratron used to fire the fuse.

I can see how it took a bit of time for some folk to be weaned away from valves.

Luap
29th Mar 2009, 04:37
I am a Magnetic Resonance Imager (and CT Scanner) field engineer and the 20Kw amplifier - admitedly narrow band - worked in the 63Mhz frequency for 1.5Tesla. The latest models are solid sate, although due to 'black box technology' I am no longer required to know exactly how. Just change the FRU.

However, 3.0 Tesla magnets are becoming more popular for clinical use and those amps have gone back to thermionic valves - 35Kw at 127Mhz.

Will all that RF cook the patient? :eek:

Loose rivets
29th Mar 2009, 06:17
I'm confused by that. Surely the 'transmit - receive' rate is not at those frequencies?

I can't imagine the vast coils being able to conduct current in Mhz, the voltages would have to be astronomic at those frequencies.

I thought they just needed to give a short rise-time pulse, and detect the response. I'd imagined this in the hundreds of Hz, not millions.

Graybeard
29th Mar 2009, 06:50
Wally Schirra, Mercury 7 Astronaut, told of visiting the Cosmodrome. He was amazed how they provided for their heroes. It should have been the best they had. All the Cosmonauts lived in the same high rise building. The rookies were given the top floor - because the roof leaked.

GB

Landroger
29th Mar 2009, 14:16
Luap.

No, that sort of output is very rarely used and the frequency is nowhere near high enough to be a big microwave. :) The output power is set automatically at the start of the scan, to be just enough to precess your protons to the required angle.

I'm confused by that. Surely the 'transmit - receive' rate is not at those frequencies?

I can't imagine the vast coils being able to conduct current in Mhz, the voltages would have to be astronomic at those frequencies.

I thought they just needed to give a short rise-time pulse, and detect the response. I'd imagined this in the hundreds of Hz, not millions.


I'm not sure if you are refering to an MRI here? The transmit pulse is a rather attractive (on an oscilloscope) waveform 'envelope' lasting milliseconds, but containing the 63Mhz carrier. Then transmit is switched off and recieve is turned on by high speed PIN diode switching.

An MRI is not one, but many coils. There is the main 'solenoid' coil, a superconducting coil with about 740 Amps at Nil Volts energising it. This does not change. Additionally, there are 18 superconducting 'shim coils' placed around the main magnet, to ensure a homogenous field.

Inside that physically is a double coil, which is actually concentric fibreglass tubes with a series of coils 'printed' on them. These are the 'Gradient coils' and they actually form the 'mechanism' for selecting the voxel being 'read'. These are resistive coils, driven by a very powerful linear amplifier for each axis, capable of pulses of 200Amps at 2000Vdc!

Inside that again, is the 'tube former' of the RF coil. I am not an 'aerial' man at all, but the coil is a 'birdcage' type antenna. I hope none of you need to be scanned on an MRI, for obvious reasons, but if you have been, you will know that they are very, very noisy.

This is the gradient coil pulsing and the minute amount of contraction the coil imposes on the two ton, resin bonded fibreglass tube, acting like a loudspeaker coil. :eek:

Sorry to go so far off thread, but the question was asked. :)

Roger.

Billredshoes
29th Mar 2009, 17:42
Hi
I can only speak for my self but with the AN2 that I fly we have a small grey box which is used to carry the spare Valves and crystals. !!!!:ok::ok::ok:

Loose rivets
29th Mar 2009, 19:02
Landroger Thanks for that, very interesting. I'd guess some of those things were happening. 200 X 2000 is a lot of watts!


I've been MRI'd several times...if there's another time, I'll be even more of a pest asking to look at the technical bits.

Loose rivets
30th Mar 2009, 05:52
BTW, chaps and chapesses, have you seen this? Note the Swiss army knife.

What dedication.

Fascinating video of a man making a vacuum tube! | The Audiophiliac - CNET News (http://news.cnet.com/8301-13645_3-9843011-47.html)


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