Posted: Wed Feb 13, 2008 5:24 pm    Post subject:   


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Ok, I have a question... what causes that blue fluorescence?

A long time ago I heard someone say with great confidence that it is from air leaking into the vacuum of the tube. That didn't really make sense to me since that meant air was rushing in there and in a few weeks the thing should be bright blue...or just not work at all cause there's no longer a vacuum in the tube. This was in a guitar store after all though...

The only thing that made sense was I do notice that the older my tubes get, the more blue glow that I notice.

So what's it from?
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Muff Wiggler
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Slashdot It! Posted: Wed Feb 13, 2008 5:31 pm    Post subject:   


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Blue glow is GOOD - it's a sign of a good hard vacuum (and other things). Better quality tubes tend to have more blue glow.

It's pink glow between the plates that is bad - a sign of air getting into the tube. The pink is caused by oxygen and nitrogen molecules being ionized. This is BAD. Your tube can blow, the amp's current can run out....bad

(thanks Eric Barbour for my lesson on all this stuff, by the way)

also see here:

http://members.aol.com/larrysb/blue_glow.html
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Slashdot It! Posted: Wed Feb 13, 2008 5:48 pm    Post subject:   


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Rad. Either I heard backwards, or the guy was retarded.

I'm glad to be properly informed now!

As payment for this wealth of information, I give you a "glow shot" of my amp that I did a few years ago.


There certainly seems to be a healthy dose of pink glow in there though
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Slashdot It! Posted: Wed Feb 13, 2008 5:53 pm    Post subject:   


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if you look at my pics - the same pink is there.

I don't think it's the oxygen pink. My cathode heaters are distinctly ORANGE (as they should be) when viewed with my eye. When I take a photo, this orange mixes a bit with the blue to create what appears to be a pinkish haze. I see the same in your pic.

I don't think this is the feared pink glow. I think it's just your normal color from the heaters.

I'm not an expert though.

GREAT PIC by the way!!! WOW!!! I assume those are 6L6's? That thing must be LOUD. Well, it's a Mesa, of course it is...
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Slashdot It! Posted: Wed Feb 13, 2008 6:00 pm    Post subject:   


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Yeah, all 6L6, the three rectifiers didn't quite fit in the shot well...you can see the slight glow of blue of one of them on the right side of the frame.

It's definitely loud, but most Dual Rectifiers are actually louder than it is. It seems to start to fully saturate around 12 oclock on the master volume (although, this is with the dirty channel...i don't think I ever checked with the clean).

Lately, I've only played the clean channel with various pedals (soda meiser is the main distortion now) and I really like the clean channel. Sounds very much like a Fender twin.

The clean channel is very loud though...so much that it's actually very difficult to get a decent practice volume...it's either too quiet or too loud.

I've had this amp for almost 10 years now; I don't want to sell it, but I'm getting closer and closer to getting some kind of small tube amp. *Maybe* even try and make one myself. Maybe.
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Slashdot It! Posted: Wed Feb 13, 2008 6:42 pm    Post subject:   


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Great pix Muff. and Thanks for the pointer to Vacuum Tube Valley, I wasn't aware of those guys. I gotta get a pic of the tube army I put together for the SuperBaby. It only needs one pre, power, and rectifier tube, so you can mix and match at will without having to shell out for matched pairs. With the 28 assorted tubes I have here, that's...uh...756 possible combinations :shock:

Right now: Telefunken 12AT7 into reissue Tung Sol 5881 with a squishy NOS Jan-Philips 5y3GT rectifier.

The Superbaby is like a tube tone test factory. I'm trying to swap only one at a time and live with it for a few sessions before giving in to the urge to change it up again. Haven't found a "bad" sound yet, although some of them break up earlier than I like.

Paul

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Slashdot It! Posted: Wed Feb 27, 2008 2:36 pm    Post subject:   


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just a quick update to further promote Vacuum Tube Valley, and Charlie's Services

After three weeks of joy, I've developed a filament rattle in one of these tubes.

These are very old tubes - over 50 years old. They are also very expensive tubes - one could buy a complete amplifier for less than the cost of this single tube that just went bad.

Anyway, Charlie immediately offered to replace the tube at no cost to me. In a very gracious and friendly fashion as well. He's an awesome guy to deal with.

This man stands behind his products and services.

Imagine getting a very expensive NOS tube on eBay, that was fine when you received it, and then developed a rattle over subsequent weeks. Do you think you would have any luck getting help from the seller?

As if you couldn't tell, I really strongly support Charlie and Vacuum Tube Valley. This is where you guys should get your tubes. If you don't know what to get, or want suggestions, or ideas on how different tubes will sound in your amp, or want the best tubes for a certain fixed amount of money - Charlie will give you excellent advice.
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BLUE GLOW in Electron Tubes
 
Explanation 1:

Transcribed for your convenience from the "Sylvania Engineering Data Service", Volume 1
 

BLUE GLOW

Blue Glows are not tube detriments per se. They are, however, suspects in the eyes of many receiving tube users for lack of a full understanding of their origins. There are several types of Blue Glow which can be described as follows:
 

FLUORESCENCE

This type of glow is usually violet in color and most noticeable around the inside surface of the glass bulb. It is most pronounced on power tubes and is the product of electron bombardment of the glass taking place within the tube. It generally has no adverse affect upon receiver performance, and in fact, tubes displaying this phenomenon are particularily good with respect to gas content.
 

MERCURY VAPOR HAZE

This is a blue-violet glow associated with those tube types which rely upon mercury vapor for proper operation. In such cases, the blue glow should be evident indicating proper operation.
 

GAS

Gas produces a blue haze, generally confined to the vicinity of the mount structure. The proper function of gas types such as thyratrons, voltage regulator and voltage reference tubes, requires the presence of this glow as an indication of proper tube operation. Some voltage regulators use neon instead of argon and as a result exhibit a pink-orange glow. It is, however, a distinct detriment in vacuum receiving types, where the presence of gas in large amounts can cause malfunction of the equipment.
 
 

Submitted by: Doug Haugen
 

Explanation 2:

Transcribed for your convenience from "RADIOTRONICS"Technical Bulletin No. 39 - 24 February 1936
Radiotronics was published by the Amalgamated Wireless Valve Co. of Australia using technologies derived from RCA USA, Marconi, Telefunken and others.

FLUORESCENCE and BLUE GLOW

With modern valves a blue colour is frequently observed (either steady or flickering) on portions of the bulb wall; this is a phenomenum known as "fluorescence" and is somewhat akin to X-ray fluorescence and is due to electron bombardment of the glass.

The colour of this glow and its intensity depend on the nature of the glass, the voltages employed, and the design of the valve.

It has no deleterious effect, however, and actually is a sign of an extremely high vacuum.

It is particularly prominent with type 42, but may also be observed on other types, such as 6A7, 6C6, 6D6, but since in these types a black coating is used inside the bulb the fluorescence is rarley noticed.

Blue glow is a glow between electrodes caused by ionised gas. It is never observed in a really hard valve, but there are certain types in which a very slight glow may be observed which, although indicating a very small amount of gas, is not deleterious to the valve; for example this sometimes applies to type 50.

When any appreciable glow is observed inside the plate, the valve is definitely defective, due to gas, and a negative grid current will be observed if a micro-ammeter is placed in the grid return circuit. A valve reading less than 1 micro-ampere for each 10 milliamperes of anode current is quite satisfactory, and even two to three times this amount of negative grid current is usually quite permissible.

When a valve becomes very gassy, the blue glow frequently extends in the shape of streamers radiating from the ends of the anode, and the valve in this condition is completely unusable.

In high vacuum rectifiers used at voltages not exceeding 400 volts, a small amount of blue glow is not detrimental and may be rather beneficial. High voltage rectifiers, on the other hand, must have an absolute minimum of gas.

Occasionally fluorescence may be seen on the inside surface of the anode or grid; this fluorescence may be easily distinguished from blue glow by the fact that it is a thin film and does not spread through the space between the electrodes.

All types of fluorescence are completely harmless."
 

Voltage Regulator Tubes
Before the zener diode there was the voltage regulator tube.

The operation and application of VR tubes, and some other gas devices considered.


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The very earliest valves were plagued by the problem of residual gas. The last major step in the creation of the modern valve was made in 1913 when I.Langmuir developed a method of producing a hard vacuume.

Most of the world then concentrated on developing hard vacuume valves, but in Germany, particularly between the wars, a lot of research was done into gaseous valves.

The main difference to a normal valve is that these generally had a cold cathode, no heater or filament. Strictly speaking these cathodes aren't actually cold but are heated sufficiently by ion bombardment by the fill-gas.

The inclusion of certain inert gasses such as neon, hydrogen and argon at low pressure in the valve has a significant effect, the most important here being electron multiplication. Under the right conditions electrons leaving the cathode strike gas molecules causing more electrons to be emitted by secondary emission, causing a cascade.

This translates into a gas amplification factor which can be very large. The Geiger-Müeller radiation detector is a good example of this cascade amplification, a single incident particle triggering a cascade current large enough to comfortably detect. In fact most gas tubes show some sensitivity to light and particle radiation so a simple neon can be used to detect nuclear radiation.

The penalty is noise, and while gas amplified valves were used in audio applications, particularly portable, their home turf quickly became industrial applications where the end result was a binary on/off.

Speed was also a problem due to de-ionisation time which limited most gas valves to high audio speeds. Initially using neon it was found that the inclusion of a few percent of hydrogen greatly sped up de-ionisation and thus valve speed.

Not all gas tubes were cold cathode and hot-cathode thyratrons in particular were used a lot in industry to operate relays directly, and in oscilloscopes as relaxation timebases.

Examples of this line of gas tube development are still all around.

The humble neon pilot light
Mercury and sodium vapour lighting, including fluros
“Neon” signs (different colours come from different gas mixes)
Xenon camera flashguns
Disco strobes
Xenon cinema projector lamps
Modern car headlamps
Thyratrons (valve SCR's)
Ignatrons (really big valve SCR's)
Geiger-Müeller radiation detecting tubes
Self-illuminating lift buttons
Telephone lightning arrestors
And of course the voltage regulator tube. Some gasses display a constant-voltage characteristic and this was employed to provide stable voltage supplies, very much in the manner of zener diodes today.

A very simple structure consisting of a cylindrical anode and a straight wire cold-cathode up the centre. The cathode is treated with an emissive coating, but unlike hard vacuume valves these cathodes have to cope with ion-bombardment without being damaged.

The only trick is the striker, a small wire attached inside the anode that ends a few mm short of the cathode. The initial ionisation of the fill gas starts in this gap and provides ions to cascade conduction to the whole tube.


This is VR150 which is, naturally, a 150 volt regulator.

Voltage  US type  UK type 
150  VR150  0D3 (0C2) 
105  VR105  0C3 (0B2) 
90  VR90  0B3 
75  VR75  0A3 

Pat Hawker takes up the story:

 
This is an example of a fairly conventional use of VR tubes in power supplies.


This illustrates the use of the internal base link to protect the regulated circuit should the VR tube be unplugged.


This not only solves a VR application problem, it's a pretty cute little circuit trick. I'll leave it to the reader to work out how this solves the problem.


Here the VR tube is being used for a fixed drop from a higher rail. Strauss used exactly this trick in one of their amplifiers.


As valves go the glow-gap divider was a rare variant even in its day.



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So what's this then? No, not a multi-voltage VR tube but another example of what you can do with gas technology that you can't do with vacuume (or not nearly so easily, anyway).


This little baby is a Decatron and was used extensively for industrial counting/display applications such as Bendix batch counters into the 70's. End-viewing, there were generally mounted in a row with their ends poking though the front panel surrounded by a numbered escutcheon.

There are thirty cathodes, ten digits and two sets of guides arrayed in a circle around a central disk anode. The cathodes are ordered Cathode 0, Guide 1, Guide 2, Cathode 1, G1, G2, C2, G1, G2, C3, and so on.

On power-up one of the numeral cathodes strikes and holds the discharge. Reset is by opening all the cathodes except zero, to which the glow will auto-transfer.

To count the first guide rail is pulsed negative, causing the glow to auto-transfer from the cathode to the first guide cathode next to it. This is allowed to go positive as the second guide rail is pulsed low, capturing the glow. When this guide is allowed to go positive again the glow auto-transfers to the next numeral cathode, now being closer than the starting cathode.

Resistors in each cathode circuit allow the detection of a terminal count. These will count up or down so may batch from zero-to-count or count down to zero. These also appeared in scalers, particle counters in neuclionics.

The smoky band on the glass around the cathodes is their main failure mode. Ion bombardment causes the glass to darken and eventually go silvery and hard to see though. Part of the service routine was swapping all the tubes around one (units to thousands, all down one) to distribute this darkening evenly across all the tubes. I encountered these in pill counters where the batch was generally small so the upper tubes didn't get exercised.

Coupling circuits could either be the old faithful 12AX7 or another gas device called a trigger tube, its operation similar to a SCR.

Counting speed was limited, about 10k counts/sec for this GS10C.

Sidebar: Where higher counting speeds were required, such as in neuclionics, another hard-vacuume tube called the Trochotron was used. These are a cathode-ray beam switching device which were very much faster. In some cases these were used as pre-scalers ahead of decatrons.

These consisted of a valve envelope like a 6L6, but with two rows of numbers, odd and even, wrapping around part of the side. In operating the classic blue/green phosphor glow appeared in a window next to each digit, counting proceeding zig-zag up and down, left to right.

Like decatron counters, these also made heavy use of VR tubes for the various supply and bias rails their operation required, the Bendix having about six different supply voltages.

5992This is a military/industrial version of the 6V6GT, and is rather rare. I have only seen versions labelled Bendix, but GE lists it in their Characteristics book so they must have made some as well. These have rugged brown micanol bases, heavy glass, a short boxy black plate structure, three mica spacers and dual top mounted D getters. This is probably the ultimate holy grail tube to have in the 6V6 world, but their scarcity today keeps them in a fairly obscure corner of that world. Clients tell me these blow away even the NOS RCA blackplate greyglass tubes! They are indeed awesome, and will never be made again. All of these tubes glow blue inside and have slightly longer warm-up time than the 6V6GT, this is normal. They are plug and play compatible with any 6V6 type tube, with normal bias checks and/or adjustments as you would do whenever replacing any power tubes. These are worth seeking out as they are built like a tank and may outlast both you and your amp!

ALL of these vintage tubes will outperform any of the modern made junk, and the demand has grown to the point where I feel many will benefit from this detailed 6V6 list on my site. Please e-mail me if you have any questions.

The 12AX7 is a high-mu twin triode which operates in typical service as a class A amplifier, in the preamp stages of high fidelity, musical instrument, and public address amps, as well as professional line and microphone preamps, and many other audio and communications devices. The demand for the vintage USA and European versions of this tube are beginning to outstrip the supply, and, as a buyer, you need to carefully consider what is available, the device you will use it in, and what your budget allows. The highly regarded vintage brands of Telefunken, Mullard, Amperex, and RCA come at an appropriate premium price over other brands. While this higher price is justified (these tubes sound wonderful and are very long lived) other vintage brands are often just as good. Brands like GE, Sylvania, Tung-Sol, Motorola, Raytheon, and Zenith are often overlooked, but usually perform just as well as the more famous vintage brands, which makes them a terrific value, especially as the stocks of New Old Stock (NOS) tubes vanishes. Indeed, some of these were made by RCA for these other labels, and others like the Tung-Sol and Sylvania have their own followers who prefer these brands over the higher priced premium labels. There is intense hype out there about the 12AX7, and even some absolute fantasies. Some of the European labels have strong snob appeal, which can cloud accurate judgment of these very good tubes, and many self-proclaimed tube experts have either praised or shot down some vintage brands which deserve neither. If you are new to tubes in general, or are replacing a set of Russian or Chinese current production 12AX7 tubes with vintage NOS, you would do well to try several different brands and hear for yourself the sonic nuances. Then you would be well advised to lay in a stock of the brand of your choice "for a rainy day".

The main differences with this family of tubes are as follows:

12AX7: the original version of this tube. This can only be used in parallel filament circuits. This is not a big deal as virtually all audio equipment is of a parallel filament design. These often have large rectangular plates with several horizontal ribs. The older versions have blackplates, which are often preferred by audiophiles.

12AX7A: This version can be used in series or parallel filament circuits. These usually date to the 1960s and have greyplates. Vintage versions of these are about the most sought after tubes of any type today. Often RCA and GE made these for electronic organ manufacturers, and have the organ brand name on the label. These are usually specially selected tubes, and are a great buy---when available! Sometimes, 12AX7A tubes made for the US Military are labeled 12AX7WA, and I have seen WB and WC versions. The W is the military type code, A,B, and C are progressively later productions. These are nice military spec tubes. DO NOT confuse these with current production Russian or Chinese crap with the suffix WX, WB, or WC! These are not military tubes and are not NOS tubes at all!

ECC83: This is the European version of the 12AX7, and is identical to it. Most tubes with ECC83 listed on the label are European, and the ones in demand are Telefunken, Amperex, Mullard, and Siemens. More difficult to find but worth seeking are German made Valvo and the early Holland made Philips Miniwatt and Philips "SQ". Telefunken tubes have a diamond shape molded into the glass on the bottom center of the tube. The most desirable of the Telefunken ECC83 tubes is the "smoothplate" or "flatplate" versions. Like the name implies, the dull grey plates are perfectly smooth. It does not matter if some of these on the market are labeled for Fisher or Dynaco, as Telefunken made these tubes for many OEM applications. They are the EXACT same tube regardless of the label. Amperex and Mullard tubes have a pair of alpha-numeric date codes printed in dark grey near the bottom of the tube. The rarest of these are the BugleBoy cartoon tube label for Amperex (especially if it also has the treble clef music symbol next to the Bugleboy image), and the older Mullard logo that looks like a shield, especially with the letters "BVA" below it. Watch for the Mullard "10M" series of ECC83 tubes in the distinctive royal purple and gold boxes. These sweet tubes were factory screened for a 10,000 hour heater life, matching internal triodes, and low noise, rather like the Telefunken ECC803S tubes. The 10M Master Series has gold plated pins, and the 10M Concert Series has standard pins. Demand and scarcity has driven the price of these babies to record heights lately. The 1950s versions of Mullard and Amperex (and some rare Siemens) tubes often had long narrow plates, with D or square getters, and are the scarcest and finest sounding of these two brands. BEWARE: the Amperex Bugleboy and Telefunken smoothplates are being faked! The fake Amperex may have the words "Bugleboy" and the cartoon tube on the box, and the label will not wash or wipe off. The fake Telefunken may have gold pins (real Telefunken ECC83 tubes had only standard pins) and a perfect, baked enamel label that won't wipe off. Our tubes are carefully screened for fakes when we obtain them, and are guaranteed genuine.

7025: This is the low noise, "industrial" version of the 12AX7. This has a spiral wound filament which reduces hum and noise. These are usually the best choice for demanding hi-fi applications, although the premium brands and blackplate types also have very low noise.

ECC803S, E83CC, E803CC: This is a premium European version of the ECC83. The "S" indicates Special, and these tubes were screened for low noise and matching internal triode elements. These are rare in the USA. Telefunken and Siemens are the brands most often spotted, although several of the Mullard factories made these for various labels. Most of the Telefunken and Siemens tubes had gold plated pins. Sometimes the E83CC also carried the 12AX7WA military type label, even though it is a European made tube.

B759: This is an ultra-low noise version of the 12AX7, with matched triode elements. To my knowledge, only Genalex (Marconi-Osram Valve Co.) made these in England for their "Gold Lion" series. Extremely rare in the USA.

6681: This is listed in some tube manuals as a "special 12AX7", otherwise the electrical specs are the same as the standard version. Being an industrial type it most likely has a ruggedized filament, and is built to withstand many on-off cycles. I believe only RCA made these, regardless of brand on the label. UPDATE: One astute customer has informed me that GE offered 6681 tubes with the typical top seams, date codes, and "Gt. Britain" marking that indicates English Mullard manufacture. Indeed, some GE 12AX7 and 7025 tubes from the 1970s were also Mullard made, so they must have made some 6681s for GE as well. Overall, the 6681 is an excellent step up in quality from the standard 12AX7, and is a bargain at current prices.

5751: This version was usually the one made for the US military. These have a lower gain than the 12AX7, but are otherwise totally compatible. Great hi-fi tubes, as the lower gain reduces noise and microphonics. These are all ruggedized and most can withstand a drop on the floor. The internal triode elements are matched as well. The early versions are more sought after as they usually have extra support rods and an extra mica spacer on top of the plate structure ("Triple Mica"), as well as blackplates, all said to contribute to low microphonics. Don't overlook the greyplate versions, they are also excellent tubes, and many of these have the triple mica as well. The GE non-military version is usually the "five star" series, intended for broadcast use. The RCA broadcast type is the "Command" series. Both the RCA and GE broadcast types are scarce, as are any of the triple mica types.

OK, SO TELL ME HOW THEY SOUND!!

A tough question if there ever was one! The best advice is to get a few types and hear for yourself the good sounds you have been missing. All of these vintage tubes are excellent, much better than the Russian or Chinese yuck that is being made today. When replacing any stock Russian, Chinese, or East Europe tube with any of these vintage NOS types, you will notice immediately that the midrange glare is gone. Gone too is that honky, boxy quality, and the tiring upper midrange screech that current production tubes are famous for. Here are some VERY GENERAL observations about some of these vintage tubes:

TELEFUNKEN, SIEMENS, VALVO, LORENZ, and other German made NOS: These tubes are usually characterized by an impressive open "air" at the top end. The soundstage is large, even in mono applications these tubes have a great 3-D image. The midrange is ruler flat, and the bass is tight and accurate. These tubes have a fine sense of dynamics, and most are impressively quiet. These are not "warm" tubes, and to some ears their lack of midrange warmth may be heard as bright. I tend to think of them as accurate, and their clean, focused sonic image is astonishing. My personal favorites. The Siemens, Valvo, and Lorenz are often priced lower than the more well-known Telefunken, and can be a great bargain since their sonics are similar to the highly regarded Telefunken. Oh, and here is a little secret: the Telefunken ribbed plates sound pretty much the same as the smoothplates! In spite of the "shootouts", and "Some Guy's Tube Lore", and other claims to the contrary, the sonic differences between these two tubes (made in the same era at the same factory) are small and very subjective. Don't let anyone tell you what tubes you should like and what you should not like!

AMPEREX, PHILIPS, MAZDA and other Holland/France/Belgium made NOS: These tubes are a great balance of a clean, airy top end, nice midrange warmth, and accurate bass. They are very pleasant, clean, and musical to listen to in hi-fi applications. Unlike other clean European tubes, these break-up impressively when overdriven in a guitar amp. The Philips Miniwatt series, usually made in the same Heerlen, Holland factory that produced the Amperex Bugleboy tubes, are a great alternative to the increasingly rare Bugleboy. They are identical internally, carry the same factory code markings, and sound the same. These Holland tubes can often be found, sometimes made for other brands or OEM labelled for amplifiers, when the Bugleboys cannot. The rare longplate versions are the same sonically, but with even more soundstage space and detail. The rare French Mazda has the air and sparkle of the Telefunken, the touch of warmth of the Amperex, and adds a nice bit of dynamic punch to the sound.

MULLARD, GENALEX, BRIMAR, and other British made NOS: Like a warm British jacket of the finest tweed, these glorious tubes have an attractive sweet warmth in their midrange and lower regions. The top end is silky and pleasant, without being rolled-off. The best of these tubes retain a fine sense of "air" at the top, and the upper midrange is smooth and liquid. These tubes reproduce the human voice, especially female voices, with haunting realism. The rare longplates and Genalex versions have an eye-popping huge soundstage, razor sharp focus and detail, and an uncolored top end while retaining the warmth of the lower priced versions.

RCA, RAYTHEON, GE, SYLVANIA, and other USA made NOS: This group is very diverse. The RCA, Raytheon, and Sylvania blackplates are among the best here. These are very musical tubes with air and sparkle at the top end, warmth in the mids, and great bass. The RCA are drop dead beautiful in guitar amps, even the lower priced greyplate and longplate versions. They also have a wonderful "phat" gritty sound when overdriven in these amps. The blackplates and most tubes made for organs are very quiet. The greyplate GE is an all-around nice-guy tube to listen to, the longplate here possibly getting the nod for hi-fi use. The Sylvania greyplate and military versions are clean and a bit bright, but the Sylvania 5751 variants are very balanced and pleasant to listen to for hours. The GE blackplate 5751 triple mica is also in great demand as a very musical vintage tube. The RCA 5751 is more like the Mullard, with a rich warmth and wide bandwidth. Currently, the USA made tubes are a nice surprise with their low prices as compared to the European types.


 
  
MULLARD





Mullard was founded by Captain S.R. Mullard in 1920 and was from the beginning one of the finest tube manufacturers in the world. Mullard tubes have a rich, warm, presentation that still supplies all of the detail and information that any "state of the art" tube does but with a suave European flavor that is perfect for many of today's more forward systems. The EL34, EL37, & Mini tubes 6DJ8, 12AX7, 12AU7, etc are some of the longest - lasting, toughest, finest sounding tubes ever made! Mullard is one of the few tubes that have the almost magical ability to separate the noise from the music, and the ability to present the two in different places on the soundstage. This is something that transistors never do! Mullard did go into a gradual slide, correlating to general "efficiency & modernizing" starting on the late 60s and got pretty bad by the late 70s. It was all over by 1981. There also seems to be a good number of tubes coming to the surface that seem to be made by someone else, but on the same machinery that are completely sub-standard. They seem to be from the early 80s. The "other guys" sell these as the real stuff and YOU suffer.

 
 

    
 
   
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Date code MC1 for tube type (MC) and batch code (1) and then B7C, for Blackburn factory and 1957 year...not sure of C, perhaps for March of 1957?


KT66

Original M-OV version of the KT66; this is from late production

KT66 is the designator for a vacuum tube introduced by Marconi-Osram Valve Co. Ltd. (M-OV) of Britain in 1937.

The KT66 is the direct descendant of the "Harries Valve" developed by British engineer J. Owen Harries and marketed by the Hivac Co. Ltd. in 1935. Harries is believed to be the first engineer to discover the "critical distance" effect, which maximized the efficiency of a power tetrode by positioning its anode at a distance which is a specific multiple of the screen grid-cathode distance. This design also minimized interference of secondary emission electrons dislodged from the anode.

EMI engineers Cabot Bull and Sidney Rodda improved the Harries design with a pair of beam plates, connected to the cathode, which directed the electron streams into two narrow areas and also acted like a suppressor grid to absorb some secondary electrons. The beam design was also undertaken to avoid the patents which the giant Philips firm held on power pentodes in Europe. Because this overall design eliminated the "tetrode kink" in the lower parts of the tetrode's voltage-current characteristic curves (which sometimes caused tetrode amplifiers to become unstable), M-OV marketed this tube family under the sobriquet "KT", meaning "kinkless tetrode".

A number of different KT tubes were marketed by M-OV thereafter. Some were versions of existing American beam tubes or European power pentodes, such as the KT66 (6L6), KT77 (EL34) and KT63 (6V6); while others were unique to the KT series.

Although the RCA 6L6 of 1936, the result of a license agreement between RCA and M-OV, was the first true "beam power tube" on the market, the later KT66 became almost equally famous. The two tubes were nearly interchangeable, except that the KT66 was much more rugged and capable of tolerating overvoltages and overcurrents that would destroy a 6L6.The KT66 was very popular in European radios and audio amplifiers. It was the standard output tube in the classic Quad II (1952, a version of which is still being manufactured today) and Leak TL/12 (1948), both among the earliest British hi-fi amplifiers. Because of their excellent electrical characteristics and overload tolerance, KT66s are preferred by some guitar players for use in guitar amps in place of 6L6GC. (Readjustment of the amplifier's bias is necessary.)

M-OV ceased glass vacuum tube manufacture in 1988, yet their old audio tube types have become valuable collectibles. As of 2004, an old M-OV KT66 (bearing the official "Genalex" marketing brand that M-OV used outside the UK), unused in its original carton, can easily bring US$250. The trade in KT66s is so intense that at least two copies are being manufactured; one at Reflektor Saratov in Russia, and one at Liuzhou in China. These modern tubes are often packaged in counterfeit "Genalex" boxes and sold as being the real thing.
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1899 1899 - In 1899, Fleming became scientific advisor to the Marconi Company and during the next few years made his most notable contributions. These included his design of the Poldhu transmitter station and the invention of the Thermionic Valve. During the First World War ...This allowed more staff to be appointed to the department including WC Clinton and JT Morris. In 1899, Fleming became scientific advisor to the Marconi Company and during the next few years made his most notable contributions. These included his design of the Poldhu transmitter station and the invention of the Thermionic Valve. During the First World War the resources of the department were placed at the disposal of the Admiralty for research to aid the anti-submarine campaign ... 
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From A History of the Department — Electronic & Electrical Engineering @ UCL
www.ee.ucl.ac.uk/about/history.html

1904 1904 - The Marconi valve was not invented by Marconi, as the name might imply, but it is a product of the fertile brain of England's foremost expert, Dr. Fleming. Fleming invented the valve in 1904, and several years later DeForest patented an instrument which he called ...The Marconi valve was not invented by Marconi, as the name might imply, but it is a product of the fertile brain of England's foremost expert, Dr. Fleming. Fleming invented the valve in 1904, and several years later DeForest patented an instrument which he called an audion, but which in reality was nothing but a form of Fleming's valve. When first used, the valve consisted of an ordinary small incandescent lamp, wtb a small metal plate suspended directly above but not touching the ... 
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From Electrician and Mechanic
books.google.com/books?id=WsYSAAAAYAAJ&pg=PA53 ...

1905 1905 - With the thermionic valve to his credit, Fleming resumed his role as scientific adviser to the Marconi Company in 1905. The following year Lee De Forest announced the audion, or triode valve, which was Fleming's valve with a further electrode included between the ...With the thermionic valve to his credit, Fleming resumed his role as scientific adviser to the Marconi Company in 1905. The following year Lee De Forest announced the audion, or triode valve, which was Fleming's valve with a further electrode included between the filament and Fleming's extra electrode. This device could function both as an amplifier and as an oscillator, enabling wireless telegraphy to make rapid progress. 
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From Brian Bowers - Wireless: From Marconi's Black Box to the Audion (review) … - Related web pages
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1919 1919 - Marconi-Osram Valve Co. Ltd. was founded in 1919, and took over the manufacture of Marconi valves from Ediswan, the trade name of the Edison and Swan United Electric Light Company Limited. The M-OV, as it was later known, was a joint Marconi-GEC valve ...Marconi-Osram Valve Co. Ltd. was founded in 1919, and took over the manufacture of Marconi valves from Ediswan, the trade name of the Edison and Swan United Electric Light Company Limited. The M-OV, as it was later known, was a joint Marconi-GEC valve-manufacturing company. It produced all of its valves at the Osram Lamp Works at Hammersmith, London. 
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From MHS | Marconi Collection | Catalogue - Related web pages
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1924 1924 - So I took a correspondence course in wireless telegraphy in 1924 with the Marconi School of Wireless. They issued me with a "REXONA GRAMOPHONE" and a record for morse practice. However, after a while I had memorised its contents, so I built a one valve ...So I took a correspondence course in wireless telegraphy in 1924 with the Marconi School of Wireless. They issued me with a "REXONA GRAMOPHONE" and a record for morse practice. However, after a while I had memorised its contents, so I built a one valve set using a 201A valve and honeycomb coils to listen to the Melbourne shore station VIM and any ships in the Bay on 600 metres. 
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From Marconi School of Wireless - Related web pages
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1926 May 18, 1926 - Captain George Gottwaldt, the wireless expert of the Norge, in describing his experiences with the s radio apparatus, says: "The Norge was equipped with a specially constructed Marconi valve transmitter, giving an aerial output of a maximum of about 200 watts. ...
From NOBILE ANALYZES FLIGHT; Normal Speed Was 57 Miles an Hour -- … - Related web pages
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1928 Nov 1, 1928 - ... -T'fi'e Marconi Valve is different. .EVERY •• ,. : ' „";■.■•■■• "-i-part of it is designed to work. As a 'i ":.;•■ result the Marconi Valve is a perfect valve. j Marconi Valves do everything that a valve should do. ■'■^./■■'■'.'.,.'i;. SEND FOR FREE BOOKLET. a Perfect set of ...
From Page 6 Advertisements Column 1 - Related web pages
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1936 1936 - 9 The patent was awarded to Barnett Levin and Nyman Levin of the Marconi Wireless Telegraph Company in England in 1936. The specification states: "The invention relates to light valves and has for its object, to provide improved light valves of great ...The first patent issued on an application of liquid crystals was a light valve device that operated at about 50 volts and at elevated temperatures using polarized light.9 The patent was awarded to Barnett Levin and Nyman Levin of the Marconi Wireless Telegraph Company in England in 1936. The specification states: "The invention relates to light valves and has for its object, to provide improved light valves of great sensitivity suitable for use as optical translating devices in ... 
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From Liquid Gold - Related web pages
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1937 Jul 21, 1937 - Rapid technical advances were made during the next few years, with many inventions by Marconi and others, including John Ambrose Fleming, an Englishman, whose valve detector was the of the vacuum tube, and Lee De Forest, an American, who added the grid. Marconi worked to perfect his ...
From … to Perfecting His Uses of Radio Waves; MARCONI PIONEER IN RADIO'S … - Related web pages
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1957 1957 - But, in 1957, a competitor appeared from England. The Marconi- Osram Valve Co. of Hammersmith, London, which already made and sold the KT66 and KT77 beam tubes under the Genalex name, introduced the imposing KT88. Its improved ratings made it a popular ...For many years, Tung-Sol's version was seen as one of the best power tubes on the market. But, in 1957, a competitor appeared from England. The Marconi- Osram Valve Co. of Hammersmith, London, which already made and sold the KT66 and KT77 beam tubes under the Genalex name, introduced the imposing KT88. Its improved ratings made it a popular substitute in hi-fi amps. 
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From Issue 6 Articles - Related web pages
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