Metal VS Glass Tubes

[tablebeast]

So, the first tubes were glass, then they turned to metal only to return to glass. Is that right? With a lot of 6 volt octal tubes there is usually a regular metal cased model and then a glass 'G' and maybe even a 'GT' model all for the same basic tube. The glass ones are supposed to sound better and from what I've heard I tend to agree. So why the metal casing? Is it just cost related? Shielding? They must have been cheaper to make so if that's the case why didn't they continue making them in metal cases? Are there things about the metal tubes that are superior? I've found a lot of info on the internet about the WHAT as far as metal and glass tubes, but I can't find much on the WHY. As in, 'why metal tubes' and 'why glass tubes?' Inquiring minds want to know.

Jesse

 

[Guest]

No sound difference between metal and glass. Metal tubes a first were developed for military purposes. Later, as soon as nuclear weapon and heavy artillery anyway kills everything it did not matter anymore too much, were tube envelopes made of glass or a metal.
When computers became available, people who wanted them for Fourex games managed to invent a "closed vacuum tax", to force the industry to produce smaller tubes.

 

[CJ]

It depends on the tube. Metal 6L6's have terrible sound.
Metal 6SJ7's sound great.

Some of them.

RCA and GE brought out the metal tube in 1934.
Developed in secrecy by GE, the new design used a steel shell with individual glass filled eyelets for the lead-in wires, on a new, convienientt octal base. This product line gave GE and other radio makers a complete tube compliment for the 1935 line of sets.

The original nine were:

5Z4 rect
6A8 and 6L7, converters
6C5 and 6F5 triodes
6F6 power pentode
6H6 duodiode
6J7 and 6K7 pentodes
A tenth type, the 6D5 power triode, was withdrawn before major production.

The other tube makers were unhappy to have their product line obsoleted.
Some, like Raytheon, signed up to adopt the new line.
Others were unhappy with RCA's invention.
The multiple eyelet seals were sure to leak, so they hurried to repackage their big pin produsts with octal bases.
Yet others like Arcturus and Triad quickly devised what they termed the "perfected" metal tube, the metal-glass (MG) design, where a small conventional stem sealed glass tube was hidden inside a steel shell, on the same octal waferbase as GE-RCA used.

The MG push was short lived. The RMA issued registration in July, 1935 on the 5Z4MG, 6A8MG, 6C5MG, 6D5MG, 6F5MG, 6F6MG
6H6MG, 6J7MG, 6K7MG, copies of RCA's line, plus the 25Z5MG rectifier for AC/DC sets.


Later registrations thru MArch 1936 covered the 6M6MG, 6N7MG, 6Q7MG, and (today unknown) 6Z6MG rectifier.
For those wanting to collect this set today, eventual adders were the 6J5MG, 6Z5MG, 25A6MG, and 25Z6MG.

 

[Guest]

[quote author="CJ"]It depends on the tube. Metal 6L6's have terrible sound.
[/quote]

The same as plain 6L6Gs.

 

[CJ]

Well, the metal 6L6 sucks in guitar amps, I have never tried it in an SE amp. Might be OK. Probably a cleaner sound. A typical metal 6L6 with 300 volts o it's plate puts out about 0.6% distortion running in triode, where=as a 6L6-GC runs at about 1.2% distortion.

The original metal 6L6 was a typical design for RCA at that time.
Metal-shell tubes were popular in the 1930's.
They were heavily marketed to the public who feared injuring their hands on broken glass,
and to radio manufactures who, among other things, appreciated the fact metal tubes were
less likely to break during shipping of theor radio sets.
An added feature was the shielding effect of the metal envelope, which improved radio performance.

The steel envelope was more expensive to manufacture and had real problems dissipating heat, so the fad was virtually over by 1950.
The metal 6L6 and it's premium version, 1614, were often used in early jukeboxe amps and in many
Zenith radio chassis, not to mention PA amps.

.
.
More on the metal progression...

A better variant on the MG idea was the Arcturus "Coronet" type, which used a new glass button stem construction, with the lead-in wires sealed in a circle.
These were used in a line of octal based replacement types, to be used with an adapter to "modernize" existing radios.
This line was:
24
27
51
56
57
58
75
77
78
85
2A6 and
25Z6


Meanwhile, RCA overcame great startup difficulties and got true metal tubes into full production, introducing additional types as well.
To overcome the high cost of the eyelet base, a buttom stem was put into use in 1936.
In this design, the glass stem disk with it's lead-in wires was sealed into the bottom of the metal envelope.
The button stem idea had great benifits in later stem designs.
Metal tubes continued to be registered, right up until the start of WW2,
and even a few variant types afterward, but other designs took over.

These included the Loktal (Sylvania), the miniture (RCA), the subminiature (Hytron), the Lighthouse Triode (GE),
the Button Stem Octal (WE) the Nuvistor (RCA) the Compactron (GE), the Novar (RCA),
the 9-T9 (Sylvania), and the Filthy Sanchez (CJ). :green:

 

[skipwave]

[quote author="Wavebourn"][quote author="CJ"]It depends on the tube. Metal 6L6's have terrible sound.
[/quote]

The same as plain 6L6Gs.[/quote]

My amp disagrees, but it always was rather loud and strongly biased. :razz:

 

[Guest]

[quote author="skipwave"][quote author="Wavebourn"][quote author="CJ"]It depends on the tube. Metal 6L6's have terrible sound.
[/quote]

The same as plain 6L6Gs.[/quote]

My amp disagrees, but it always was rather loud and strongly biased. :razz:[/quote]

Mine too, but they did not see any difference in envelope materials.

 

[CJ]

Once again proving that sound is subjective.
I have been comparing metal 6L6's in git amps starting back in 71 when I couldn't afford a nice glass RCA black plate.
If you are on a limited budget and have siome metal 6L6's, fine. Be happy.
But why mess with them when you can have something that sounds better?

Some interestinf stuff:

http://www.jt30.com/jt30page/micKtubes/Metal-6V6s.html


"BTW, if you ever want to experiment, put in a set of the old METAL 6L6's. These do sound AWFUL, but you can experiment and get lots of weird tones."

(www.poprecords.com/cowabunga/gearfaq.html)

OK, were both wrong, this is the real reason why metal tubes suck ( and watch that pin 1!):


"The problem I'd have using metal tubes today is: 1) I can't enjoy that tube glow and 2) its difficult to observe any failures that may be brewing in the tube."


:grin:

 

[Guest]

I sold on ePay a matched quad of metal pre-WWII RCA JAN 6L6s, they sounded excellent and measured better than new 6L6 and 6P3S. However, if to put them instead of 6L6GC you will not see hor red plates that melt from power they can't dissipate. The same with glass 6L6, but at least you will see arching and red plates...

 

[CJ]

There is no glass 6L6, only the 6L6G.

Just flip em upside down in an oil bath like those crazy hammies did.\

I hope your not the guy who lured the poor kid with the Mac 30 into buying those.
:shock:

 

[Guest]

[quote author="CJ"]There is no glass 6L6, only the 6L6G.
[/quote]

Glass 6L6 are called 6L6G.
6L6GC is a tube that is rated for more dissipation than original 6L6.

 

[CJ]

Right.
Then there's the STR, some freaks used to run 750 on those guys.