Hello all.What is the signifigance of metal power tubes such as 6L6 or 6V6.Were the metal ones invented first or are they special purpose?Do they actually sound different then the glass ones(frequency response etc.)Also what about metal or glass radio tubes like 6SA7 etc???Thanks for any info!

:Hello all.What is the signifigance of metal power tubes such as 6L6 or 6V6.Were the metal ones invented first or are they special purpose?Do they actually sound different then the glass ones(frequency response etc.)Also what about metal or glass radio tubes like 6SA7 etc???Thanks for any info!

Chris: My understanding is that glass tubes came first. I'm not aware of any difference in performance except that the metal envelope also acts as a shield, so some radios were designed to use metal tubes and didn't include metal shields around the tubes seen in most other radios. Mark from Kalamazoo

No internal difference, electrically. Of course, metal envelopes are essentially unbreakable. However, they can rust.

I have a theory that metal tubes may run a little hotter, but know of no proof. Since they are in a vacuum, tube internals dissipate their heat by radiation. With a glass envelope, some of that radiation will zip right through the glass?
Doug

Just speculation: Perhaps to use in military equipment where signal radiation was of a concern. Or perhaps a tube that was less breakable...

Radiodoc
***************

:No internal difference, electrically. Of course, metal envelopes are essentially unbreakable. However, they can rust.
:
:I have a theory that metal tubes may run a little hotter, but know of no proof. Since they are in a vacuum, tube internals dissipate their heat by radiation. With a glass envelope, some of that radiation will zip right through the glass?
:Doug

Hi All,
The 5, 6 and 7 prong tubes from the early 1930's were never made in metal, they were all glass.
RCA started making metal glass tubes in 1936 expecting their better heat dissipation would cause them to last longer. It didn't work out that way, though.
The link http://www.radiophile.com/rcat10-1.htm is to the source I have on this. It is a great on line radio museum. The guy who has it hasn't updated it since 2007, though.

Best Regards,

Bill Grimm

I suspect that metal tubes were, in part, an RCA marketing gimmick.

Prior to WW2, when the Navy developed the proximity fuze for anti-aircraft projectiles, RCA made a strong pitch for their metal tubes - to better withstand the G forces. Sylvania jumped in, claiming that they could ruggedize their glass tubes. The Navy went with glass tubes - and the proximity fuze was very successful.
Doug

Some metal tubes seem to be smaller than their glass equivalents. I was just assuming they were less expensive to make, like a glass Coke or beer bottle vs. the modern aluminum can or plastic one.

Seems they would be less prone to breakage, but I prefer to see to tube "glow" through the holes in the chassis instead of hiding it inside a can.

Does anyone know the story of the evolution from ST "fat glass" (usually 'G' suffix) to the straight 'GT' suffix style? Was this a space saving move?

I found it interesting when I found out that the metal was actually the tube envelope. I had thought it was just a shield over a glass tube albeit slimed down tube. Elements within tube same as glass with glass base which is the seal at bottom of metal tube, then vacuum applied. One thing about metal tubes to bear in mind is that pin 1 is grounded to metal shield/tube. Some manufacturers use that pin as a connecting point as most glass tubes do not use that pin. Given that any signal/voltage at the connecting points would be on the metal shield if one is used where glass was before. That could cause safety/RFI problems and shorts if tube came in contact with chassis. I picked up that info on here. PL

:Some metal tubes seem to be smaller than their glass equivalents. I was just assuming they were less expensive to make, like a glass Coke or beer bottle vs. the modern aluminum can or plastic one.
:
:Seems they would be less prone to breakage, but I prefer to see to tube "glow" through the holes in the chassis instead of hiding it inside a can.
:
:Does anyone know the story of the evolution from ST "fat glass" (usually 'G' suffix) to the straight 'GT' suffix style? Was this a space saving move?

I have a price list on tubes dated 1983 and made by Sylvania.
Glass tubes seemed to be less expensive. For example 6SQ7 is $22.40 but the 6SQ7GT is $17.70.

Best Regards,

Bill Grimm

Prices have gone down. RadioDaze sells NOS 6SQ7GT for $4.00.
Doug

:I have a price list on tubes dated 1983 and made by Sylvania.
:Glass tubes seemed to be less expensive. For example 6SQ7 is $22.40 but the 6SQ7GT is $17.70.
:
:Best Regards,
:
:Bill Grimm

Hi Doug,
Funny isn't it. Getting those tubes in 1983 as an investment would have really been bad.

In 1983 6SQ7 was near obsolete for replacement and had been obsolete for new designs for many years. I have noticed even now that when an electrical component gets near end of life its price goes up. I suspect that in 1953 the 6SQ7 would have been closer to $4.

Best Regards,

Bill Grimm

::Hello all.What is the signifigance of metal power tubes such as 6L6 or 6V6.Were the metal ones invented first or are they special purpose?Do they actually sound different then the glass ones(frequency response etc.)Also what about metal or glass radio tubes like 6SA7 etc???Thanks for any info!
:
:Chris: My understanding is that glass tubes came first. I'm not aware of any difference in performance except that the metal envelope also acts as a shield, so some radios were designed to use metal tubes and didn't include metal shields around the tubes seen in most other radios. Mark from Kalamazoo

According to the 1937 Sylvania Tube manual, and other service literature of the time, extensive work was done prior to 1937 to develop smaller tubes, increase factory yields, and allow set manufacturers to eliminate tube shields, and put stuff closer together. The first metal tube on the market was the 5Z4 in 1935. With metal tubes, the octal tube base and smaller top cap came to be the standard. Soon after additional metal tubes were introduced, tube manufacturers began to manufacture the "G" line of tubes with octal bases. The glass tubes turned out to be less expensive than the metal tubes, and some set manufacturers and service men preferred them.

Metal power tubes were thought by some to be more rugged and more reliable. Glass does reflect some of the heat back inside the tube, and the black metal cans of metal tubes are a better heat radiator. The massive Crosley Model WLW used 37 metal tubes and no glass tubes except for the tuning eye.

The metal shell is typically the tube envelope. One exception is the 0Z4, which has a glass tube surrounded by cardboard inside the metal shell to keep the OZ4 parts at a higher temperature.

Later came the GT/G or GT (T for Tubular) tubes. 41 and 80 tubes were never made as metal tubes, but came to be made as both the traditional 'ST' glass tubes, and a smaller tubular version. Loctal tubes came later, about 1939.

The glass-to-metal seals, and actuating the getter in metal tubes came to be more of a problem than was anticipated, raising the manufacturing costs of these tubes above what had been projected.

Ed M