Origin of the 6L6GC?

[Rhodesplyr]

Does anyone know which tube manufacturer originally came up with the 6L6GC and when it first became available? GE and Sylvania both have 1959 6L6GC datasheets, while the RCA datasheet available online is from 1960.

[JoeM]

RCA

http://www.vacuumtubevalley.com/Magazines/VTV04a.pdf

See the 6L6 article by Eric Barbour. Gives some of the historical background of the 6L6.

[bob p]

I'm not sure if your question is asking who developed the 6L6, or who developed the 6L6GC. I think the answer to both questions is RCA.

the History section of this Wikipedia article might help.

6L6 - Wikipedia, the free encyclopedia

[Rhodesplyr]

Sorry I was vague. I just assumed that everyone (on here) knew RCA developed the original 6L6 (as Tung Sol developed the 5881).

I read Barbour's article, but he equivocates on the origins of the GC version:

"In 1959, a five-ply combination metal sandwich type plate design and a different maximum rating system allowed the 6L6GC to raise the plate dissipation from 19 to 30 watts."

It's an oddly structured sentence in which he avoids directly stating which manufacturer came up with this five-ply plate design.

This 1960 General Electric document that touts their five-ply plate material makes it sound like it's GE's innovation.

http://n4trb.com/AmateurRadio/GE_Ham...5%20No%201.pdf

I found a couple of other references online that suggest that GE developed this plate material, including a post by Ned Carlson who wrote that GE introduced the 6L6GC in 1961. But then, there's a '59 GE 6L6GC datasheet that states in a note that it supercedes an earlier '58 document. Sylvania published a 6L6GC datasheet in 1959. The earliest RCA 6L6GC datasheet I can find is 1960.

[Enzo]

My original 1959 Sylvania book - a looseleaf - has the 6L6, 6L6G, 6L6GB. The 6L6GC pages are later updates added in, but they are still also dated 1959.

[Rhodesplyr]

My original 1959 Sylvania book - a looseleaf - has the 6L6, 6L6G, 6L6GB. The 6L6GC pages are later updates added in, but they are still also dated 1959.

All the data seems to circle around that date, including the schematic of Fender's Brown Vibrasonic 5G13, which, on a quick scan of the Fender Field Guide site is the first to list 6L6GC rather than 5881 on the schematic.

What brand did Fender tend to use as original equipment? I would guess that the 5881s would all have been Tung Sol, but I'm not sure.

[Rhodesplyr]

Another source claims that the five-ply plate material that made the GC possible was developed by Texas Instruments via an "explosion welding" process and would, thus, have been available to all the tube manufacturers, not just GE.

I have *no* idea if this is true, but it sounds interesting and might explain why the 6L6GB's made by different manufacturers all got a wattage bump around the same time.

[Rhodesplyr]

Confirmation:

The five-ply material used for 6L6GC plates was developed by Texas Instruments. They filed for a patent in 1959, and it was granted in 1963: US Patent 3112185.

[Tom Phillips]

Very interesting. so the plate in a 6L6GC isn't just a simple piece of thin sheet metal. Things like this show how many specialized processes were used in the manufacturing of vacuum tubes. The text of the patent mentions "oxygen free copper." Until now I had not run across that term in old documents or at least never noticed it.
The attached figure is from the patent.

[km6xz]

The technical details of how the materials were processed and handled were trade secrets and are probably lost today. The layered plate material is a good example. It is simple to state but how to actually make it when each of those materials have very different coefficients of expansion and surface charge migration. The devil is in the details and why modern tubes do not perform as consistently or as long as back in the day when some of the best minds in engineering and materials science were working on these problems.
Now, all the new companies, are market driven rather than process or technology driven. Taking apart an old tube and measuring element size and positioning is not enough to duplicate them. Original research on materials and tubes ended many decades ago and no current company bothers with such details as QC tests of finer parameters. The only tube company which probably has the equipment to analyze detailed characteristic of tubes is Svetlana but they are essentially non-players now. I have two tube curve tracers but I know from talking with people in two of the common brands, that capability is both not available and not cared about. Make a tube that looks like old tubes and sell a bunch.....that is the "research" in tube manufacturing today.

The most profit comes from the balance between just enough quality to get by and less than needed to hurt sales of replacements. There is no incentive to make a tube that lasts 10 years.

[Jazz P Bass]

From the patent it appears that the main reason for the multilayer plate was thermal.
That would explain the RCA 'graphite' black plate.

[km6xz]

Graphite was used well before that time for high power tubes as anode material, or graphite or zirconium coated tantalum so it would not be the first application of graphite coatings, by almost 3 decades of common usage. The popular 813 came in metal or graphite plates and was used from the mid 35s on. Graphite plate 833's used as modulators and rf power output stages in AM broadcast not uncommon. The lower power( 125 watts dissipation) 805/VT-143 had solid machined graphite plates without a metal substrate. The most common graphite tube found in the home might have been a pair of the 3-500G, the graphite version of the more popular 3-500Z. Both would dissipate 1000 watts as a pair. There were used in a lot of commercial ham linear amplifiers. Dozens of power tubes had graphite coating on a metal substrate in stead of solid machined graphite like the ones I mention above. Most people thought the Z in 3-500z was because they could run with Zero bias. They did but the Z actually stood for Zinc plated. Both the graphite and zinc were zero bias tubes.

[Rhodesplyr]

In my search for the 5-ply patent, I ran across a discussion of carbon-coated plate materials, and, based on the discussion, there seems to have been some problem with the carbon actually diffusing into the metal over time, leaving bare spots on the surface. There was some mention of nickel-plating as a way to prevent this, but the cost was high to do it that way.

[big_teee]

Maybe it's been said already?
Which is oldest the 6L6, or the 5881, and what is the big difference?
T

[Tom Phillips]

Which is oldest the 6L6, or the 5881, and what is the big difference?

The 6L6 was first. The 5881 was a ruggedized 6L6*. The ruggedization was mostly mechanical rather than for higher operating voltages. Edit: The plate and screen dissipation rating was increased ~20% over that of the 6L6 and 6L6G. The bottle size of the 5881 was smaller too.
Cheers,
Tom