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JOC, why did several manufacturers go to metal cased tubes in the 1930-40's? John Stokes' book on the history of tubes discusses the variants and manufacturers --- but neither the reasons to get away from glass envelopes. Nor how the vacuum is maintained.
Thanks!
Follow Ups:
I think the primary purpose was for shielding in RF circuits. Most have pin one connected to the case.
Be careful using metal 6V6 and 6L6. Lots of amps (Fender included) use pin one as a tie point for the screen resistor. You can get a nasty shock by touching the metal case.
Besides that, they can sound very good. AFAIK, they have the same internals as blackplate 6v6 and 6L6.
Perhaps, the tightness of the envelope has sonic differences...
Many older audio folks liked the metal 6L6 family...
/.
It's a beam-power pentode.
R.
...thin bottle like the EL34. The 6L6, metal cased or glass on the outside, will always be a beam power tube.
I found the R-Type site pretty informative because it allows you to at least draw a grainy picture
http://www.r-type.org/static/contents.htm
*UK catkin valves* http://www.r-type.org/addtext/add012.htm
*US metal valves* http://www.r-type.org/articles/art-018.htm
According to what I see here, metal envelope valves were released as a follow up to the older balloon or coke-bottle style valves, mostly as a fact of WW2 needing a better tube (smaller, higher dissipation, better reliability, electrical shielding).
I *think* the metal envelope is more of a stop gap solution until a better glass could be made and a better vacuum could be achieved. The metal tube seems to be glass base formed and sealed into a metal envelope. There's no glass beyond the pin-base to tie-in and anchor the internals (may have acted like insulation).
My question it... knowing what we know of making glass tubes... How do you evacuate the metal can? *brain explodes*
And one hopes that the metal never goes live! *heart explodes*
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
May your tubes be lively and long-lasting. Holy be thy heater.
"How do you evacuate the metal can?"
Like most other octal tubes, they have a glass tip that extends down into the center keyway/locating pin.
I think that you have hit the nail on the head -
Better reliability, smaller envelope, and war time use...
I'll look back on some samples of 1614 power tubes that I have from a pair of old MC-30s - IIRC the envelope was evacuated through the glass base...But That could be incorrect...
Happy listening
Underneath that metal is glass, my friends...the metal is like a shielding and protective turtle shell...
You may be getting confused between the steel-shelled tubes which have the glass base and aluminium cased types such as EF50 which had an all-glass type enclosed within a metal shell
Al
but the old octal style base metal tubes have glass only at the base.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
.
Have Fun and Enjoy the Music
"Still Working the Problem"
"Underneath that metal is glass..."
Not true of the 6sh7 I just destroyed.
The only glass is at the base. The rest of the envelop is just metal.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre is correct, in fact not long ago someone posted a link to a You Tube video on how tubes are made and it featured metal shell tubes. No glass guys!
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