# Metal clad tubes



## J-75 (Jul 29, 2010)

Anyone know why metal jacketed power tubes (6V6, 6L6) are never employed in music amps? Aren't they electrical equivalents to glass envelopes?

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## dcole (Oct 8, 2008)

They are electrically equivalent. Most guys like to see the insides of tubes though. You can tell a lot by seeing the plates and filaments while the tube is running.

Just last week I looked at an Orange OR120 that had a bad heater on one tube. You could see it so I swapped the tubes location and the problem persisted with that tube. In a metal envelope I would have never seen the bad heater.

I have though of doing a "Stealth" amp using metal envelope 6SF5's and 6L6's.


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## WCGill (Mar 27, 2009)

Early ('30's, '40's) amps are loaded with metal tubes. As glass became the industry standard manufacturers used them.


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## Lincoln (Jun 2, 2008)

and maybe because metal tubes haven't been made since the 50's? Stock is running low by now and what's is left is junk.

I thought it would be cool to build an old fender Pro amp using the old metal tubes (even the pre-amp tubes) so I did. Now I can't find good tubes for it. Every time I use the amp for any length of time, another pre-amp tube goes microphonic. And I'm ordering "NOS" tubes......not used ones........although it's pretty hard to tell without the glass. 
Everybody says the octal preamp tubes are "sweeter" than 12AX7's. I couldn't hear the sweetness. I'm going to rebuild the amp using 12AX7.


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## dtsaudio (Apr 15, 2009)

I suspect that metal tubes just faded away due to manufacturing costs. Underneath the plastic base there is a glass plate used to seal the metal can. It's a lot easier to make a total glass envelope. Metal tubes while they pre-date WWII were used for the military primarily during the war.
It's important to note a metal can 6L6 is not the same as a glass 6L6G or GC. The ratings are significantly lower. There are a lot of other examples of this as well.


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## nonreverb (Sep 19, 2006)

On the whole, metal tubes tend to be more microphonic as well more costly to manufacture. Once they had the glass tube perfected it replaced most metal tube types pretty quickly.


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## nonreverb (Sep 19, 2006)

dtsaudio said:


> I suspect that metal tubes just faded away due to manufacturing costs. Underneath the plastic base there is a glass plate used to seal the metal can. It's a lot easier to make a total glass envelope. Metal tubes while they pre-date WWII were used for the military primarily during the war.
> It's important to note a metal can 6L6 is not the same as a glass 6L6G or GC. The ratings are significantly lower. There are a lot of other examples of this as well.


The 6L6G hourglass style is pretty close to spec as the metal 6L6. You cannot use 6L6G's in any new style amps or to replace in an amp designed for 6L6GC. They'll fail pretty quick.


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## dtsaudio (Apr 15, 2009)

You are of course correct. When it comes to 6L6's and their many versions care must be taken in replacing new with old. Even the 5881 has lower ratings than a GC


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## nonreverb (Sep 19, 2006)

I have several of those 6L6, and 6L6G's. It's unfortunate there's not much use for them now. I suppose the 6L6G's might work ok in a 6V6 amp.


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## Lincoln (Jun 2, 2008)

Well this thread just solved the problem I've been having with my 5C5. Feeling awfully stupid right now. I didn't realize there was such a thing as 6SC7GT tubes. All this time I've only been searching for 6SC7 tubes because that's what the schematic called for.


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## gtone (Nov 1, 2009)

Lincoln - PM'd you about some 6SC7 tubes I have. Yours if you want 'em as you've helped me out in the past with schematics and such...


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## gtone (Nov 1, 2009)

As voltages and gain factors crept up in amp designs over the years, the metal tubes quickly became obsolete (by early 50's, it seems). If you have an older circuit or want to build one, you could do so based on the metal tubes and snag a lifetime supply of NOS metal tubes on the cheap - hell I'm giving mine away!!


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## Lincoln (Jun 2, 2008)

gtone said:


> As voltages and gain factors crept up in amp designs over the years, the metal tubes quickly became obsolete (by early 50's, it seems). If you have an older circuit or want to build one, you could do so based on the metal tubes and snag a lifetime supply of NOS metal tubes on the cheap - hell I'm giving mine away!!


Much appreciated gtone. Those old tubes are getting harder to find every year.


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## Lincoln (Jun 2, 2008)

The old saying "be careful what you wish for" doesn't apply on this site. The heavens opened up and rained vintage tubes on me. Thanks to Steve (gtone) I now have enough 6CS7's to keep my 5C5 roaring forever more.
So many great people in this place.


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## gtone (Nov 1, 2009)

Gotta take care of our good neighbours in AB...


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