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Old 3rd May 2008, 05:07 PM   #1
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Default Metal Can 6CA7s

For audio use, how do GE full metal 6CA7s compare to the glass version?

I have a few of these and I'm wanting to know whether they are worth keeping for DIY amplifier use.

Thanks, Greg
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Old 3rd May 2008, 06:14 PM   #2
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Yo Gregg,

In all my years I've never seen or heard of a metal 6CA7. Did you mean 6AC7? Can you post a pix of one?

Victor
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Old 3rd May 2008, 06:21 PM   #3
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Victor, Yes they are 6AC7s.

Excuse me while I turn a nice shade of red.

I'm glad you replied or I might have not noticed that for a while. Funny how you see what you want to see sometimes.

Thanks!
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Old 3rd May 2008, 07:49 PM   #4
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Default Re: Metal Can 6CA7s

Quote:
Originally posted by 69CamaroSS396
For audio use, how do GE full metal 6CA7s compare to the glass version?
No good at all. I strongly suggest you send them to me for proper disposal.

But seriously, the only difference is that you have that nice orange glow with the glass versions, and you don't with the metal ones. In one project, I'm using metal 6J5s (Westinghouse JAN NOS) which are considerably less expensive than the glass ones, and they sound just great.

The only other pecularities of this type are that you need DC heater power if using for low level audio amplification. You also need a stiff screen supply. A series dropping resistor doesn't cut it since this gives a variable-u characteristic that might be handy for AGC in an IF strip, but leads to inferior sonics. You definitely want at least a stiff voltage divider, or perhaps, those small panel neons or Zeners for screen regulation.

The only other consideration is that the metal envelope is never allowed to become more positive than the cathode. Otherwise, it'll act like a second anode, and you definitely don't want that.
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Old 3rd May 2008, 08:15 PM   #5
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Quote:
But seriously, the only difference is that you have that nice orange glow with the glass versions, and you don't with the metal ones.
As discussed earlier no one has any metal 6CA7's in captivity. I have however done some extensive testing on metal 6L6's. These experiments occurred back in 1969 in my technical high school electronics class. We had hundreds of metal 6L6's to play with thanks to the Homestead Air Force base. I melted more than my share. I can share some more items learned during these "testing sessions".

It IS possible to get over 100 watts out of two 6L6's if you hook enough Eico power supplies together.

It IS possible to get the metal can glowing red. This happens after all of the paint peels off of it! They don't live long under these conditions.

Most metal tubes have the metal can connected to pin 1.

Some amplifiers that were not designed for metal tubes may use pin 1 on the socket as a tie point for something like B+. (Stromberg Carlson PA amp)

The foil on the cover of Steppenwolf's second album DOES conduct electricity, and it will fly all the way across the classroom when it touches the metal can of said 6L6! One of those moments that you don't forget!
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Old 3rd May 2008, 09:09 PM   #6
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Mr. Anderson, that's some funny, funny stuff.

I feel this bit of levity was, in part, due to me ineptly identifying my tubes. I insist I be given credit.
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Old 3rd May 2008, 09:52 PM   #7
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I imagine that another problem with metal can power tubes is that you can't look inside and see if the grid wires are glowing.
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