American metal valves - UK Vintage Radio Repair and Restoration Discussion Forum

John M0GLN · 21st Jun 2019, 4:00 pm

Hopefully in the right section

Found something out last night when testing an American metal cased 6L6, I had always assumed these valves were glass inside the metal envelope similar to the EF50, anyway I just touched the outside of the valve and found they aren't, I then had a look online and found this,

http://www.r-type.org/articles/art-018.htm

which explained the construction and also said,

“Although the RCA design of metal-envelope valve was far more versatile, and commercially far more successful, than the British Catkin type, it had a serious limitation in that the metal envelope had to be kept at a voltage below that of the cathode. Otherwise the envelope would behave as an anode and collect electrons. For most purposes this limitation did not matter since the envelope was normally 'earthed' to chassis and hence to the negative return of the HT voltage. “

Looking at the data sheets for the 6L6, pin1 on the metal valve is the envelope and on the glass version it is NC, so really the AVO valve data book should have shown pin1 earthed for the metal version, it also makes substituting a glass valve with a metal one slightly more involved if one wanted to.

John

ionburn · 21st Jun 2019, 7:14 pm

This came at a good point. I have been experimenting with a 6K7 today and am finalising a circuit. I saw the 'M' connection in the data book and connected it to ground naturally but it is good to see that is advised. Thank you!

G6Tanuki · 21st Jun 2019, 7:57 pm

The metal envelope is traditionally grounded: indeed one of the pins on the original octal-base is 'reserved' for this purpose! [later glass-body valves often usurped this in order to accomodate a dual-triode like the 6SL7GT]

I always liked US metal-envelope valves. They were sometimes wantonly overdriven - I remember seeing a design of a "120-Watt" HF transmitter using a pair of metal 6L6 which included water-filled cans welded over the metal envelopes to dissipate the heat!

Terry_VK5TM · 22nd Jun 2019, 3:50 am

By pure coincidence, I had just archived this article on American Metal Valves yesterday.

Argus25 · 22nd Jun 2019, 8:10 am

I have been using American metal valves all my life. They have distinct advantages with the shielding afforded by the metal envelope. There are metal versions of most types.

If they are stored in humid conditions the metal rusts under the paint. If they are smoothed down with 1000 grade paper and treated with organic rust converter and re-sprayed with HOLTS black lacquer that can be restored to an excellent finish with no rust. However if they have been stored well often there is no rust at all. They were/ are much more robust to surface damage that the typical metallised glass types.

RCA also made a 1" cathode ray tube in the same case as a metal 6L6, with the metal to glass bond for the faceplate.

Many manufacturers went straight for a metal 6k7 for a radio's IF just to avoid a shield assembly required for a glass type.

Also, one of the most revolutionary metal valves ever produced was the 6AG7. It was created especially for video output stages in pre-WW2 TV sets. Prior to that the valves used were borrowed from other types such as the 6AC7 or the 6V6. It's very difficult to out class a 6AG7.

John M0GLN · 22nd Jun 2019, 9:08 am

Quote:

Originally Posted by Terry_VK5TM

By pure coincidence, I had just archived this article on American Metal Valves yesterday.

Very interesting article.

Thanks

John

John M0GLN · 22nd Jun 2019, 9:27 am

Quote:

Originally Posted by Argus25

Also, one of the most revolutionary metal valves ever produced was the 6AG7. It was created especially for video output stages in pre-WW2 TV sets. Prior to that the valves used were borrowed from other types such as the 6AC7 or the 6V6. It's very difficult to out class a 6AG7.

I've built a crystal oscillator with a 6AG7 and I'm using this as my excuse for not checking whether the metal shells needed earthing, the 6AG7 shell is internally connected to the suppressor grid and automatically goes to earth so I've never had any shocks off it.

John

G4XWDJim · 22nd Jun 2019, 10:05 am

The 6AG7 is one of the valves on my list to try as a suppressor grid modulated QRP PA transmitting valve. I have a few ready to try when I can get round to it. Has anybody else tried it - was it effective. Is g3 big enough to exercise control.
Jim

John M0GLN · 22nd Jun 2019, 1:36 pm

I was going to reply to this question by saying that as g3 and the shell were connected, it probably wouldn't work as they would both rise to the anode voltage now they were not earthed, however I thought I'd better check first and have found this out, there seem to be 2 variations, the General Electric data sheet which you would assume would be the definitive one shows g3 and M on pin 1 with pin 3 NC, however I also found another diagram showing g3 on pin 1 and M on pin 3, I checked my valves and I have both, the one in my oscillator has the GE base with both on pin 1, but the 2 others I have are wired separately on pins 1 and 3.

John

Argus25 · 22nd Jun 2019, 2:11 pm

I'm pretty sure the valve was designed by the television department of RCA, not GE. Looking at RCA's data sheet, G3 is connected in the valve to the "shell" and to pin 1 and therefore behoves the user to ground pin 1.

I have not seen any variation of the 6AG7 that was different from this, and every 6AG7 I have seen was a metal valve, they were never made as a glass variant, as far as I am aware.

Pin 3 is N/C on a 6AG7.

John M0GLN · 22nd Jun 2019, 3:42 pm

My apologies as I seem to have got my pins mixed up, I've re-checked my 3 valves and M is on pin 1, but to confuse things for me both the Radiomuseum,
https://www.radiomuseum.org/tubes/tube_6ag7.html
and the Valve Museum
http://www.r-type.org/exhib/aaa0042.htm
show M on pin 3, however the Valve Museum also have a pdf which shows pin 3 as NC.
Someone's mixed things up like I did.

The RCA data sheet is as you correctly write showing pin 3 NC and also that they were the developer.

So it looks like modulating g3 as G4XWDJim would like to do is impossible as it's going to rise to the anode voltage without the earth.

John

Argus25 · 23rd Jun 2019, 10:47 am

Here is a project with an American metal valve and a British radio:

http://worldphaco.com/uploads/THE_ED..._640_RADIO.pdf

The reason the chassis in this radio looks much better than anyone else's 640, is because it gets rubbed down with WD-40 every year. But because people don't understand WD-40 very well they are reluctant to use it for certain applications and there are many myths about it that are believed.

I have recently performed an "accelerated corrosion experiment" involving approximated steel and brass, with a control, in a hostile environment that has confirmed the excellent corrosion inhibiting effects of WD-40. I will post it within the next two weeks.

21st Jun 2019, 4:00 pm	#1
John M0GLN Octode Join Date: Jan 2004 Location: Southport, Merseyside, UK. Posts: 1,156	American metal valves Hopefully in the right section Found something out last night when testing an American metal cased 6L6, I had always assumed these valves were glass inside the metal envelope similar to the EF50, anyway I just touched the outside of the valve and found they aren't, I then had a look online and found this, http://www.r-type.org/articles/art-018.htm which explained the construction and also said, “Although the RCA design of metal-envelope valve was far more versatile, and commercially far more successful, than the British Catkin type, it had a serious limitation in that the metal envelope had to be kept at a voltage below that of the cathode. Otherwise the envelope would behave as an anode and collect electrons. For most purposes this limitation did not matter since the envelope was normally 'earthed' to chassis and hence to the negative return of the HT voltage. “ Looking at the data sheets for the 6L6, pin1 on the metal valve is the envelope and on the glass version it is NC, so really the AVO valve data book should have shown pin1 earthed for the metal version, it also makes substituting a glass valve with a metal one slightly more involved if one wanted to. John

21st Jun 2019, 7:14 pm	#2
ionburn Heptode Join Date: Dec 2014 Location: Lincoln, Lincolnshire, UK. Posts: 583	Re: American metal valves This came at a good point. I have been experimenting with a 6K7 today and am finalising a circuit. I saw the 'M' connection in the data book and connected it to ground naturally but it is good to see that is advised. Thank you!

21st Jun 2019, 7:57 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: American metal valves The metal envelope is traditionally grounded: indeed one of the pins on the original octal-base is 'reserved' for this purpose! [later glass-body valves often usurped this in order to accomodate a dual-triode like the 6SL7GT] I always liked US metal-envelope valves. They were sometimes wantonly overdriven - I remember seeing a design of a "120-Watt" HF transmitter using a pair of metal 6L6 which included water-filled cans welded over the metal envelopes to dissipate the heat!

22nd Jun 2019, 8:10 am	#5
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: American metal valves I have been using American metal valves all my life. They have distinct advantages with the shielding afforded by the metal envelope. There are metal versions of most types. If they are stored in humid conditions the metal rusts under the paint. If they are smoothed down with 1000 grade paper and treated with organic rust converter and re-sprayed with HOLTS black lacquer that can be restored to an excellent finish with no rust. However if they have been stored well often there is no rust at all. They were/ are much more robust to surface damage that the typical metallised glass types. RCA also made a 1" cathode ray tube in the same case as a metal 6L6, with the metal to glass bond for the faceplate. Many manufacturers went straight for a metal 6k7 for a radio's IF just to avoid a shield assembly required for a glass type. Also, one of the most revolutionary metal valves ever produced was the 6AG7. It was created especially for video output stages in pre-WW2 TV sets. Prior to that the valves used were borrowed from other types such as the 6AC7 or the 6V6. It's very difficult to out class a 6AG7. Last edited by Argus25; 22nd Jun 2019 at 8:28 am.

22nd Jun 2019, 10:05 am	#8
G4XWDJim Octode Join Date: Oct 2008 Location: Littlehampton, West Sussex, UK. Posts: 1,465	Re: American metal valves The 6AG7 is one of the valves on my list to try as a suppressor grid modulated QRP PA transmitting valve. I have a few ready to try when I can get round to it. Has anybody else tried it - was it effective. Is g3 big enough to exercise control. Jim

22nd Jun 2019, 1:36 pm	#9
John M0GLN Octode Join Date: Jan 2004 Location: Southport, Merseyside, UK. Posts: 1,156	Re: American metal valves I was going to reply to this question by saying that as g3 and the shell were connected, it probably wouldn't work as they would both rise to the anode voltage now they were not earthed, however I thought I'd better check first and have found this out, there seem to be 2 variations, the General Electric data sheet which you would assume would be the definitive one shows g3 and M on pin 1 with pin 3 NC, however I also found another diagram showing g3 on pin 1 and M on pin 3, I checked my valves and I have both, the one in my oscillator has the GE base with both on pin 1, but the 2 others I have are wired separately on pins 1 and 3. John

22nd Jun 2019, 2:11 pm	#10
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: American metal valves I'm pretty sure the valve was designed by the television department of RCA, not GE. Looking at RCA's data sheet, G3 is connected in the valve to the "shell" and to pin 1 and therefore behoves the user to ground pin 1. I have not seen any variation of the 6AG7 that was different from this, and every 6AG7 I have seen was a metal valve, they were never made as a glass variant, as far as I am aware. Pin 3 is N/C on a 6AG7. Last edited by Argus25; 22nd Jun 2019 at 2:16 pm.

22nd Jun 2019, 3:42 pm	#11
John M0GLN Octode Join Date: Jan 2004 Location: Southport, Merseyside, UK. Posts: 1,156	Re: American metal valves My apologies as I seem to have got my pins mixed up, I've re-checked my 3 valves and M is on pin 1, but to confuse things for me both the Radiomuseum, https://www.radiomuseum.org/tubes/tube_6ag7.html and the Valve Museum http://www.r-type.org/exhib/aaa0042.htm show M on pin 3, however the Valve Museum also have a pdf which shows pin 3 as NC. Someone's mixed things up like I did. The RCA data sheet is as you correctly write showing pin 3 NC and also that they were the developer. So it looks like modulating g3 as G4XWDJim would like to do is impossible as it's going to rise to the anode voltage without the earth. John

23rd Jun 2019, 10:47 am	#12
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: American metal valves Here is a project with an American metal valve and a British radio: http://worldphaco.com/uploads/THE_ED..._640_RADIO.pdf The reason the chassis in this radio looks much better than anyone else's 640, is because it gets rubbed down with WD-40 every year. But because people don't understand WD-40 very well they are reluctant to use it for certain applications and there are many myths about it that are believed. I have recently performed an "accelerated corrosion experiment" involving approximated steel and brass, with a control, in a hostile environment that has confirmed the excellent corrosion inhibiting effects of WD-40. I will post it within the next two weeks.