It’s always interesting to try something new – especially vacuum tube. Sometime you can get something from unknown thing.
I’m quite sure we are familiar with 6SN7 vacuum tube, which is commonly used in pre amplifier and even a lot of amplifiers are using this tube in input/driver section. Not surprisingly to see the price is sky rocketing, especially the red base 5692.
I just realize that I got some old tube which is “compatible” with 6SN7, but with different base. Here we go, the RCA 7N7.
RCA 7N7 is practically an 6SN7 in Loctal socket, instead of common Octal socket used by 6SN7. Loctal socket might not be too familiar for most of us, but it’s widely available. We can search the socket in ebay.com at really affordable price or for premium option, we can always go with Yamamoto Teflon Tube Socket.
Be noted that even this tube has “7” prefix, it actually has 6.3 Volt filament. The same applies to 14N7 which is using 12.6 Volt filament. It’s different with let’s say 7DJ8 which is using 7 Volt filament supply.
There are some other tubes with this kind of “exception”. Another example is 1A6 which is using 2 Volt filament (if I remember correctly, most tubes with 2.0 Volt filament will be using “1” prefix, while “2” prefix will be used by tube with >2 Volt filament, example: 2A3 which is using 2.5 Volt filament supply).
Anyway, it’s up to you to use 7 Volt as sometime higher filament voltage could alter the sound (some people likes it).
This RCA 7N7 is a tube with metal base (which sometime I prefer over plastic base). Sturdier construction I would say.
Below shot was taken with my Nikon D90, Sigma Macro 150 mm F/2.8 lens, and two remote controlled flashes (Nikon SB-800 and SB-900).
The tubes itself has some sort of punch “stock control” card. An interesting way to see how this kind of stuff was taken care at that time (somewhere on 1966).
It would be interesting to see how this tube will compare with most 6SN7 and probably with the famous 5692 red base – if I can get some!
John Atwood
February 10, 2015 14:08Loctal tubes were developed by Sylvania and Lansdale tubes (Philco’s tube division) in the late 1930s as an alternative to RCA and GE’s metal tubes. They were the first consumer glass tubes to use a “button base” which had much more shorter pin-outs than the “press base” ubiquitous at the time, which made them better for VHF applications. The lock-in feature made them attractive for mobile applications. Other manufacturers that produced loctal tubes were Raytheon and National Union.
In Europe, Philips made physically compatible 8-pin loctals, such as the EF22, although none were direct 2nd-sources to the American types. I am not sure if there was a licensing agreement between Sylvania and Philips. 9-pin loctals, such as the EF50, were not marketed in America, but Sylvania made many EF50s during WW2 to support the British war effort.
RCA worked hard to suppress the loctal tubes – mainly through a “Preferred Tube Types” list that they pushed throughout World War II to reduce the number of types manufactured. Nonetheless, a fair amount of WW2 equipment used loctal tubes, both the 6 and 12 volt versions and also the 1.4V battery types (such as 1LC4, 1LE3, etc.) Sylvania pushed loctals after the war and a moderate amount of equipment used loctals, such as the famous REL 646 FM tuner, which was all loctals, except for the 5U4G rectifier. Most post-war loctals were used in FM police radios. However, by 1950, nearly all designs were using miniature tubes.
The 7N7 you show is a late type manufactured by Sylvania. It is essentially the same as a late Sylvania 6SN7GTB, which is quite a good tube. RCA never manufactured pure loctals, although for a while before the war they made some “-LT” and “-LM” types that were essentially metal tubes with octal bases. After the war, all tube companies had to provide a complete set of replacement tubes to their franchised dealers, so tubes were purchased from other companies and marked with company’s logo. This is why your RCA tube was actually made by Sylvania – the only company making loctal tubes after the late 1940s.
Good luck with your 7N7s – they should sound very good. Another good type is the 7F7, which is the same as the 6SL7GT. I am a fan of your site and like your tube explorations.
– John Atwood
Auw Jimmy
March 1, 2015 17:21Hi John,
Thanks for your comment. I’ve been to your website to (heard your name from several DIY site).
E.A. Heras
February 13, 2016 11:56Great info on the 6SN& & 7N7. I would like to know all the tube base types & dates when introduced such as the Octals, Octals, 4 pins, 7 pin, 12 pin, space charge tubes , triodes, tetrodes etc. Could you direct me to a web site. Thanks
The Chief from Amarillo,Texas.
Auw Jimmy
February 21, 2016 12:43Hi Heras,
Maybe Google can help with those kind of historical information.
Thanks.