Sylvania made some of the best 6sn7 tubes ever made. Their metal base 6sn7w tubes are the stuff of legends. They made a similar 6sl7w as well, pictured below:
Both of these versions now command huge amounts of money. They have other highly regarded versions of both the 6sn7 and 6sl7 but the original metal based tubes are still considered the best they ever made. Me being a cheapskate, decided to pursue those tubes in a more cost effective manner by buying Loctal and 12v versions of the tubes.
Loctal was a base that Sylvania made to compete with RCA’s Octal base. It had the aded feature of being able to be secured in the base, to be locked in as it were. This was supposed to be an advantage in vibration prone installations like cars and airplanes. Common wisdom has it that the 6sn7/12sn7 and the 7n7/14n7 were the same tube internally. But I noticed that the 14n7 looked different to the regular 6sn7.
Yes, the plates look the same as the tall bottle 7n7 and look similar to the bottom getter JAN 6sn7 but it also has the copper support rod of the sought after metal base 6sn7w. On top of that, the 14n7 sounded noticeably better to me. I think these are the equivalent of the metal based 6sn7w! Curiously, no other 6sn7 or 7n7 share this construction.
When I started rolling 6sl7 tubes I once again took a look at the loctal equivalent 7f7. I have several that look identical to the bottom getter JAN 6sl7 but then I found a pair that look like this:
Once again, these are a dead ringer for the sought after metal base 6sl7. There are a few differences in the mica but the plates and copper support rod are the same. I also found the same construction in the 14f7 and 12sl7 but not in any 6sl7. Like the 14n7, I found these copper rod 7f7 to be noticeably better than the regular, well regarded, bottom getter JAN 6sl7. Once again, I think these are analogs of the metal based W variant.
So what’s going on? Why did Sylvania make their “best” versions of their 6sn7 and 6sl7 in 12v and loctal and not in the regular octal format? My theory is that the initial run of the metal based 6sn7w and 6sl7w were made for a contract with the government for the war. According the Vacuum Tube Valley, the metal based 6sn7w was released in 1942. They then used the same tooling for their own proprietary base, the loctal. One pair of the 7f7 copper post tubes I have came in boxes labeled as being made for the US Signal Corps and labeled as Hygrade Sylvania. The Hygrade was dropped in 1942 so I think that dates these tubes pretty well. I have other 7f7 with the same construction labeled GM (not Delco) and Motorola both of which made car radios.
By all accounts Sylvania did not make tubes with the loctal base for very long. I have only ever seen the older version of the 6sl7 in the 7f7. There were a wide variety of 7n7/6sn7 tubes made over the years that mirrored each other. I do own a 7n7 with a non copper support rod but that appears to be a rare tube. Seemingly only the very earliest metal based 6sn7w had the copper rod. They eventually switched to some other material in that tube but all of the 14n7 tubes I have seen have the copper rod. I’m guessing that copper was more expensive and in scarcer supply during the war so they went with a less expensive material as soon as they could.
So if you’re looking for the best twin triode tubes Sylvania ever made but don’t want to spend the money on them, check out the 14n7 and the copper rod 7f7, 14f7, and 12sl7.