This months tube is an electrically equivalent tube to the 6SL7 which was presented last month. The Loktal type 7F7.
Like the 6SN7 which has a Loktal pendant in the 7N7, the 6SL7 also has a Loktal equivalent. When Sylvania introduced the Loktal base, they made equivalents to the most popular Octal tubes.
So a Loktal replacement for the 6SL7 was a logical choice to make. The 7F7 has the same symmetrical arrangement of the pins as the 7N7. See pinout on the left. All electrical parameters are the same as for the 6SL7. An amplification factor of 70 which classifies it as high mu double triode. A plate resistance of 44kOhm at an operating point of 250V/2.3ma. See the data sheet for all technical parameters. The heater voltage listed in the earlier data sheets is 7V which seems not equivalent to the 6SL7. Also the heater current at 0.32A is slightly higher. In later data sheets the heater voltages of Loktal tubes was specified at 6.3V. This may seem confusing but there is a simple explanation. The Loktal tubes were primarily meant for use in mobile equipment like car radios. So Sylvania rated the heater voltage at 7V which is the nominal voltage of lead acid cells when being charged.
This enabled the use of 7 as prefix for this line of tubes to distinguish them from their Octal counterparts. The heater wires quite certainly have been the same as used in the Octal tubes. Most likely the entire tube design has been shared between the Octal and Loktal lines. So they stretched the heater rating to the upper limit. This also shows how robust the heaters are. Not really necessary to be anal about exact heater voltages. The 7F7 shares the same problems with other Loktal tubes which is corrosion of the pins. Keep that in mind when using these and make sure to scrape off the pins. There is also not such a variety of sockets available as for Octals. But nonetheless Loktals are a cheap alternative, especially in case of 6SL7 and 6SN7 which command rather high prices due to their popularity. Their Loktal counterparts are available at much lower cost. And you get the same quality for that. See the very linear curves listed in the data sheet:
And actual measurements of a tube:
Just as nice as those of the 6SL7.
Only a few manufacturers made Loktal base tubes. According to a reputable source they were only made by Sylvania, Raytheon and National Union. With the vast majority sold coming from Sylvania. All other manufacturers who offered Loktals bought them from Sylvania and rebranded them. That's why you will only come across very few variants in construction style.
So let's start with photos of some Sylvania tubes. The oldest variant shown above.
The next photo shows a later version, but it basically only differs in tube box and printing on the glass.
These boxes werde made such that the tube could be tested without removing it from the box.
The top flap can be folded out.
So that the tube inside could be pushed down a little to expose the pins:
The latest version in yellow/black boxes:
This version also differs in internal construction style with the plates spaced further apart compared to the older ones:
A JAN-7F7 made for the military:
Zenith traditionally sourced it's tubes from Sylvania, so of course they also rebranded their 7F7:
The same with Philco:
As can be seen in the next photo the correct spelling of the Loktals is with a 'K'. Often they are also named 'Loctal' with a 'C'.
Another Philco 7F7 with red printing on the glass:
Although Sylvania introduced the Loktal base as an alternative and competition to the Octal tubes, even the large manufacturers like RCA obviously did not want to exclude it from their offering and sourced Sylvania tubes:
Also General Electric :
On the left the oldest one, which also collected quite some patina on the metal base, a common issue with Loktal tubes.
The later one has the same internal construction with nice round plates, closely spaced to each other. The amount of getter is a bit reduced.
And the latest version with the two triode systems spaced further apart:
Here a brand which I never came across before 'Rex Cole':
Apparently for the military.
But inside the boxes I found a General Electric and a Sylvania. Not sure if they only had the packaging branded or if somebody simply put in old used tubes.
The Sylvania was obviously heavily used.
Interesting rain bow pattern which developed on the getter of this one:
A Mullard-branded 7F7:
This one has a different internal construction as all other 7F7s I have, the plates are in a triangular shape:
The National Union 7F7s in my stock also look very similar to the Sylvanias.
Motorola offered 7F7s as well:
As did Tung-Sol:
Again opbiously from Sylvania as they share the same construction styles.
For the tube of the month post of the 6SL7 I did not want to open up a pricey tube to show the internals. But the 7F7 is rather cheap and I will sacrifice one for dissection.
Since the 6SL7 and 7F7 were most likely pulled from the same production line with just different bases applied, it is sufficient to just open one of these two types. This comparison between later versions of 6SL7 and 7F7 show how similar their construction is:
I picked an earlier Sylvania for inspection of the internals:
The base of the tube:
Removing the glass gives a nice view of the plate structure:
Beautiful round plates.
A close up to the top mica and alignment of electrodes:
The heater wire is very thin, here partially pulled out of one of the cathodes:
Removing the upper mica disc:
Grid and cathode of one of the two triode systems removed:
Removing the plate of the other half:
The grid wire appears to be gold pated!
Grid in comparison to a match.
Another nice example of the marvellous vacuum tube engineering of the past.