Got a question fer ya'll there:

I am thinking of making a really nice kit table model radio for my nephew, out of an old chassis I got from a really junky consol radio that literally exploded from water damage to the wood. I want to use all "G" style tubes because I like them. I want to build the type of oscillator circuit that requires a 6SA7 tube (one coil with tap in middle...unlike the two coil system that uses a 6A8). However, if there ever was a 6SA7G tube made, they are very hard to come by. I've seen a few 12SA7G tubes, but never a 6SA7G tube. The 6L7G tube has the same configuration as the 6SA7 tube (the way the elements are tied together). The only problem is that it is only listed as a pentagrid mixer/amplifier, but never as an oscillator. Is the second grid in the 6L7 a grid/plate like those in the 6SA7 and 6A8 or is it just a regular grid? If it is just a regular grid, it probably wouldn't work as a combination oscillator/mixer. I suppose that the 6A8 might work for a 6SA7 type of circuit, but the grid connections are a bit different and lend themselves more to the two coil system.

Thanks in advance,

T.D.

Hi Thomas

No reason you couldn't use a tapped oscillator coil circuit with 6A8G. I looked for examples and found Stewart Warner R-134 & R-136 both use a 6A8 with 3 wire oscillator coil, not two separate windings.

You are right about 6L7. Tube manuals mention using it with a separate oscillator tube but it should work as osc/mixer. Both input grids have good gain. Unlike 6A8, the one that would normally be used for oscillator is on top in a 6L7.

An 6J7G could also be made to operate in this circuit. Grid #1 and #3 can be used as inputs. You should be able to find examples using a 57 tube. (Similar to 6K7 with different base & 2.5 volt fil.)

Norm

:Got a question fer ya'll there:
:
:I am thinking of making a really nice kit table model radio for my nephew, out of an old chassis I got from a really junky consol radio that literally exploded from water damage to the wood. I want to use all "G" style tubes because I like them. I want to build the type of oscillator circuit that requires a 6SA7 tube (one coil with tap in middle...unlike the two coil system that uses a 6A8). However, if there ever was a 6SA7G tube made, they are very hard to come by. I've seen a few 12SA7G tubes, but never a 6SA7G tube. The 6L7G tube has the same configuration as the 6SA7 tube (the way the elements are tied together). The only problem is that it is only listed as a pentagrid mixer/amplifier, but never as an oscillator. Is the second grid in the 6L7 a grid/plate like those in the 6SA7 and 6A8 or is it just a regular grid? If it is just a regular grid, it probably wouldn't work as a combination oscillator/mixer. I suppose that the 6A8 might work for a 6SA7 type of circuit, but the grid connections are a bit different and lend themselves more to the two coil system.
:
:Thanks in advance,
:
:T.D.

Thanks!

Since I am working with an existing chassis that has seven tube sockets, I want to go with two IF amplifiers and a push-pull output using screen grid inversion. I want to keep the tube count down to the alotted socket spaces, so I have to keep the oscillator circuit to one tube. I would rather have two IF tubes which improves sensitivity than have an oscillator tube and a converter tube (using the 6J7 or 6K7 as an oscillator). I am reusing the original oscillator coil which is designed for a 6SA7 tube. The tubes will be "pending oscillator," 6U7G, 6U7G, 6Q7G, 6F6G, 6F6G (or 2 6v6...I haven't decided), and a 5Y3G, and possibly a 6U5/6G5/6E5. Since you say that the 6A8G works with the three wire oscillator coil type of circuit, I will go with that.

Just a curious question: What is the benefit, if any, of using a separate oscillator tube? Does it simply increase the tube count, or does it really help? I know that pentagrid tubes have trouble with oscillator grid negativity, so perhaps keeping the oscillator as a separate unit improves this situation.

Thanks a lot for your help!

Thomas

:Hi Thomas
:
: No reason you couldn't use a tapped oscillator coil circuit with 6A8G. I looked for examples and found Stewart Warner R-134 & R-136 both use a 6A8 with 3 wire oscillator coil, not two separate windings.
:
: You are right about 6L7. Tube manuals mention using it with a separate oscillator tube but it should work as osc/mixer. Both input grids have good gain. Unlike 6A8, the one that would normally be used for oscillator is on top in a 6L7.
:
: An 6J7G could also be made to operate in this circuit. Grid #1 and #3 can be used as inputs. You should be able to find examples using a 57 tube. (Similar to 6K7 with different base & 2.5 volt fil.)
:
:Norm
:
::Got a question fer ya'll there:
::
::I am thinking of making a really nice kit table model radio for my nephew, out of an old chassis I got from a really junky consol radio that literally exploded from water damage to the wood. I want to use all "G" style tubes because I like them. I want to build the type of oscillator circuit that requires a 6SA7 tube (one coil with tap in middle...unlike the two coil system that uses a 6A8). However, if there ever was a 6SA7G tube made, they are very hard to come by. I've seen a few 12SA7G tubes, but never a 6SA7G tube. The 6L7G tube has the same configuration as the 6SA7 tube (the way the elements are tied together). The only problem is that it is only listed as a pentagrid mixer/amplifier, but never as an oscillator. Is the second grid in the 6L7 a grid/plate like those in the 6SA7 and 6A8 or is it just a regular grid? If it is just a regular grid, it probably wouldn't work as a combination oscillator/mixer. I suppose that the 6A8 might work for a 6SA7 type of circuit, but the grid connections are a bit different and lend themselves more to the two coil system.
::
::Thanks in advance,
::
::T.D.

Thomas

Being new to this I am curious, do you know the history of the "G" type tubes? What is their claim to fame?
Also I was wondering, what was the purpose of metal cased tubes? Was it for ruggedness, shielding?
They had a relatively short commercial life. Do you know why?

Gary

:Got a question fer ya'll there:
:
:I am thinking of making a really nice kit table model radio for my nephew, out of an old chassis I got from a really junky consol radio that literally exploded from water damage to the wood. I want to use all "G" style tubes because I like them. I want to build the type of oscillator circuit that requires a 6SA7 tube (one coil with tap in middle...unlike the two coil system that uses a 6A8). However, if there ever was a 6SA7G tube made, they are very hard to come by. I've seen a few 12SA7G tubes, but never a 6SA7G tube. The 6L7G tube has the same configuration as the 6SA7 tube (the way the elements are tied together). The only problem is that it is only listed as a pentagrid mixer/amplifier, but never as an oscillator. Is the second grid in the 6L7 a grid/plate like those in the 6SA7 and 6A8 or is it just a regular grid? If it is just a regular grid, it probably wouldn't work as a combination oscillator/mixer. I suppose that the 6A8 might work for a 6SA7 type of circuit, but the grid connections are a bit different and lend themselves more to the two coil system.
:
:Thanks in advance,
:
:T.D.

Norm probably knows a lot more on the glass/metal issue than I do. As far as the metal tubes go, both reasons apply. They are rugged and the shell serves the double purpose of a shield. It makes such a great shield that interferance entering and leaving the tube is virtually eliminated. They came about in the late 1930s and after the end of the 1940s peanut tubes took over, though there still were radios produced with glass tubes and metal ones (octal tubes), like some of the AC/DC sets with the standard octal string (35z5GT, 50L6GT, etc.). This explains their short life, though. Most high end systems used either all peanut tubes or a mixture of glass power output octals (and a glass rectifier) and the rest were peanut tubes. Glass tubes are cheaper to manufacture, and metal tubes are not really required for shielding purposes in the output and rectifier sections. Metal shields are more expensive to make and add to radios/amplifiers than just putting in metal tubes, so in the cases where you find early guitar amplifiers (1950s early, not the ones from the 1930s and 1940s), you'll find metal tubes in the pre-amplifier sections and glass ones in the output sections. Some manufacturers managed to use all glass octals, but hum and feedback are problems in high gain amplifiers. When you think about it, though, a lot of the tube styles were short lived and lasted about the same length of time. First there were globe tubes, which took on all sorts of shapes and sizes for a long time before the 1920s. Then, by the 1920s, they were mostly of standard size and shape (small variations as all tubes go). They also had the standard base system, mostly 4 pin, but more pins were common by the late 1920s. By the early 1930s the shoulder type tubes (Coke bottle, or "G" style when referring to octals) started taking over. These went strong until the metal tubes came about in the late 1930s. Then they were mixed in with eachother. By this time the standard base tubes started to really fade out and octal bases started to take over. A few years after metal tubes came about, around 1939-1941, the "GT" tubes started replacing a lot of "G" styles in many radios. By the late 1940s, peanut tubes started creeping in. "G" type tubes started fading out. Hold-outs were 5Y3G, 6L6G, 6BG6G (19BG6G, etc.), and tubes like that. By the 1950s, G type tubes really faded out, though they still were found in some larger instruments (televisions, high end radios, etc.), especially in power and output sections. Metal tubes sort of went down, too. Peanut tubes really started finding their way into electronics. It goes on and on. Peanut tubes probably stayed strong longer than any of the other tubes, though you could find a few octals here and there right up until tube stuff really died off. You also found a lot of compactron tubes in televisions. Most tubes had roughly a 7-10 year life span of popularity, perhaps less.

As for the G style, I am clueless to that one. The original bulb shape of a tube leaves the elements swinging in mid-vacuum (can't say mid air because there's no air). There must be a bunch of wire supports to hold everything in place, along with those fancy glass blobs and stuff. The G style, with its tubular top, allows a mica support or wire supports to rest evenly against the glass at all edges. This is my attack at it, but I really don't know. There's a whole big explanation that I've heard about glass strength and all these things and stuff, like that they had to keep the bulb shape to some extent, so only made the top tubular. Still, you find some tubular tubes in the 1920s and earlier. So, like I said, I am clueless here.

Thomas