05-21-2009, 11:38 PM
Here's a 'lightweight' explanation of the 70. I'm using the diagram at the top of the 3rd page at http://www.nostalgiaair.org/PagesByModel...013833.pdf with correlation to the radioremembered page
1st 24 to the left - thats a basic RF amplifier like the 6K7 at other other site.
2nd 24- Gets tricky here. This tube is a mixer. In the old days they usually called it a 1st Detector...read on...
The 27 below that one. Thats the local oscillator.
Before going further I'll point out that although radioremembered's block diagram shows two individual tubes for the 1st det (mixer) and the local oscillator, the schematic shows a single 6A8 tube. Long story short, the 6A8 is providing both functions. He provides as good of an explanation as I can. The 6, 5 and 8 pins on that 6A8 translate to your 27 in the older Philco. In fact, many texts will refer to that pin 6 grid as the 'oscillator plate' since that is its function compared to an older two tube scheme.
Using two tubes versus a single multi-purpose tubes is a subject in itself. Short version is that the multi-purpose mixer or converter tubes were not around at the time of the Model 70.
The next 24 is an Intermediate Frequency (IF) amplifier. It equates directly to the one at radiorem.
The last 24 is the equivalent of the detector/avc/1st audio stage at radiorem. There's a significant number of differences here. Older sets like the 70 are sort of 'crude' in comparison to later circuits.
The 47 is the Audio Output amplifier stage which is fairly straightforward.
So you see that even though the circuits are quite different, the basic building blocks are similar. This holds true for many superhet radios. Some have different frills, some have less, some have more, but the basic blocks will be there.
Soldering. First tip is to keep your tip clean. Second tip is to keep your tip clean. Third tip...you guessed it. Old radios often have a lot of oxidation on the joints and it takes a little extra effort to break thru it since it impedes the heat transfer. That also will nasty up your tip in a heartbeat. What are you using for a soldering gun/pencil? There might be some specific advice to make life easier.
Me, project wise, not much. I've got a 3-tube homebrew project going on and I'm diving into the realm of solid-state for the first time in 25 years with a little regenerative rcvr project.
Adios,
Bill
1st 24 to the left - thats a basic RF amplifier like the 6K7 at other other site.
2nd 24- Gets tricky here. This tube is a mixer. In the old days they usually called it a 1st Detector...read on...
The 27 below that one. Thats the local oscillator.
Before going further I'll point out that although radioremembered's block diagram shows two individual tubes for the 1st det (mixer) and the local oscillator, the schematic shows a single 6A8 tube. Long story short, the 6A8 is providing both functions. He provides as good of an explanation as I can. The 6, 5 and 8 pins on that 6A8 translate to your 27 in the older Philco. In fact, many texts will refer to that pin 6 grid as the 'oscillator plate' since that is its function compared to an older two tube scheme.
Using two tubes versus a single multi-purpose tubes is a subject in itself. Short version is that the multi-purpose mixer or converter tubes were not around at the time of the Model 70.
The next 24 is an Intermediate Frequency (IF) amplifier. It equates directly to the one at radiorem.
The last 24 is the equivalent of the detector/avc/1st audio stage at radiorem. There's a significant number of differences here. Older sets like the 70 are sort of 'crude' in comparison to later circuits.
The 47 is the Audio Output amplifier stage which is fairly straightforward.
So you see that even though the circuits are quite different, the basic building blocks are similar. This holds true for many superhet radios. Some have different frills, some have less, some have more, but the basic blocks will be there.
Soldering. First tip is to keep your tip clean. Second tip is to keep your tip clean. Third tip...you guessed it. Old radios often have a lot of oxidation on the joints and it takes a little extra effort to break thru it since it impedes the heat transfer. That also will nasty up your tip in a heartbeat. What are you using for a soldering gun/pencil? There might be some specific advice to make life easier.
Me, project wise, not much. I've got a 3-tube homebrew project going on and I'm diving into the realm of solid-state for the first time in 25 years with a little regenerative rcvr project.
Adios,
Bill
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