03-31-2018, 02:20 AM
"Hmm! I can't see anything wrong with this idea, except if it were that easy, it's pretty hard for me to understand why Philco put the bias cell in there in the first place. Any of you other Philco techs have a thought?"
We have really no way of knowing why Philco did or did not do things with a particular model in a particular year, it could have been because they were given a deal on some parts, it could have been to avoid paying patent royalties, or it could have been that whatever engineers they had designing the set got up one morning and thought it was a good thing to do. A common motivation behind many engineering decisions at Philco was to avoid paying patent royalties, often when RCA was the recipient, hence why they used weird circuits like autodynes in the model 89s, and eschewed using eye tubes, since the arrangement was to pay a royalty for each set that was sold. Then you have things like the modular chassis idea, which had little reason behind it other then David Grimes liked the idea, I think that it was probably sold on the idea that the modules could be assembled into different models by mixing a matching parts.
With regard to bias cells, those seem to have become the rage around 1936-37 or so, as that was roughly around when they started to appear in Canadian built Rogers and Marconi radios, there was a pair of them under the chassis, and if the cells had dried up the radio would go dead since one was typically between the grid of the first audio tube and the wiper of the volume control. How long they lasted in service I don't know, but probably longer then the life of the average pilot lamp as I find vintage cells still in place in many radios that were so equipped, which is not surprising since there was no current draw on them. I don't think the lifespan of these is something you need be concerned with in terms of a future owner, I would be more concerned about the ballast resistor failing at some point, a capacitive dropper would be a good substitute if there is room, or a combination of a pair of 3 amp diodes (in series) and a smaller resistor.
Just find a schematic of an AC/DC set that also uses a #75 tube as first audio, such as an International Kadette, and see what they used to bias the tube if you want to eliminate the bias cell.
Regards
Arran
We have really no way of knowing why Philco did or did not do things with a particular model in a particular year, it could have been because they were given a deal on some parts, it could have been to avoid paying patent royalties, or it could have been that whatever engineers they had designing the set got up one morning and thought it was a good thing to do. A common motivation behind many engineering decisions at Philco was to avoid paying patent royalties, often when RCA was the recipient, hence why they used weird circuits like autodynes in the model 89s, and eschewed using eye tubes, since the arrangement was to pay a royalty for each set that was sold. Then you have things like the modular chassis idea, which had little reason behind it other then David Grimes liked the idea, I think that it was probably sold on the idea that the modules could be assembled into different models by mixing a matching parts.
With regard to bias cells, those seem to have become the rage around 1936-37 or so, as that was roughly around when they started to appear in Canadian built Rogers and Marconi radios, there was a pair of them under the chassis, and if the cells had dried up the radio would go dead since one was typically between the grid of the first audio tube and the wiper of the volume control. How long they lasted in service I don't know, but probably longer then the life of the average pilot lamp as I find vintage cells still in place in many radios that were so equipped, which is not surprising since there was no current draw on them. I don't think the lifespan of these is something you need be concerned with in terms of a future owner, I would be more concerned about the ballast resistor failing at some point, a capacitive dropper would be a good substitute if there is room, or a combination of a pair of 3 amp diodes (in series) and a smaller resistor.
Just find a schematic of an AC/DC set that also uses a #75 tube as first audio, such as an International Kadette, and see what they used to bias the tube if you want to eliminate the bias cell.
Regards
Arran
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