03-28-2018, 02:37 PM
Hi, All:
New guy. I retired last October and have taken up restoring old radios. I've done several Philcos, so thought I'd try this out!
My current victim is a Model 604. My question concerns the strange little "bias cell" on the grid of the 75 tube. What the heck? Probably was a good source of income for the service guys, as it is not easy to get at and replace. I guess with no transformer, there isn't an easy way to get a negative supply, but tons of radios were built with no power transformer. WHATEVER the solution might be, if anyone knows why they did this, the curiosity is killing me. How long did they last? What was the chemistry to get a ONE volt cell? Never heard of that.
Anyhow, I've read several obscure references to this. One person said he put an LR44 cell in there, but those are really little. How long could my customer expect that to last before needing replacement? I saw another post where a guy just put a 3V coin cell in there. Seems like that would leave the bias pretty far off. Lastly, I saw a post where somebody suggested just replacing it with a 6 meg resistor. Not sure, but looking at the schematic, seems like that would basically leave the grid at ground or maybe even put a slight positive bias on it.
I've got the resistors and capacitors replaced, the speaker fixed, the tuning cap drive repaired, and have straightened out several screwups by predecessors. I'm going to want to power it up fairly soon and would love to have an idea what to do here. I see several of them restored online, so there must be a way. I will likely tack some wires to a AAA for test purposes, but need a permanent fix, hopefully not requiring a battery. Maybe a diode and resistor network?
Thanks!
Mike
New guy. I retired last October and have taken up restoring old radios. I've done several Philcos, so thought I'd try this out!
My current victim is a Model 604. My question concerns the strange little "bias cell" on the grid of the 75 tube. What the heck? Probably was a good source of income for the service guys, as it is not easy to get at and replace. I guess with no transformer, there isn't an easy way to get a negative supply, but tons of radios were built with no power transformer. WHATEVER the solution might be, if anyone knows why they did this, the curiosity is killing me. How long did they last? What was the chemistry to get a ONE volt cell? Never heard of that.
Anyhow, I've read several obscure references to this. One person said he put an LR44 cell in there, but those are really little. How long could my customer expect that to last before needing replacement? I saw another post where a guy just put a 3V coin cell in there. Seems like that would leave the bias pretty far off. Lastly, I saw a post where somebody suggested just replacing it with a 6 meg resistor. Not sure, but looking at the schematic, seems like that would basically leave the grid at ground or maybe even put a slight positive bias on it.
I've got the resistors and capacitors replaced, the speaker fixed, the tuning cap drive repaired, and have straightened out several screwups by predecessors. I'm going to want to power it up fairly soon and would love to have an idea what to do here. I see several of them restored online, so there must be a way. I will likely tack some wires to a AAA for test purposes, but need a permanent fix, hopefully not requiring a battery. Maybe a diode and resistor network?
Thanks!
Mike
