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
Well, this lends credence to the guy who posted to just replace it with a 6 meg resistor, (this one is 4 meg) except I would have to come up with those two big, honking resistors in the power line to bias the cathode. I kind of tried to work it out and my head started to hurt trying to figure out where the filter chokes and filter caps would have to be connected in the 604 to accommodate these. At first glance, it looks like a pretty major rewiring job. I might try to figure out a way to just stick in a 600v silicone diode and a couple of resistors to get - 1 volt or so. Fired it up today and briefly picked up some short wave spanish, but no broadcast. Put in a better 75 and a better 43 and now I got nothing. Then I remembered I forgot to recap the RF unit. Took the cover off. Holy Crap!! No idea how I am going to get those replaced. Can't even get a soldering iron at 2 of them. Doesn't look good. I'm going to go up to the house, get a shot of scotch and rustle up some supper.
It's strange that I thought that I posted about this earlier but the post isn't here, no matter. These bias cells were introduced by Mallory back in the 1930s, in my opinion they were sort of a solution looking for a problem that nobody really had. In any event these are basically small carbon zinc cells, that put out about 1.4 volts originally, and since they were connected in a control grid circuit for bias they basically lasted their shelf life since there was no current draw, so the same would be true of a modern cell in the same circuit. There are three ways of handling this, one is that you use a modern watch battery as a replacement, but your have to reverse the connections on the bias cell holder, and either gut the bias cell and use the outer shell as an adapter for the watch battery or short across it in some way such as with a short piece of wire running from the cup section of the holder, or by wrapping the old bias cell in aluminum foil. The reason you have to reverse the connections on the holder is that on a watch battery the outside of the case is positive whilst the center is negative, whereas bias cells are more like a conventional battery, at least this is one of the better holders with the cup rather then one of those flaky ones that holds the cell in place with a stiff "U" shaped piece of wire. Another method is to rewire the tube inserting a resistor and capacitor between the cathode and common negative so that the tube will now be self biasing, this will take some research to select the right values, but flipping through some Riders schematics should help, including other Philco ones. The third method would be to boil the bias cell in water in the hope that some water will get inside and re-activate the electrolyte, or to drill a small hole in the cell and inject some distilled water, I don't know how successful or long lasting this method would be.
Regards
Arran
Had to replace a battery in a old lap top. Not the big one but a small button cell original had solder tabs and lasted about 15 years. Sorry I do not recall the output voltage or number. Had to purchase like 10 at one time was cheaper than buying only one cost was less than five dollars for the whole pack. Have other items that used the same battery without the tabs so just removed the tabs and good to go. David
> I'm going to go up to the house, get a shot of scotch
It's much better to keep it in the shop like most tools it's good to have it on hand although it does seem to evaporate quickly!
![[Image: icon_eek.gif]](http://philcoradio.com/phorum/images/smilies/icon_eek.gif)
Thanks, all, for the help with this. I am going to recap the rf unit, (not sure how, yet) and then tackle this battery issue. Since I am going to sell the radio, I am loathe to pass on a battery problem to the next owner. Among the suggestions, I like the coin cell idea as much as any. If I go that way, I will probably obtain one of those little plastic coin cell holders and put a 2032 in there. Still, I'd be giving my new owner a service liability sometime in the next 5-10 years.
Does anyone know what the symptoms are for a dead bias cell? No audio? Garbled audio? Early tube failure? Wondering how a person would know when it is time to replace the cell.
I will throw this out there as an experiment. What if you were to put a resistor with bypass capacitor in the cathode Of the 75 tube. You would need to experiment with what value of resistor needed to give you +1 volt on the cathode, which would be equivalent to giving you -1 volt on the grid. It would be an easy cure needing to add 2 components to the cathode circuit! This would make it self- biasing!
Ron
(03-29-2018, 07:44 AM)Radioroslyn Wrote: [ -> ]> I'm going to go up to the house, get a shot of scotch
It's much better to keep it in the shop like most tools it's good to have it on hand although it does seem to evaporate quickly!
Exactly what I do. I just keep it across the door, so I have to take about 10 steps to reach for it. Keeps me from doing it excessively often. But it is there, close, in case I get cold or frustrated.
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?
One of my many hobbies is the production of distilled spirits, for medicinal purposes and general disinfecting. I keep some of that handy in the shop in case I spot a stray germ. You can never be too careful.
Well look at it this way, for years they would use the center tap on the power transformer and a candohm resistor to ground to get negative bias for the audio output stage. Then later on, abandoned that by using a resistor in the cathode circuit giving the same results with less complexity. Give it a try, would be curious to know if it works!
Ron
Hi, Ron!
Yes, this is definitely a simple thing to try. Currently I have cobbled a 1.5v battery in there so I can tune and troubleshoot. If I ever get it working, I'm definitely going to try this. I don't want to send it out to someone and have that battery be a problem for them.
Currently, my I.F. is working ok. I'm injecting through a .01 cap to the grid of the first and second i.f., and they tune ok. RF is not going so well. The sensitivity is poor, I can't get the frequency dial to calibrate, and it just acts wiggy. For instance, I can set the tuning dial to 800 kc, dial my sig gen from 550 to 1500 and pick it up at 6 different places. Almost like the local oscillator is running spurious or some such. I don't know what to do. My 6A7 is pretty lame, and I have one on order, so I guess I'll try to give up on it until that comes in.
I'll bookmark this thread and let you know how your idea works if I ever get the set working. Right now, it doesn't look good.
Thanks!
Mike
Hi, All:
OK. Don't put DeOxit on rotary switches. I bet you guys all knew that already. I found out accidentally. I decided to let the tuning problem rest and work on something else. One of my dial lights was intermittent, and it didn't seem to be the socket, so I opened up the RF and sprayed some contact cleaner on the switch (which I had put DeOxit on a few days ago while doing the recap.) Well, the light worked better, and so did the radio. So I got some flux remover and scrubbed the whole switch with a soft round brush and rinsed it with contact cleaner. Radio tuned up fine and is playing well. Short wave sensitivity leaves a little to be desired, but it picks up quite a bit of stuff on my long wire, so I'm calling it good. There is a horrific tuning procedure in the Philco service manual involving slapping a separate tuning cap across the oscillator side of the main tuner and then trying to zero beat it with a signal generator or some such. NOT going to happen.
Ron, I'm going to try your idea tomorrow morning. I have to say, it seems too simple, and it is hard for me to imagine Philco would not have done that instead of the stupid bias cell idea.
Many thanks to all who are trying to help. It's coming along!