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Philco 604 Bias Cell - Printable Version

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RE: Philco 604 Bias Cell - mboessen - 03-29-2018

Hi, again, everyone:

I have another issue with my 604 that I'm wondering if anyone has addressed.  The dial lights are awfully dim.  In a brightly lit room it is very difficult to see the little slit light image on the back of the tuning dial.  When I first turn on the set, they are really bright until the filaments get warmed up, so I am afraid to try a lower voltage light.  The Philco parts list doesn't actually tell me the lamp type, so I picked a couple of 47s and put them in there.  Is it possible I have the wrong lamp?

Also, the series resistor in the filament circuit gets really hot, like unpleasant to put my finger on the back side of the chassis.  Is this normal?  I have a variac on it, running at the rated 115 v.

Again, Thanks!

Mike

RE: Philco 604 Bias Cell - Radioroslyn - 03-29-2018

< the series resistor in the filament circuit gets really hot, like unpleasant to put my finger on the back side of the chassis.  Is this normal?
Yes it's dissipating about 17wts of energy (55v @.3A). That is about the same as two 50L6's.

<  Is it possible I have the wrong lamp?
To be honest I don't remember (I have a 604 but it's been a while since I've had a look at it.) Chuck had a list of the Philco p# vs standard bulb but his site lately has been just a mere shadow of what it was.There are 3.3v @ 150ma bulbs mostly used in post war RCA sets.


GL

RE: Philco 604 Bias Cell - 462ron - 03-29-2018

I will be waiting for your results!  Icon_clap

Ron

RE: Philco 604 Bias Cell - Radioroslyn - 03-29-2018

Took a few minutes and found that the 34-2068 lamp is a #47. The Philco lamp is frosted.

RE: Philco 604 Bias Cell - mboessen - 03-30-2018

Hi, All:

About the lamp question, thank you Terry, for looking that up. I am going to see if I can figure out a way to cut the lamp wire from the big resistor and try putting in a separate resistor straight from the AC line to the lamps so they won't be subject to the higher voltage during filament warmup. Might not work, but I'm going to try it. It is a little bit of a tricky thing, since the lamps are stuck in that metal socket with no ventilation, so cranking up the brightness will crank up the heat as well. Don't want to shorten lamp life too much, and there is also the spectre of melting the sheet of plastic that makes the slit. More later on this.

About the bias cell, it might be next week, Ron. I'll post something about it after I figure out what to put in there. I have a decade box, but it might be just 1/2 watt resistors. Not sure if that is enough. Also a tricky situation.

RE: Philco 604 Bias Cell - 462ron - 03-30-2018

I would think 1/2 watt would be sufficient. The 75 isn't a power output amp that passes more current plus if you are developing only 1 volt of bias on it, that's not much. Don't forget you might need a bypass cap across it too. Also, since the 75 is the second detector too, I hope the long leads of the decade box don't do anything funky with it. It will be interesting to see how this works out!

Ron

RE: Philco 604 Bias Cell - Radioroslyn - 03-30-2018

>separate resistor straight from the AC line to the lamps so they won't be subject to the higher voltage during filament warmup.
Not a great idea as you will need to drop abt 115v @ .15A = 770 ohm @ 18W eekkkkkk! Which means you'll need abt a 50w resistor so it doesn't get flaming hot.

RE: Philco 604 Bias Cell - mboessen - 03-30-2018

OOps. Yep. I multiplied 6.3v X .15a, and it is the difference, or closer to 110v. Rats. It's a darn 1 watt bulb. Shouldn't be this hard.

RE: Philco 604 Bias Cell - Arran - 03-31-2018

"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

RE: Philco 604 Bias Cell - mboessen - 03-31-2018

Hi, Arran:

Thanks for taking the time to think about my problem.  Someone here did send me a schematic for a Philco 38-14, where they threw in a couple of resistors in the "power supply" kind of sort of in parallel with the ballast resistor, to bias the cathode.  I considered it, but the way it was done looks like a major rewiring job.  I fooled with this for 3 hours last night and decided I will just modify it for an LR44 cell and move on.

I read the part about replacing the ballast resistor with diodes or capacitors.  Did I read that correctly?  I've never heard of that.  Can you elaborate a little more on that?  Maybe post a sketch?

I'm having trouble with my audio in this set as well, which was one of the reasons I have been trying so hard to get this bias thing right.  My audio is not clean.  It is a little garbled, sounding like there is 60 cycle hum mixed in with it.  I replaced all the caps and any resistor outside 10%.   I've swapped out all the tubes.  There is about 2v of ripple on the B- relative to chassis, and I see it on the 75 tube grid and plate.  Not so much on the 43 plate.  I don't know if this ever happens to you, but I have become so frustrated with this set that I am going to set it aside and cool off for a while.  I've got a model 20 to start on, but first I need to repair the transmission in my brother's tractor/backhoe, which should keep my mind off of this for a couple of weeks.  If I don't get inspired beforehand, I will try to post more details about the audio problem in a separate thread.  I am beginning to suspect a problem with my bench setup, as I have had this problem on 2 other sets and never could get it satisfactorily resolved.

Again, Thanks!

Mike