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philco: Bias Cell??

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » philco: Bias Cell??
           
Michael Boessen
Michael Boessen
 
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27.Mar.18 20:23

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Hi, All:

Working on a very unique Philco 604 I paid too much for on ebay.  I was working on a recap and ran into something I didn't recognize, so downloaded the schematics and parts list for the philco 604C listed on this web site.  Turns out it is called a "1 volt bias cell".  Seriously?  There is a battery in the darn thing to bias a tube?  You gotta be kidding.  Not easy to get to either.  I wonder how often that had to be replaced?

Anyhow, does anyone have any idea for a solution to this?  Fairly sure they are not available.  I don't even know what chemistry a 1 volt cell would be.  Mercury? 

The schematic I downloaded from the Radio Museum is in PDF, but it won't let me upload it to this post, so please search the 604 C listed in the RM database.

Also, if anyone knows where I can get a 604 schematic, that would be nice.  This thing has been worked on quite a bit, and I think I might just need one.  There appear to be a few differences to the 604C schematic I am using.  Of course, there could be work-on-it-itis, as my first boss used to say.

Many thanks in advance!

Mike

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David Carroll
 
 
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28.Mar.18 03:44

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Hi Mike,

Have a Philco radio with those type of cells for bias.  It's a model 37-34 Farm radio.  Mine had three of those type in series (1V each) and so I mounted a 3V button cell/holder in the chasis and it worked just fine.  As far as I could tell they were made by Mallory originally.  There was an pdf I found somewhere about them.  I don't remember seeing any 1V button cells, just 1.5V.  Maybe a voltage divider to get 1V?  The bias for the tube probably does not draw much current.  Hope this helps.

Dave

Michael Boessen
Michael Boessen
 
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28.Mar.18 14:57

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Thanks, David!

I am interested to see if I can get a conversation started on this practice.  I thought of button cells also, but there is nothing out there at 1 volt.  I just assumed that 3v would be too much bias and cause grid cutoff in the stage.  I find it odd that Philco could not find any way to bias this tube in the usual fashion, with a voltage divider.  Why would you put in a battery, that is very difficult to replace, and requires a service call and considerable dismantling of the radio?  I have never seen this practice in any other radio brand that I am aware of.  There must be some reason why they would do this.

While we are here, I saw one obscure mention on a Philco forum of replacing the cell with a roughly 6 megohm resistor, but I couldn't contact the person and no one else seems to know anything about that.

I'm going to order a couple of 2032 coin cell holders and experiment with that, but I just wonder if any of the more experienced technicians on this huge website can explain why Philco did this in the first place, other than to generate revenue for their service people.  Also, what would the end user's symptoms be if the battery goes dead?  Distsorted audio?  

There has to be someone really smart out there that understands this.

Thanks!

Mike.

Rüdiger Walz
Rüdiger Walz
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28.Mar.18 23:40

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The bias cell is really mysterious here. Bias cells have been used in Germany in portable transistor radios in the 50ies. Here it is recommended to replace them by a silicon-diode which stabilises 0,7 volts to avoid leackage of the cell and damage of the printed circuit board. 

In the case of the LF valve in your radio a 2 MOhm resistor should do also. However, for a non-battery substitute you have to take two szenarios into consideration:

1) electrons reach the grid and it becomes negative or

2) the grid becomes hot and emits electrons. Subsequently it becomes positive.

In both cases the bias cell works well and stabilises 1 V. I propose two silicon diodes inversely arranged. They would stabilise 0,7 V for both szenarios. You may add two Germanium diodes to get 1 V or use two Zener diodes. 

Rüdiger Walz

Michael Boessen
Michael Boessen
 
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29.Mar.18 02:49

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Hi, Rudiger:

This is interesting, but I have a question:  If the grid doesn't draw enough current to forward bias the diode, I won't get the .7 voltage drop, will I?  Basically, I need the grid to be -1 v relative to B- and I am not sure how this solution will do that.   I'm wondering if I can just put a 600v silicone diode on the ac line with a couple of resistors in a voltage divider arrangement to give me -1v.  I guess it would need a filter cap as well, but seems like it would be more certain to give me -1V bias on the grid.  I am a terrible engineer, but I think I can do it.  I won't lie, negative voltages always confuse me.

Rüdiger Walz
Rüdiger Walz
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29.Mar.18 19:24

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You do not need much current to get the 0,7 V drop at the diode. It is not working like a resistor. Just test it. Take two common diodes as a substitute for the cell and listen to the sound. You may also use a high resistance elektronic voltmeter to check the grid bias.

Rüdiger Walz

Michael Boessen
Michael Boessen
 
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29.Mar.18 20:55

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Hi, Rudiger!

Currently I have a LR44  battery temporarily connected in there while I attempt to do alignment and troubleshooting.  My I.F. is working ok and I get audio at the speaker.  My R.F. is acting very strange.  Poor sensitivity, and tunes very strange.  For example, with the tuning dial set to 800, I can dial my sig gen from 550 to 1500 and get 400 hz tone in the speaker at 6 places on the sig gen dial.  This is too odd for my limited experience.  I don't know what to do.  I'm ordering a new rf/oscillator tube, as mine is pretty weak, but I'm not optimistic.

I am very interested in your proposed solution.  If I get the rest of it working, I am definitely going to try it.  I will let you know here how it works.

Thanks for the help!

Mike

Michael Boessen
Michael Boessen
 
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30.Mar.18 02:56

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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.  

So, tomorrow I'm going to take a shot at Rudiger's diode patent.  Sorry, Rudiger, I have my doubts, but it would be an elegant fix if it would work.  I still don't think there will be enough current to forward bias the diode.  Right now it is running with a 1.5v cell in it.  So, 2 silicone diodes should give me 1.4, right?  

Another suggestion I found elsewhere involves putting a resister with a bypass cap in series with the cathode, so it would self bias.  That also seems too simple, and it is hard for me to imagine Philco would not have done that instead of the stupid bias cell idea. But,,,if the diode idea works, I won't even fool with it.

Many thanks to all who are trying to help.  It's coming along!

Jörg Sigg
Jörg Sigg
 
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31.Mar.18 00:33

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These Bias Cells were also installed at the end of the 30s at the Swiss Paillard receivers. For example, the models 406 and 59. These batteries are charged very little and hold accordingly long. They have the advantage that very little ripple voltage enters the audio circuit. Also note the negative feedback coil in series with the speaker coil on your Philco radio. This bias cell can easily be replaced with a standard 1.5 volt AAA cell.

 

Diese Bias Cells wurden ende der 30er Jahre auch bei den schweizerischen Paillard-Empfängern eingebaut. Zum Beispiel die Modelle 406 und 59. Diese Batterien werden sehr wenig belastet und halten dementsprechend lange. Sie haben den Vorteil, dass sehr wenig Brummspannung in den Audiokreis gelangt. Beachten sie auch die Gegenkopplungsspule in Serie mit der Sprechspule des Lautsprechers bei ihrem Philco-Radio. Diese Bias-Zelle kann problemlos durch eine Standard AAA-Zelle, 1,5 Volt, ersetzt werden.

Michael Boessen
Michael Boessen
 
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31.Mar.18 02:40

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Hi, George!

Yes, I have several ideas from the forum for putting a battery in there.  I am planning to sell the radio and I did not want to give a battery problem to my customer to have to change every few years.   I have operated the radio both with the cell connected and without.  There is very little difference.

Rudiger:  With the cell in, -1.5 volts on the grid.  With no cell in it, .9v on the grid.  I put in a 1n4003 silicone diode.  Grid bias .43.  I tried 2 in series and 2 in parallel at opposite polarity.  Little or no difference in the sound or the bias voltage.  I suspect my 1 meg ohm input resistance voltmeter is probably loading it down a little.  

At this stage, It is not clear to me that this battery is even worth the trouble.  Not sure where I will go from here.

It's been fun playing with it, though, and has sparked a good discussion, which is one of the best parts of this forum!

Thanks everyone!

Mike

Michael Boessen
Michael Boessen
 
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31.Mar.18 02:43

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Sorry.  Note to Rudiger:

I mis-typed one thing in my previous post.  With no cell and no battery the grid reads -.9v, not .9v as I typed it in the previous post.

Thanks again!

Mike

Michael Boessen
Michael Boessen
 
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31.Mar.18 15:14

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Rats.  It was too late at night when I typed that last post.  ALL of the bias voltages I typed in there were supposed to have a "-" in front of them.    Sorry if this has become tedious.  I'm still new at this.

Michael Watterson
 
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06.Apr.18 22:53

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Only use a Silver Oxide button cell.

It was the original application of a Mallory Mercury cell (Mallory is now called Duracell). They are actually 1.35V. They were never 1V except at end of life, as is an Alkaline cell.  A modern silver oxide button cell is a better replacemant than the Alkaline button cells. It might last five years to ten years.

Do not use a Lithium button cell as it's twice the voltage. The Alkaline buttons wiil work but not as well as they droop considerably and start too high at nearly 1.6V with no load. They also only last half as long in this application. Silver and Mercury buttons are very flat discharge curves.

Do not add a diode. Pointless as they need current draw to have a voltage drop. The Silver Oxide is a little higher  voltage. There is almost no current draw.

Do not use a zinc air hearing aid battery as they self discharge. They like the thier big Fencer batteries are designed for continious use once the seal is removed. The big fencer packs are five Flag sized cells.

The idea was that it would be cheaper than a cathode resistor and the large value Electrolytic decoupler (or else the resistor would reduce the audio gain by negative feedback. It was not a smoothing issue.

Michael Boessen
Michael Boessen
 
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07.Apr.18 17:24

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Hi, Michael!

Thank you for taking the time to read this and write this thoughtful and helpful reply.  I had some LR44 cells on hand, so I swapped the connections to the battery holder and put one in.  The set plays fairly well this way.  I agree with you that the diode Rudiger suggested can't work because there has to be current flow through a diode to get the voltage drop.  I tried it, and there is no difference in the audio with the diode in or out of circuit.  There is very little difference with the battery in or out for that matter.

I have decided to go with your suggestion to get an SR44 cell and put it in there.  I was going to try a diode and voltage divider, but these DC radios always leave me at a loss as to how to connect something like that.

Again, thank you for your time.  I have an audio problem that I am going to start another thread on,   Hoping someone has an idea about that, as I have it in another set as well.

Best regards

Mike

  
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