[bdi1976]

Hi Phorum members-
I am in the final phase of restoring my 1934 84B and am still confused about putting a modern speaker into the wiring. Specifically, how the wires get rerouted after removing the FC. I have attached a scheme of my 84B and a second one to the right with my wiring. If one of you could clarify if I'm correct, that would be awesome- Thanks guys,
BDI

[bdi1976]

The schematic only shows the copied pic and may not show the edited part I added. I had to click the message at the top of Microsoft Word stating "Enable Editing" to see my drawing at the top right. Thanks and sorry-

[morzh]

BDI

Since it is a bit of a pain to try to see these images on my tablet, here is the general idea: replace the field coil with a resistor of the same or greater value that is the ohmic resistance of the coil. The power rating should be adequate, that is whatever the voltage drop is, U-square times R applies and then make it 2 times that.

The capacitor after the coil could i crease as due to the lack of the inductance the ripple will i crease and so will the hum.
There is pratxically no change in wiring other than that coil for resistor swap.

[bdi1976]

Thanks Morzh- That is a clearer explanation than previous threads I've read. Should have the speaker tomorrow and hopefully she'll be singing (instead of humming) tomorrow night. If it don't it blow up, I just got a 90 with an original cabinet in incredible condition that I will be working on next. Thanks for the help-
bdi

[Raleigh]

If I understand your drawing correctly, you are putting your new resistor or choke between the secondary of the output transformer and the speaker. This is not the right place.

Your new resistor or choke should connect electrically between the positive end of the 8 uF filter cap and the positive end of the 4 uF filter cap. The positive end of the 4 uF cap should also connect to the one of the wires of the PRIMARY of the output transformer, the one that does not go to the plate of the output tube.

[bdi1976]

Thanks Raleigh - That set me mostly straight on the wiring except for one wire. Without a field coil, what happens to the Green and White wire coming from the 8uF which used to go to the FC? It should be no longer needed...is this correct?
Could someone explain the basic flow of voltage from the rectifier to the filters running, then to the FC. I thought the DC voltage comes from the rectifier, gets filtered for smoothing by the 8uF filter to ground, then it feeds the FC...is this correct? How does the 4uF function if it does not connect to the rectifier at all (note: part 32 is not in my model, so I am assuming the 4uF line does not connect to the rectifier)? I think my lack of understanding of how this end functions is making me confused on the wiring. Thanks for the help guys!!
bdi

[Eliot Ness]

BDI, I think you are confusing the field coil with the voice coil. The field coil is # 27 in the original drawing, that component is what you're replacing with the resistor. So green/white goes to one end of the (new) resistor and white goes to the other end. Find a value close to 1140 ohms and preferably in a wire wound version.

What you are seeing is a basic pi type of filter. You have two capacitors with an inductor (in this case the field coil) between them for added filtering. See below for the basic concept:

http://en.wikipedia.org/wiki/Capacitor-input_filter

[morzh]

Your basic flow is this:

http://en.wikipedia.org/wiki/File:VacRect2E.png

This above is a simple full-wave tube rectifier.
It is not filtered.

http://www.radiomuseum.org/tubes/tube_5y3.html
This is now filtered.

Now, this above will still have pulsations which are

Vpulse=I/(2*f*C)

Where Vpulse is the ripple voltage, f - Frequency (60Hz) and C - capacitance in Farads (say 12uF is 0.000012F).
Which at say 50mA of current will yield 34V of ripple.
One would say why not increase the cap 100-fold and get rid of ripple.
Valid suggestion but with tube rectifiers the capacitance is limited by what the tube can handle.

The best way of getting rid of the ripple is adding the second stage of the filtering. Most typical is LC. The inductance L is either a separate choke or the field coil used as such. And then there is the second cap (say 8uF cap).
The L will resist the change of current and so your current will become triangular and not suqare-wave type. The capacitor will smooth the voltage. Good thing is there does not have to be a small limit on the second cap size as the current is limited by the choke.

Choke also has a parasitic parameter, the resistance which is quite large - it is from hundreds to thousands of Ohms. This brings another degree of filtering due to the time constant RC. It is not very effective in your case as the time constant has to be larger than your ripple's period, so the output cap has to become say 80uF to have some good effect with 400 Ohm resistor.

Problem with purely resistive filtering is the resistor needs to be large and will dissipate lots of power whereas the choke only dissipate the power due to its parasitic resistance and if it were pure inductance it would not dissipate anything at all. Things being equal the capacitor increase and not resistor is desirable, but then you should see if you overload your tube when your resistor is small and the second cap is large.

Another problem with increasing the resistor is it will drop the voltage, and if you were dropping say 30V before you threw out your field coil, and you decided to increase the resistor twice, you will be dropping 60V. Not only this will increase the dissipated power 4 times, but it will also drop the Vbb by the same 30V which might adversely affect the operation of the tubes, especially of the audio output stage which typically works at full rectified Vbb voltage.

What would I do.
If I had extra space, I would rather install a separate choke of comparable inductance and parasitic resistance (which are always listed in datasheets) to your field coil.
And then increased the output capacitance to further attenuate the ripple.

[Raleigh]

BDI1976, is the audio output transformer part of your original Philco speaker assembly? If it is, then the schematic makes more sense to me. On some older Philcos, the output transformer, the field coil, and the voice coil are all part of the speaker assembly. Later Philcos have the output transformer inside or on top of the chassis.

Before you decide exactly how to make the connections to install your modern PM speaker, you have to know if the output connector is attached to the chassis or if it is attached to the speaker assembly.

I think the little "V" shaped notations on the schematic represent connectors. If I'm interpreting this right, then there is a long bundle of 3 wires connecting the chassis with the speaker. The colors of those 3 wires are not specified on the schematic. The colored wires specified in the schematic are part of the speaker assembly, not the wires from the chassis.

The connector lines up so that the green wire on the speaker assembly connects to the plate of the 42 tube, the green/white wire of the speaker assembly connects to the + of the 8uF cap, and the white wire of the speaker assembly connects to the + side of the 4uF capacitor. All other connections are internal to the speaker assembly.

Now, if the output transformer is mounted on the speaker assembly, then you have to mount it or a replacement somewhere else.

Let me know if the transformer is mounted on the speaker (or post a picture), and then we can go from there. I think I can be very specific about which wires go where once I know that.

[bdi1976]

Thanks again everyone for the helpful info. I believe I have it right now, but the speaker didn't arrive yet to test. The output transformer is a Hammond universal, which fits next to the rectifier tube or I may mount to the speaker itself. I have the 1800 Ohm resistor attached to both + for the 8uF and 4uF where the FC used to be and I upped the 4uF cap to 10uF to eliminate the potential hum issue. I really enjoyed rebuilding this first set and I am anxious to start on a 90 next. Thanks guys...I'll let you know how it goes.

[Arran]

What happened to the original speaker? Was it missing? If you still have it hold onto it, they can usually be reconed or rebuilt. I'm with Morzh on this, a proper filter choke is the better option for the power supply as opposed to a resistor. Though you can get away with using a resistor in a Pi filter arrangement in a 4 tuber like a model 84, and it will work fine if you use larger filter capacitors to compensate for the lower inductance, don't try it in a larger tube count set using 8 tubes or more. It's still preferable to use a filter choke (or a solid state substitute) as opposed to a resistor.
The reason is simple, a filter choke or field coil is an inductor, and inductors resist fluctuations (or changes) in voltage through inductive reactance, but they pass DC. A resistor resists a given amount of voltage depending on the current load, so the more current load there is the more voltage drop you will see across that resistor. Resistors also don't filter and regulate as well as chokes, so you will need larger caps to not only compensate for the voltage drop across the resistor but to filter out the extra ripple.
In the early AC sets high value capacitors were bulky and expensive, the only types they had were large value paper capacitors, so they used paper capacitors of 1-2 uf but used two or three filter chokes to get rid of the AC ripple. When they came out with wet, and later dry electrolytic capacitors, they suddenly had inexpensive high value capacitors in a compact package, so they could reduce the size and number of the filter chokes needed, usually incorporating the filter choke into the speaker field coil making it perform a double duty.
Regards
Arran

[morzh]

As Arran said, "Though you can get away with using a resistor in a Pi filter arrangement in a 4 tuber like a model 84, and it will work fine if you use larger filter capacitors to compensate for the lower inductance".

Now, about increasing capacitances.

Your 80 tube can handle up to 32uF direct capacitive load, so you should not be exceeding that. Go say 20uF for starters.
This will 1) decrease ripple and 2) increase the output voltage some but then some of it will be lost on your resistor as you might want to go a bit higher.

Then the resistor. Go at least with the value of your field coil loss resistance which is 1.14 kOhm.
Then choose your second cap from the ratio

R*C >> 1/60, where R is in Ohms, C is in Farads, and the results are in seconds.
So if your resistance is roughly 1K, your output cap ideally should be 160uF.

This is a bit too much, as this kind of cap will be too large and expensive. However if you go with say 2K resistor and 47uF (50uF) 500V cap you will stay within size and fairly low cost. And the time constant is still acceptable.

Choose your resistance wattage so it could handle whatever the voltage drop you will measure, formula V-square/R for dissipated and two-three times that for rating of the resistor so it does not self destruct too soon.
Or if you know your current consumption in advance, go I-square * R for the dissipated (same as above for rated).

You will need to experiment a bit to see if your output voltage is the same or close to recommended; should not be much lower or much higher.

------------------


This said, the choke is still a preferred solution.

[K7Sparky]

I agree with Arron in asking what happened to your original speaker?

In post #4 you spoke of hum and seemed to imply swapping to a PM speaker will cure it. It will not.

The cap / choke (Field coil) / cap is a pie filter (as in the Greek letter as it looks like it).

A resistor is a poor substitute for an inductor in a pie filter. Your output voltage will be higher without the L unless you up the R and that will cause wider voltage variations with load - volume. You will have more hum without the L all else equal.

If you are fishing for a hum solution, replacing the speaker will make it worse not better. Jacking up the caps will help or totally mask the real problem.

As Morzh pointed out watch the cap size you are asking the 80 to operate into. No use to stress the old bottle.

EDIT: Just noticed from your schematic the original speaker VC is 89 Ohms probably 11:1 to your PM speaker. Therefore the transformed impedance to the plate of the 42 is about 1/10 of design.

[morzh]

I think it is 0.89 Ohm.

I am not even sure what one has to do to create a VC with 89 Ohms of resistance in a regular cone short of winding it with a Constantan wire
Which will make the turns ratio closer to 100 or so.

[K7Sparky]

Edit: Got the speaker impedance wrong from the print
Rest of post deleted as it maded no sense for actual speaker.