Hi, I have a restored chassis for an old AWA model 163 Fisk Radiola. A copy of the circuit can be found at http://www.kevinchant.com/uploads/7/1/0/8/7108231/52_163.pdf. (which I have to say is an excellent source of Australian valve radio schematics - this had the actual AWA Technical Info and service data with an extremely helpful wiring diagram - the AORSM only has the schematic).

Anyway, I bought the chassis to learn from and got it working after replacing every cap and several resistors and removing a number of incorrect connections that someone else had previously incorporated including a direct connection from the B+ off the mains transformer to the chassis making the chassis live!

It didn't come with a speaker. I have the table model which is the model 52 and the circuit is identical and attached the speaker through its plug into the console chassis.

How do I go about selecting a speaker for this chassis to match? The schematic shows a 1600 ohm field coil and a 'T.G. 113' Loudspeaker Transformer. As I have never had to replace a speaker before I have no idea on how to go about it. I assume that the requirements change if I use a 6.5 inch speaker (what is in the table model) or a 12 inch (which I think is what was used in the console). I will make a cabinet myself to house it all in.

Any assistance is appreciated. I assume you can't use 'modern' type speakers because they don't have field coils?
Regards
Angelo

You can use a modern speaker, but it is imperative that the 1600 Ohm coil is replaced.

The field can be substituted by High wattage Wire wound resistors. Done it several times.

Not looked at circuit.

Marc

How do I go about selecting a speaker for this chassis to match?

I'm not sure what typical nominal impedance the electrodynamic speakers were. Marcc may know from experience.

Otherwise, you can work from first principles, as it were:

First, you need to determine the impedance ratio of the output transformer. Use the following procedure:

1. Disconnect both sides of the output transformer.

2. Attach a variable AC supply to the primary (outer leads only; ignore the centre tap).

3. Attach and AC voltmeter to the secondary.

4. Slowly increase the variable supply from zero volts and stop when the voltmeter on the secondary reads 1 volt.

5. The voltage on the primary at that point gives the winding/turns ratio (e.g. if it reads 30 volts then the turns ratio is 30:1)

6. Calculate the impedance ratio by squaring the winding ratio (e.g. a turns ratio of 30:1 equates to an impedance ratio of 900:1)

7. Once you have the impedance ratio, you can determine the effect that any secondary impedance (in the form of a speaker coil) will have on the reflected impedance in the primary and hence what the output pair "sees". For instance using the 900:1 ratio, 8 notional ohms load on the secondary will reflect 8*900 = 7200 notional ohms impedance load in the primary. A 4 ohm speaker load reflects 4*900 = 3600 ohms.

8. The reflected notional load determines the output power and quality of sound (e.g. where distortion kicks in and the bandwidth of the frequency response). Without the design specs for the circuit in question, it is difficult to know what the optimum reflected impedance should be, however, according some data I have, a pair of 6AQ5s in push-pull should be happy operating into a reflected impedance around the 10,000 ohms mark.

Most of those were around 3.5 Ohm, there may be a reference some where?

That would be irrespective of the diameter of the speaker
that someone may have one of & knows?

Speaker AE5 used TG52E transformer speaker AJ1 used the TG 113D.

I have a Bandmaster table here with an 8" speaker but it's Rola & dynamic.

Marc

Marc, is there a list of the AWA codes for their parts with details of the characteristers of the part? Eg. the schematic refers to a Loudspeaker code AJ1 for the console and AE5 for the mantel and a TG52E for the mantel loudspeaker transformer and TG113D for the console.

I have searched the internet but nothing - was there an official AWA parts list that anyone knows about?

Sorry GTC but being new to this, I would struggle with your suggestions.

I have the speaker and transformer for the table version which works well - can't I just check the ohms on each and match a speaker and transformer that is close from an old scrap console radio?

Angelo

Marc

Will power resistors as suggested give the same filtering as the field coil? I have read on the US antique radio forum that this is not an ideal solution and the better option is to use a filter choke in place of the field coil? I believe you can buy filter chokes pretty easily? Question is what type to replace a 1600 ohm field coil.

I think it may be easier to salvage a field coil from a scrap radio and attach that to a modern speaker?

I once got a Kriesler beehive working using a resistor to replace the filter choke. The hum level was quite acceptable and this radio is still going 20 yrs later.

It needs to be say 10watt ceramic type as it will dissipate quite a bit of heat. In the Kriesler it was mounted above the chassis (with well insulated wiring as it has a couple of hundred volts DC on it) where it was well ventilated.

Can be a temporary fix until you find a replacement dynamic speaker or choke, and enable you to troubleshoot the rest of the circuit. And you can use a permanent magnet speaker as well.

With a choke, it is actually a superior filter. I would need to know the voltage drop across it to get a current rating.

With that info you can also get the wattage, for resistors.
Choke must have a similar DC resistance.

The cost is usually the factor and they were phased out in many latter radios. Do not be tempted to use bigger caps. or you may damage the rectifier.

The Quality of the caps is also a strong factor. I would not go below 500V with 80 and seriously consider Spragues in that situation for their better specs.

That speaker transformer was pretty comon in AWA Models 38, 41, 51, 52, 48 Bandmaster 457DE.

All with 42 / 6F6 output. Many would have a set with that and someone may be prepared to disconnect the Voice coil & measure it


Marc

Marc

With the transformer, do I simply measure the ohms on the primary and secondary and get a replacement one with similar readings? I assume that the voice coils on the speaker will be pretty much the same regardless of size?

I have a Tasma "Baby" radio, the schematic from the Tasma company's service data showing how to replace the field coil speaker with a perminant magnet speaker. It specifies a 2K resistor, though no wattage rating, I'd use at least a 5 watt power resistor, as the radio circuits must draw 43ma to get 6 volts across 140 ohms (parts 32 and 33). Which would make for 3.6watts in a 2K resistor. I don't understand the addition of a 50K resistor and 0.1μF cap to ground at points A and B, I'd think it would kill the audio signal, so I'd skip it. My radio still has its FC speaker, and I haven't measured its resistance.

You can add capacitence after this resistor, as the resistor will limit what the rectifier tube's cathode will see. You should avoid adding capacitence right at the rectifier tube's cathode, as it will cause excessive peak currents on that cathode, killing the life of that rectifier tube.

As for the impedance of the speaker voice coil, you could measure the DC resistance of the broken FC speaker (assuming that that part of it is what is not broken) , and pick a new speaker with close to the same DC resistance. The voice coil doesn't much care that the magnetic field comes from a magnet or a field coil. To a first order approximation, a speaker's impedance is about 1.2 times the DC resistance.

Not interested in the transformer. It is the field that is of interest.

I have replaced several chokes with resistors. In order to get the Wattage of resistors one needs to know what power it is trying to handle

That also applies to the chokes which have their current rating as part of the spec. If you use a 60mA choke and the set draws 100mA, it will cook.

This is why the voltage drop across the choke, or the voltage either side of it needs to be known.

Looking at the circuit voltages etc. They are saying a current draw of 50mA with 50V drop across the choke.
So a 60mA choke should work if the DC resistance is 1600 Ohm (You may get away with 1500)

That gives 4 watts of heat.


Marc

Marc, where do you get the 50V from? I can't see it in the schematic. The only voltage given for the 'L.S.' (Loudspeaker?) field is 80 volts.

Yep! misread that one. 80V drop across field. It's all on the page with the element voltages

Still 4Watts

Couple of choices if you go resistor if EVATCO* has 3,3K at 3Watt, parallel two and with an 80 try not to go below 500V with the filter caps. * If buying from them.

Give the resistors plenty of air space.

Marc

Is the speaker transformer simply designed by reference to the size of the speaker or does it need to match the radio circuit too? Eg. can you use the transformer that specifically comes with any second hand electro magnet speaker?

does it need to match the radio circuit too?

See Post #3, point 8.