In many cases the speaker and transformer were supplied by the one company as a unit.

You cannot use an OP transformer "Willy Nilly" it has to match to the output tube. Single ended Outputs can have more than one Primary impedance with the same valve.

The recommended for 6AQ5 is 5000 Ohms (That is impedance, not DC resistance). It is not unusual to see 7000 Ohm on 6V6 which electrically,the same valve.

The secondary winding should also be the same as that of the speaker voice coil.

Marc

The recommended for 6AQ5 is 5000 Ohms

Yes, for for single ended. As I noted in post #3, for a push-pull pair it's 10,000 ohms (according to my GE data sheet).

However, not sure now how I got onto pairs of 6AQ5s. I think that was another thread asking about OP transformers?

Yours has a 42 / 6F6

I believe Brad also is working on a set that should have a 42 but has a 6V6, which is not helpful, as the bias requirements are different

6F6 is a 42 with a different base & plenty of that went on.
eg 6A7, 6A8.

Hopefully if that one is original the speaker Voice coil could be measured, as could several others as that transformer was common too.

There is no point tossing the OP trans if it's ok. Recommended is 7K primary for 42.

Most speakers close to 3.5 Ohm, now will be 4Ohm

Marc

Thanks Marc and GTC

if I were to find a field coil speaker on ebay with attached output transformer (so I don't have to worry about matching transformer to speaker) but the field coil is say 1000 ohms instead of the 1600 the circuit requires, can I add say a 600 ohm power resistor (eg 10watt) in series with the field coil? Do you need an exact match - eg sometimes I see field coil speakers with more than 1600 ohms, can you still use or will it affect the tubes?

Is it like resistors/caps where there is a 20% margin either way or are field coils not as flexible? Is adding a power resistor in series or in parallel (to reduce the total resistance) an acceptable solution?

Regards
Angelo

If the circuit calls for 1000 Ohms then there is every likely hood that that is not the the right speaker. As I said that OP transformer was common to a wide range of models.

The difference will overload the B+. The set was designed for 1600R. All of the current must pass through the field.

You can add 600R, check there is not a second "added" choke.

If the speaker is not in a repairable state, I think 1600R and a 4 Ohm Dynamic speaker is looking less of a hassle. And less costly.

Marc

Summary:

Given that, in this case, you already have the correct OP transformer, then as long as it has continuity on both primary and secondary you should go ahead and use it.

Replace the original 1600 field coil (which is required as a power supply ripple filter) with resistance as discussed above, and look for a 4 ohm (as Marcc suggested) speaker of the appropriate physical size.

Dear GTC

I have neither a speaker nor an OP transformer at this stage. I have plugged the chassis (which is a console version) into a similar AWA radio that is a table model that consists of a 1600 field coil. The speaker is about 6.5 inches in diameter. Sounds fine but volume is not that great.

I am looking around on ebay for a replacement field coil speaker. Most of those on ebay have the attached output transformer. The only differential is the resistance of the field coil. I need 1600 based on my circuit. That is why I wanted to know if I could 'up' the resistance or 'reduce' the resistance on a field coil with an appropriate power resistor.

What I still don't quite understand (and I know it is just my inexperience) is whether, if I need to do something to the output transformer that comes with the replacement speaker - it was obviously designed to work with that speaker. Or, is it the case that it may not be appropriate for the chassis because it is affected by the output tube used?

Okay, from post #18 I got the notion that you had the OP transformer.

In the first instance, output valves must work into the appropriate impedance, therefore the circuit designer specifies the primary of the output transformer to provide the required impedance for the chosen output valve.

That done, the secondary is wound to match the desired/chosen speaker impedance.

Back in the heyday, there were only so many output valves and so output transformers were manufactured (either by the radio manufacturer themselves, or by specialist transformer manufacturers) to suit the popular valves in the various amplifier mode combinations they were used in.

Similarly with speakers. Their coil impedances eventually standardised around the range of 4, 8 and 16 ohms and transformer manufacturers provided those impedances for secondaries.

As discussed in post #3, output transformers "reflect" the attached secondary impedance into the primary, so if you mismatch the speaker, then you'll create a mismatch in the primary and affect the conditions facing the output valve. How badly such a mismatch affects performance depends on the degree of the mismatch and the tolerance of the circuit to it.

Now as Marcc said in post #18, "Recommended is 7K primary for 42. Most speakers close to 3.5 Ohm, now will be 4 Ohm", so you should try to find an OP tranny with 7Kohm primary and 4 ohm secondary, then create a speaker combination that presents and impedance of 4 ohms to the secondary.

As Marcc also said, the transformer type specified in the notes for that set was common to a number of sets, so you may find one in a "junker" set or available for sale separately if you attend radio society swap meets, etc.

If you intend to make this a continuing hobby, I'd strongly suggest your joining the HRSA so that you can attend their functions and "network" locally.

Thanks GTC. That was very helpful. It is probably why the chassis 'works' with the speaker in the other radio I have that has a 1600 ohm field coil but volume is not great and the Mains transformer runs hotter than normal.

The output transformer in each set for the speaker is a different number - I assume because the chassis came out of a console model, it used a much bigger speaker than the 6.5 inch in the table model and that is part of the issue and this affects the operation of the radio overall.

It matters little as to the diameter of the speaker itself albeit the tendency was to use a bigger cone diameter , speaker, on low power sets to get more interface.

PA speakers generally have provision for a line transformer to be mounted on them.

TV sets are being scrapped by the thousands, bound to be able to get a speaker there.

Mitubishi used a 6.5" speakers in some of their car doors, see a wrecker. I used one in a Philips 132L ......

OP transformers are gettable new.

Footnote: Beware with some modern speakers. Used in table sets etc. the magnets are so powerfull, they can have interaction with the power transformer.

Marc

If you do get and install a replacement field coil speaker, it may make a differeence which way the field coil is connected to the radio's power supply. Manufacturers often used the fact that they'd be some variation of the magnetic field the coil would produce, related to the hum ripple of the power supply. And one can phase that variation to counteract ripple hum coming from the audio output of the radio, this counteraction happening between the field coild and the speaker voice coil. So be prepared to swap the connections on either the field coil or voice coil or the audio output transformer primary if the radio seems to have excessive hum.

Lots of digging around and I found the answer!
Thanks Wa2ise, your post above was exactly what I have been hunting for. Some people might call it waxing lyrical or waffling; I call it spreading the knowledge. Someone, somewhere, will need the information that at the time may seem insignificant. trite or off topic. Not that Wa2ise comments were any of these. Point is any info you guys can impart through these forums is gonna be Gold to someone. Please don't forget that.

You're on the right forums Phil. There's quite a few threads of a high-tech nature here. The search feature will unmask the gooduns.

Taking a few steps back. If the other sets speaker has the same field coil resistance, and the same output valve, and works pefectly in the radio that you swatted it from.

If is does not work in this radio and the transformer is getting hot. Something is wrong in that set causing it to draw excessive load.

Check the wiring & make sure the electrolytics are correct.

There is no guarantee that the plugs, if the same, are wired the same.

Marc