Field coil speaker and PM speaker on the same amp (alternatively)

[lo-fi]

I have an old Oahu (by Valco) amp from the '50s that's still using 10" field coil speaker (schematic here: https://elektrotanya.com/gretsch_g61.../download.html) .
Everything's working fine after a cap job.

I love its tone and I absolutely do not want to get rid of the old Rola field coil speaker. It's a great recording amp as is (2×6V6, octal preamp tubes, field coil speaker).
Anyway, I'd like to gig with it in small venues without the risk of blowing the rare (and nice voiced) stock speaker.


My idea is to install a switched jack connected to original OT secondary to allow an external speaker, having:


- permanent shared negative connection (external and field coil speaker);


- hot wire connected to the old Rola when no input jack is inserted, otherwise connected to modern PM ext. speaker.


By doing that I could keep the field coil circuit in place and fully working (no need to deal with field coil replacement/emulation).
At the same time I could easily use an external 8 ohms cabinet.

Weight is not an issue with that old little combo amp. No problem in carrying the unused stock speaker when gigging.

I successfully installed a similar jack on a Float-A-Tone amp who already had been equipped with an ext. speaker input. I wanted speaker selection to be strictly alternative and the switched jack worked out fine.
Now the stock field coil speaker makes it all a bit more complex (maybe not!).

I have a very basic knowledge of how field coil speakers actually work. I'm assuming that the field coil speaker can actually live with no signal from OT secondary whenever external PM speaker is connected. In any case the amp would keep on seeing a field coil as expected. The OT would always be connected to a 8 ohms speaker. It looks like a win-win.

Is it a crazy idea? Am I missing something?

Of course, I don't want to damage any component. The purpose is to keep the amp stock with some added flexibility (and reliability) for gigging usage.
Am I fooling myself?

Any opinion would be appreciated.

Thanks!

[Helmholtz]

Your plan should work (assuming your amp corresponds to the Gretsch schematic provided).
Speaker field coil and voice coil are independent. The amp uses the field coil as a PS filter choke, so that needs to stay in place.

What DCR does the voice coil have?

[Chuck H]

As far as I can tell what you propose should work fine. The field coil does not rely on signal to the voice coil to do it's job as a choke. My only caveat would be that if the intention is to preserve the field coil speaker it should be considered that the field coil is part of the speaker. By continuing to use the field coil you are still using a part of the speaker you wish to preserve. And possibly using it more because you'll be gigging with it. Should anything happen to the field coil, having preserved the voice coil and cone are moot. But to take the entire field coil speaker out of the circumstance is too complicated for a simple switching jack.

This idea is nothing I would ever do for a customer. But if you do it for yourself then the risks are all yours, so...

You could rewire the field coil speaker connection with some sort of four pin power plug. One with connections isolated when unplugged would be best for the obvious reason of high voltage on one of the contacts. Not that you'd ever have a reason to unplug the speaker when the amp is on, but it's just best practice I think. Anyway, then you could build yourself an external cabinet with a choke (or even just a resistor, but a choke is better) and speaker inside that uses the same connector you used for the field coil speaker. More complicated, sure. But it does absolutely preserve the original speaker.

[vintagekiki]

My idea is to install a switched jack ...

[mozz]

I have a pre WW2 Supro with the field coil speaker. I'm not sure of it was that amp or another i have, but sometimes they also have a humbucker coil tied into the circuit also. Some of those schematics are wrong/missing parts. You are best off just swapping out the speaker to preserve it. Add a resistor in place of the inductor. Anyway, if it's working now, measure the dc resistance of all speakers parts before it blows. Sometimes the dc resistance of the field is not shown on the schematic and if it has to be rewound they are gonna ask the approx. value. If the voice coil, humbucker coil, or the field coil go open, chances are you will be sending it out for a not so cheap repair if it is even possible. You can still find field coil speakers on Ebay fairly cheap unless you are talking Western elec.

[lo-fi]

Thanks everybody for your kind reply.

Helmholtz : No, I still don't know the DCR of the voice coil. I guess it's 8 ohms but I'll have to check it. I do not expect anything but 4 or 8 ohms from a single 10" speaker. I recall having tried to read impedance between speaker's terminals a few weeks ago with all the wiring in place. I guess I have read something like 3.x ohms. So, it could actually be a 4 ohms speaker.
Yes, that Gretsch schematic is identical to my 1955 Oahu's but for a small cap across volume pot not present in the Gretsch amp. I have another one with a 6J7 as V1 (1951) cosmetically identical. I call those two the Blues Brothers since they're perfect for that old school Willie Johnson / early Muddy Waters blues tone. The 6J7 one has more headroom and it's not as brutal as the one with 6SQ7.

Chuck H : You're absolutely right about continous field coil usage even with external PM speaker connected. According to my solid ignorance of field coil speakers I assumed that the field coil itself was quite a rugged component, endurable as a magnet in a PM speaker. I understand it is not. And I also understand the attempt to preserve a field coil speaker while keeping on stressing its field coil is useless at best. Thanks for the precious advice. As you said, definitive solution will have to take into account the field coil circuit, installing a choke to be engaged when connecting a PM speaker. Probably it's not worth the hassle and I'll save that old little amp for home/studio pleasures.

vintagekiki : Thanks, but isn't it a standard ext. speaker jack wiring, not bypassing the internal speaker? If so, it doesn't work for me cause I'm (actually was) looking for a solution to easily alternate the two speakers (field coil vs. PM) depending on specific needs.



So, although technically feasible I realized a proper and effective solution would require more effort than I expected. I'll leave that cool old amp alone.

[lo-fi]

...You are best off just swapping out the speaker to preserve it. Add a resistor in place of the inductor. Anyway, if it's working now, measure the dc resistance of all speakers parts before it blows. Sometimes the dc resistance of the field is not shown on the schematic and if it has to be rewound they are gonna ask the approx. value. If the voice coil, humbucker coil, or the filed coil go open, chances are you will be sending it out for a not so cheap repair if it is even possible. You can still find field coil speakers on Ebay fairly cheap unless you are talking Western elec.

Thank you! I will measure everything as you suggested for future reference. But I guess I'll keep it stock, including still perfectly working field coil Rola speaker. Caps job, 3-prong cord and death cap removal is all the amp did need. It's not meant to be a gigging amp for more than one reason and I finally decided to enjoy its tone respecting its limitations.

[Helmholtz]

The DCR of the voice needs to measured with speaker disconnected from the OT. Actual voice coil impedance is about 20% higher than DCR.

BTW, a field coil doesn't age like a tube or an ecap. I don't think a field coil failure is much more likely than an OT or filter choke failure, both being rare.
It would require prolonged operation with a power tube or filter cap short to overheat/damage the field coil - but that should be prevented by the mains fuse.

[vintagekiki]

vintagekiki ...

Carefully analyze the schematics. Jumper on the switched jack select internal spkr or ext spkr.
The field coil of internal spkr in both cases still works as a HV choke.

[g1]

Carefully analyze the schematics. Jumper on the switched jack select internal spkr or ext spkr.

Yes. The ext.spkr. jack in the diagram VK posted is a switching jack doing exactly as described in the first post.

[lo-fi]

Carefully analyze the schematics. Jumper on the switched jack select internal spkr or ext spkr.
The field coil of internal spkr in both cases still works as a HV choke.

Ops, you're right. Definitely not a good idea to read schematics late at night, giving for granted you can understand what you're seeing.
In the morning it may look way different!

[mozz]

"BTW, a field coil doesn't age like a tube or an ecap."
But they age like a piece of enameled wire. Many old radios use field coil speakers and they often have to be rewound. Current through cheap thin wire doesn't help. Then you have enamel which cracks, you have speaker vibrations. If you repair old radios from the 30's -40's you would know it is fairly common to have open field coils.

[vintagekiki]

Open spkr field coil (choke) is caused by a faulty power tube or faulty elcap after choke.

[Chuck H]

lo-fi has already "accepted" that this effort is probably not greater than the reward.?. But I still think it should be easy enough. A plain old four pin connector could be spliced into the existing amps speaker connection. Then the extension speaker cabinet would use the same connector. Simple. And the particular connector prevents any other application so the amp could only be used with it's own speaker or the purpose built cabinet.

The hitch with the cabinet is the substitute choke. I don't suppose it HAS to be the exact specs of the field coil to work. Which is good because a choke capable of input filtering the whole power supply that has a DCR of 1000 is probably hard to come by. A rough guestimate would be about 40 or 50H @ (at least) 100mA. Getting even close to that would be one of those expensive, potted hi fi models sold is small numbers to "enthusiasts". That amp uses very modest capacitor filtering too. So I'm not sure substituting a plain resistor would give satisfactory results. However...

Something like the Hammond 159M would probably work sufficiently. 15H, 100mA and a DCR of 256. They can be had for around $30 US. The lower DCR would mean marginally increased voltages. If that's a concern an additional series resistor could be used. The 15H rating isn't going to smooth the power supply like the bigger H field coil. I'm not even certain it's sufficient. Perhaps someone with greater technical understanding could chime in the particulars here. Helmholtz?

[Helmholtz]

Something like the Hammond 159M would probably work sufficiently. 15H, 100mA and a DCR of 256. They can be had for around $30 US. The lower DCR would mean marginally increased voltages. If that's a concern an additional series resistor could be used. The 15H rating isn't going to smooth the power supply like the bigger H field coil. I'm not even certain it's sufficient. Perhaps someone with greater technical understanding could chime in the particulars here. Helmholtz?

I have no information about field coil inductance.
Max. DC supply current in the amp might be around 100mA.
This means that the 1000R field coil wouild drop around 75V more than a 256R choke at full output. As you said, the difference could be compensated by a 750R series resistor.

There is an interesting aspect of such field coil circuits: As speaker sensitivity/efficiency increases with field current and amplifier current increases at high output, there will be an expansion (as opposed to compression) effect.