Hammond M3 Electrodynamic speaker

[shaun]

Hi All,
 I have a very nice 12" electrodynamic speaker from an M3. It is in such nice condition that I'd like to use it in a simple build, something akin to a single channel PP style build.

I've read many of the posts in this forum that make it seem possible that I could do this. I think PRR may have mentioned in one post about retaining the entire power supply where possible, but in this case it seems difficult to do because the M3 PS was designed to power half the planet, and I doubt I'll have room.

So my question concerns where and how to insert the field coil. As you see, in the original schematic it seems to be connected between CT and ground, along with a bunch of filtering. It has 100v across it (I assume) and is rated at 700ohms.

Also, I'd prefer to cathode bias the 6V6s; right now, they seem to be negative biased from the power supply somehow.

In hoping for a simple solution, I may be barking up the wrong tree. Or just barking :).  But is a nice condition speaker, and I've heard they can sound good, so I feel the need to use it or banish it to ebay forever so its mystery will no longer torment me.

As always, any help is much appreciated. This forum has educated me in all sorts of ways over the past two years., for which I am very grateful. My only regret is that I still don't seem to know enough to contribute effectively.  But let's hope that day will come.

[sluckey]

Do you also have the AO-29 amp that was in that same M3?

First off, 100V across 700Ω is 143mA! That's more current than most 6V6 amps need.

With that said, the PT that powered that FC speaker lives on a Hammond AO-29 amp chassis. There are two versions of the AO-29, one does not supply a FC speaker. The AO-29 also has a nice push/pull 6V6 amp on it. Together with the FC speaker, it would make a nice platform for a 5E3.

If I had that speaker and wanted to use it in a guitar amp project, I'd buy the correct AO-29 amp from eBay and go from there. There are several AO-29s on eBay right now. Your first challenge is to buy the correct one. And that will require a good bit of research on your part.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=hammond+%28ao29%2Cao-29%2Cao+29%29+-set&_sacat=0&LH_TitleDesc=0&_sop=10&_odkw=hammond+%28ao44%2Cao-44%2Cao+44%29+-set

[shaun]

Thanks for the guidance, Sluckey, and thanks for the links - didn't know they could be got so cheap. So, you're suggesting I use the original configuration, which I could do; I actually have the PT and OT from the same unit, so I could build according to the schematic to get something close to the original. But I was hoping to build something simpler.

If using the original M3 schematic is the only way to use the field speaker, I'll forgo the field speaker and design something easier. I guess it was just wishful thinking on my part, but the more I look at the schematic, the more I realize how involved the design around that field speaker is.

[sluckey]

Since you have the matching PT, OT, and field coil speaker, it would be very easy to adapt to a 6V6 amp. Here's how you could build a 5E3...

[shaun]

Shazam! Look at that. A thing of beauty, and so simple. I assumed that, if I tried something like that, I'd get way more than 100v across the field coil because of the VDC on the plates. But I really must learn to think in terms of current rather than voltage.

You already pointed out that the field coil offers 145mA of draw potential, so I guess that's what matters most.

This will be a super-fun experiment. Thank you Sluckey. I will post my results. I often refer to your schematics when building, and they're always great. Much appreciated.