How do I replace field-coil speaker with magnet speaker?

[waspclothes]

Hey guys,
I have an old Valco amp and the speaker is farting out, I can hear the voice-coil rubbing while I move the speaker.

I'm wondering, what is the procedure to replace those old field-coil speakers with modern magnetic speakers?

I'm under the impression that a choke should replace the field coil and then the speaker gets hooked up with the other two wires? What's the criteria for selecting the choke?

Can I mount the choke onto the inside of the cabinet to avoid modifying the original chassis, or is it best to mount the choke to the chassis?

Last question would be, what sonic differences would I expect if I'm replacing an old field-coil with a modern speaker? Were the field-coil speakers more/less efficient than today's speakers?

I can't find the exact schematic right now for this amp, will post it if I can - or trace it out if I need to once I crack the amp open. Thanks guys!

[The Dude]

Where is the coil now? In other words, does it attach to the speaker and is removable, or is it inside where it can't be gotten to?

[Chuck H]

I think wasp is implying that the field coil is acting as the magnet for the speaker... I think.

I can't say about the choke specs. It probably matters less than you think. The new speaker will almost surely be more efficient (good thing IMHE). This alone will make the amp sound and feel more lively. Any difference the choke specs make will be peanuts by comparison. If the choke is between the plates and screens I'd just pick any OEM Fender choke for a similar power amp. If the choke is feeding the plates as well then you'll need to know your amps whole current draw to source a part.

And you've got it right as to how it hooks up. If you can accurately locate where all the leads go then the coil in series with the power supply is the choke and the other leads go to the permanent magnet speaker.

[The Dude]

Right. The field coil does act as the magnet for the speaker. The replacement speaker will not need it to operate. The amp will. The reason I asked about the original coil is because you can sometimes remove it and mount it elsewhere saving the hassle and cost of a new coil or high wattage resistor.

[Chuck H]

+1 That would pretty much remove the need to select the right part

EDIT: I've never done it myself. I'd expect you'd need to include anything acting like a "core" to preserve the inductance. I guess you pretty much just remove the speaker from the frame, cut off the cone and don't hook up the speaker coil leads? Might need some creative framing with aluminum plates or something but it seems like a good solution.

[The Dude]

I haven't done it for quite some time. But I recall some speakers (Jensen maybe?) where you could remove the rear bell of the speaker and easily get to the field coil. The pole piece of the speaker would be the "core" I guess. The times I did this, I didn't (maybe wrongly) worry about it (the core) and everything worked just fine. I don't think it's that critical. I mounted the field coil with large washers and screws rather like they mount toroidal transformers in some amps these days.

[The Dude]

It just occurred to me, you may want to check into what the speaker is worth first. For whatever reason, sometimes these old buggers bring some good money- even nonworking. If that's the case, just sell it as is and buy a coil.
Another consideration would be to recone it.

[Leo_Gnardo]

You might get that FC speaker reconed. My local recone guy was able to do an excellent job an an early Jim Lansing (later known as JBL) in a 30's Rickenbacker amp. Ask your reconer or you can PM me and I'll direct you to mine in upstate NY.

If you do swap in a PM speaker, and substitute a regular choke, be aware that the voltage drop across a field coil is typically a lot more than a standard choke, and you'll wind up with a noticeably higher DC voltage to run the amp. To compensate you will have to increase the value of the output tube cathode resistor to keep bias current within reasonable limits. Some say they hear a difference in tone with higher voltages, brighter & more "taut". Despite the chart-readers' claims that it's rubbish, I do hear that difference. Another way to diminish the B+ is the zener diode trick. You put a high wattage (5W should be sufficient) zener between center-tap of the hi voltage winding and ground. I've never used this technique but if you're interested just mention it & I'm sure one of our members more experienced with this technique will fill you in.

[Diablo]

Hey guys,
I have an old Valco amp and the speaker is farting out, I can hear the voice-coil rubbing while I move the speaker.

I'm wondering, what is the procedure to replace those old field-coil speakers with modern magnetic speakers?

I'm under the impression that a choke should replace the field coil and then the speaker gets hooked up with the other two wires? What's the criteria for selecting the choke?

Can I mount the choke onto the inside of the cabinet to avoid modifying the original chassis, or is it best to mount the choke to the chassis?

Last question would be, what sonic differences would I expect if I'm replacing an old field-coil with a modern speaker? Were the field-coil speakers more/less efficient than today's speakers?

I can't find the exact schematic right now for this amp, will post it if I can - or trace it out if I need to once I crack the amp open. Thanks guys!

Post up a picture of the speaker, and also measure the resistance of the field coil. As long as the field coil isn't burned up, the speaker can likely be reconed (contact Weber Speakers) and be good as new. I just reconed two field coil speakers, a Rola and Jensen F12N. They are high efficiency speakers because they have a small gap and a strong electromagnet which makes for a large BL product - high flux density in the gap makes for a strong speaker motor.

[52 Bill]

The most common version of Valco amp with a field coil speaker used a 10 inch Rola. The field coil was used as the choke for the power supply and was about 1K ohms in resistance.

I've seen people use a separate choke and I've also seen a simple 1K power resistor used to replace the field coil. Normally the output transformer is riveted to the speaker basket, so if yours is mounted this way you will also need to remove it or replace it. Reconing is probably easier in some ways.

Tell us what model amp you have or at least what tubes it uses.

[waspclothes]

Hey guys, thanks for all the replies so far.

The amp is a "Bronson Electric Sound Amplifier" that says "Detroit 26, Michigan" on the front. It uses two 6V6's as the output.

Here are some pictures of the amp and speaker:





So the output transformer is also mounted onto the basket, that's pretty wild. It might be kinda neat to bolt the transformer (and choke?) onto the basket of the new permanent magnet speaker - any caveats on doing something like that? Just tossin' that out there.

[waspclothes]

You might get that FC speaker reconed. My local recone guy was able to do an excellent job an an early Jim Lansing (later known as JBL) in a 30's Rickenbacker amp. Ask your reconer or you can PM me and I'll direct you to mine in upstate NY.

If you do swap in a PM speaker, and substitute a regular choke, be aware that the voltage drop across a field coil is typically a lot more than a standard choke, and you'll wind up with a noticeably higher DC voltage to run the amp. To compensate you will have to increase the value of the output tube cathode resistor to keep bias current within reasonable limits. Some say they hear a difference in tone with higher voltages, brighter & more "taut". Despite the chart-readers' claims that it's rubbish, I do hear that difference. Another way to diminish the B+ is the zener diode trick. You put a high wattage (5W should be sufficient) zener between center-tap of the hi voltage winding and ground. I've never used this technique but if you're interested just mention it & I'm sure one of our members more experienced with this technique will fill you in.

Thanks Leo, reconing does seem like the easiest option. I don't know enough about reconing to know if voice coil problems are something that can be fixed as well? If you can PM me your recone person's info that'd be great, but I should probably try to find one in Ontario Canada to save on shipping if anyone knows one - or how I can find one.

[TomCarlos]

This type of speaker is new to me... interesting. I just found a video where a guy shows how to test one of these speakers using a computer power supply. Click here...

[Leo_Gnardo]

The amp is a "Bronson Electric Sound Amplifier" that says "Detroit 26, Michigan" on the front. It uses two 6V6's as the output. >>snip<< So the output transformer is also mounted onto the basket, that's pretty wild. It might be kinda neat to bolt the transformer (and choke?) onto the basket of the new permanent magnet speaker - any caveats on doing something like that? Just tossin' that out there.

Looks familiar- I worked on one just like it 2007. OT mounted on basket was standard back in the good ol' days. Some baskets still have provision for this but not many. I wouldn't go to the trouble of drilling a speaker & take a chance/probability of having steel debris find its way into places it shouldn't be, stuck there magnetically. Either find a spot to bolt a new OT to the chassis, or mount it to wood on the base of the cab if possible, or on the side if not.

It was a nice mellow "old" sounding amp in its original form, but the owner wanted to boost it a bit & get a brighter tone. VC was scraping as you describe on the FC speaker. So, as I described above, new PM speaker, new filter choke, higher B+ voltage, higher value Rk to compensate & he got what he wanted. Bit of bad luck for me, not only did he get what he wanted but he got it free, never paid and moved 2000 miles out of town. Grr.... so much for trust.

[g1]

A couple places in GTA Ontario you could check with:

Santon Audio Online | Bootstrap-based responsive Shopify theme

Fab Audio Speaker Repairs & Speaker Reconing | Toronto, Ontario (416) 792-2171