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

[Silvertone Jockey]

More thread drift, high end PM speakers with north and south double voice coils

http://www.makeitlouder.com/loudspeakers.html

[The Dude]

More thread drift, high end PM speakers with north and south double voice coils

http://www.makeitlouder.com/loudspeakers.html

Oh the car stereo kids will eat that shi+ up. You know,......the ones who think wattage = SPL...... the "mine's bigger than yours" type.

And, by the way, IMO, it's only important to have an amp that's flat to 10Hz if you're trying to mate elephants.

[The Dude]

Since we're rambling a bit here, how many of you remember or have worked on these?

[Mickey]

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!

I didn't read all the replies so your question may have been answered. I guess the first question is do you want to retain originality? If so you would be best to re-cone the speaker.
If originality is not important, buy a decent speaker of the same size and impedance and install as required. Would have to locate the output transformer in a suitable position but keep it away from the power transformer, then you will also require a filter choke which doesn't have to be to fancy as long as it can handle approximately 100mAs. I would suggest that you replace that output transformer as well as it looks a litter tired, and while you're at it replace those horrible looking paper capacitors and also the filter capacitors.
That field coil shouldn't be called a hum-bucking coil as in many old radios there was another coil on the speaker which was wired in reverse polarity which actually did hum-buck the 60Hz hum. But that is another story!
The field coil acts as a filter choke and at the same time magnetically energizes the speaker so that the voice coil of the speaker can vibrate (backwards and forwards) with the incoming audio signal producing the sound that your ears hear.
Spend some money and time on this amplifier and you will end up with a nice classic that you can actually use.
Good luck, Mickey

[waspclothes]

I didn't read all the replies so your question may have been answered. I guess the first question is do you want to retain originality? If so you would be best to re-cone the speaker.
If originality is not important, buy a decent speaker of the same size and impedance and install as required. Would have to locate the output transformer in a suitable position but keep it away from the power transformer, then you will also require a filter choke which doesn't have to be to fancy as long as it can handle approximately 100mAs. I would suggest that you replace that output transformer as well as it looks a litter tired, and while you're at it replace those horrible looking paper capacitors and also the filter capacitors.
That field coil shouldn't be called a hum-bucking coil as in many old radios there was another coil on the speaker which was wired in reverse polarity which actually did hum-buck the 60Hz hum. But that is another story!
The field coil acts as a filter choke and at the same time magnetically energizes the speaker so that the voice coil of the speaker can vibrate (backwards and forwards) with the incoming audio signal producing the sound that your ears hear.
Spend some money and time on this amplifier and you will end up with a nice classic that you can actually use.
Good luck, Mickey

Hi Mickey, sorry I thought I updated this thread.

I replaced the speaker with a Weber AlNico, I kept the original output transformer and mounted it inside the cabinet along with a new choke.

Can capacitor was replaced and film caps were replaced with Mallory's. Sounds great!

[JSB MD]

I have the same amp, but is a Gretsch Electromatic 1955. There is 1000-1300 ohms through the field coil. It is may favorite all time amp. It is Holy Grail! I am considering doing a scratch build reproduction with a magnet speaker. I would prefer using a choke instead of resisters. I cannot find one that looks right. You said you were going to use a fender deluxe style. The ones I see are 150 ohms. Which choke (exactly) did you use. Thanks

[TimmyP1955]

When Hammond moved to PM speakers, they replaced the field coil with a 250 Ohm, 20W resistor. Of course this does not mean that 250 would be the correct value for your amp.

[Chuck H]

Right. I wouldn't sweat it beyond actual tube safety and performance. The high DCR of the field coil speaker is no doubt because of it's dual purpose as an electromagnet. Since that is no longer necessary the thing to do to preserve the tone is focus on the impedance and voltage drop in the circuit. I'd bet that most guitar amp chokes will be ok for impedance and any resistance for DC voltage drop can be achieved with an additional resistor. Probably getting very close to stock performance from the amp but with the freedom to use a PM speaker.

[Enzo]

Replacing the coil with a choke will likely bring higher voltages, the circuit design was based around the resistance of the coil. I might suggest using a 1K resistor and observe the results. Then decide if you really need to added expense of a choke. WOuld the choke really add anything? On the other hand, it is just a guitar amp. It isn't going to make a ton of difference.

The speaker is the single thing in any amp that has the most effect on the sound. SO while a magnet speaker might sound great, it probably won't sound like the original field coil speaker.

I have a large 5k resistor in my tool drawer, a large ceramic one with screw terminals. I have a couple clip wires soldered to it. I have used it for decades in old jukebox amp repair as a generic replacement for speaker field coils. (I would receive amp modules, while the speaker assembly remained in the jukebox.)

[Chuck H]

I really think Enzo is right on this. I only mentioned the impedance you would get from a choke because some of my recent bench tests for design have shown it to be a significant parameter for clipping performance. But it's nitpicking on my part.