Ampeg (?) 15" bass speaker 5815026

Can you post a photo somewhere?

My first bass amp had a 40W valve push-pull output stage, home-built by an EE friend of my father as a (mono) sound system sometime in the '50s. It drove a 15" speaker and a mid-range horn in a beautifully constructed case the size of a washing machine. The speaker had a powered field coil!

The whole thing was terribly inefficient by today's standards. If yours is anything like it I recommend you chuck it and buy a driver that has modern magnetics.

Also, Phil Allison can probably tell you more about your Ampeg. His knowledge of such things is legendary.

Clifford Heath.

Those old powered magnets worked fine, but if the speaker is bad and needs to be replaced, that magnet usually also served as a choke in the power supply. This means you cant just eliminate it. You have to either keep the speaker magnet connected (but not the voice coil), or install a choke where the speaker coil was connected.

Installing a choke across those two wires is not difficult, but I am not sure how to determine the choke's size (capacity). Maybe someone else here will know how to do that. I personally never changed one, but I saw another guy do it (many years ago), and he explained why. He did not explain how to get the right size choke though and back then, I did not bother to ask. I dont know how critical this is..... maybe it's not critical at all, as long as the choke can handle the voltage and amperage.

Then again, maybe all those speakers had the same magnet/choke rating.

Any speaker can be reconed too, as long as the frame is not distorted.

I guess in your hurry to post, you missed the part where John said it had a large _magnet_.

--
Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com

Mine was an electromagnet... which is still a magnet. But I didn't suggest that his was. Just that magnets are so much better now, which means efficiency is.

Also... as a filter in the power supply? Because yeah, the one place you really want all that filtered-out

100Hz energy is in your speaker :) Or perhaps it was arranged so that enough got through to the amp to cancel it out? Crazy stuff anyhow.

Clifford Heath.

---------------------

** 1960s Ampegs were not sold in Australia, we had a thriving industry in home grown guitar and bass amps back then.

The model in question may well be an Ampeg B15N using a B15NC cabinet fitted with a 15inch Jensen or CTS speaker with square ceramic magnet.

See pics and details here:

.... Phil

Electro-dynamic speakers had the filed coil which doubled as the power supply filter choke. Alon with the voice coil, there was a hum-bucking coil that canceled the residual hum from the field winding.

Several of the vintage radios I have, have electro- dynamic speakers.

--
Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com

-------------------------

** See link for discussion and details of ED speaker background hum.

..... Phil

Interesting, thanks Phil. I figured there must be some hum-bucking.

so this is interesting and I never thought about it before.

Any ripple in the field winding acts as a MULTIPLICATIVE or MODULATION hum.

What I mean is....consider that there is no audio for a moment applied to m oving voice coil. Ripple in the field winding would not then create any au dible hum because there is no second field and no force applied. The one f ield is modulated at 120 Hz but there is no second field from the voice coi l. When audio is applied to the voice coil, the modulating field would add AM modulation to the audio.

I imagine it would not sound the same as an ordinary hum.

Anybody actually heard the hum from one of these speakers?

Mark

m.

moving voice coil. Ripple in the field winding would not then create any audible hum because there is no second field and no force applied. The one field is modulated at 120 Hz but there is no second field from the voice c oil. When audio is applied to the voice coil, the modulating field would a dd AM modulation to the audio.

Yes, and have deliberately produced lf modulation in transistor amps to giv e them a richer sound. 50/100Hz modulation is largely responsible for the w arm sound of old valve radios.

NT

-------------------------

m.

moving voice coil. Ripple in the field winding would not then create any audible hum because there is no second field and no force applied. The one field is modulated at 120 Hz but there is no second field from the voice c oil. When audio is applied to the voice coil, the modulating field would a dd AM modulation to the audio.

** There is background hum caused by injection of field coil ripple current INTO the voice coil of the same speaker. If the VC were left unconnected, there is no circuit and so no hum.

When the VC is connected to the OP tranny, hum is audible from this source PLUS residual from the ripple on the supply rail that is not rejected by a singl e ended tube stage.

Modulation of the audio because of varying filed strength in the magnetic g ap is small, since the iron parts are pushed well into saturation.

.... Phil

good point, thanks

ok, thank you

m