1920s vintage speaker with a guitar amp (Impedance problem)

[bzblues]

Hello everyone!

(I apoligize in advance if I ask noobie quiestions, but I'm a beginer in this matter)

I have a 1920s vintage speaker that is in a pristine condition! I intent to use this speaker with a small powered guitar amp that I've built. The guitar amp is called "ruby" wich is a 9v powered 1watt amp and it works well with 4ohm and 8 ohm speakers.

This vintage speaker is 1000 ohms and obviously doesn't work out porpperly with the guitar amp because of the impedance. So, I started to do some research and I found out that you would have to use a "step down" or "step up" impedance transformer. Now, here is when I hit the wall because its an unknown territory for me (yet)

I found out a website that allows you to calculate the type of transformer that you would have to use, but it seems that the results are not "realistic" but still, maybe I'm missing a few things.

What would you recomend to do in this particular scenario? again, its a small guitar amp (1 watt, 9V powered, works well with a 4ohm speaker) and I want to use it with this 1920's vintage speaker that is a 1000 ohm.

Is it possible to do?

Thanks a lot in advance!

Brian.

 

[rayma]

This has been done in public address systems for many years. All you need is the right transformer,
and flip the primary and secondary. This should work well enough to get started.
https://www.parts-express.com/70V-10W-Line-Matching-Transformer-300-040

 

[Rikaro]

I suppose it's a field coil speaker, so transformer won't help.

 

[rayma]

Can't he just power the field coil from a DC power supply?

Ok, how many wires are there on the speaker, 2 or 4?

 

[bucks bunny]

"I suppose it's a field coil speaker, so transformer won't help."
No, at that time there were no moving coils speakers on the market but so called "Freischwinger" in Germany.
They had this classical horse-shoe magnet on the back.
Btw these were used in the "Volksempfänger 'RE301" as well.

 

[bucks bunny]

https://www.radiomuseum.org/forum/freischwinger.html

 

[Rikaro]

A pic of the OP's speaker would clarify.

 

[rayma]

Here is a driver with a horseshoe magnet.

 

[steveu]

As Rayma suggested, a 70V transformer is a good choice. Note that 5Watts at 70V is 980 Ohms. Some amps will require a large series capacitor to drive a transformer. 4 Ohms to 980 is a turns ratio of 15.6 and your amp may work better using the 8 Ohm to 1960 Ohms ~= 2.5Watts because the inductance is higher.

Others have mentioned field coil speakers. Some very old speakers did not use a permanent magnet. Instead of a permanent magnet, they had a second connection for an electromagnet which were also part of the tube amplifier power supply. Powering a field coil would be a huge drain on a 9V battery, ie not suitable for a portable system.

 

[bzblues]

Hello, everyone! thanks for the replys

Here are some pictures. I have 3 speakers of this kind and they all have the same type of coil and magnet. The screw in the back acts as a On/off switch I would say. You screw it and it makes contact and you can hear the speaker work.

Sorry for the messy pics, its not easy to shove the cellphone. The back lits are not possible to open except for one.

Here is more info about one of the speakers: http://earlywireless.com/celestion_c12_picinfo.htm

Cheers!

 

[Rikaro]

If 1000 Ohm is the DCR, the impedance @1kHz might be larger by a factor 5.

 

[Rikaro]

From what I've read these speakers were directly connected as plate load (no OT) to the output tube of a low power SE amp.

The impedance is mostly inductive, so rises with frequency.
If not driven by a "current source" (tube), treble response will suffer.