Hi All, sorry but I have another question. The dynamic speaker on my 1941 - 5 valve Music Master 'Wagner' is shot, and the output transformer is open circuit. I don't have any info on what the output transformer impedance should be or the winding ratio.

I have ordered a replacement output transformer from RS components (1237241) which has a primary rated at 5000 Ohm. The outputs tappings are 4 and 8 Ohm.

The speaker field coil is OK and reads 1820 Ohm DC resistance. It has written in pencil "2000" on it so maybe 2000 Ohm impedance?

The output valve is a 6V6G, the transformer is 355V either side of the centre tap. The rectifier is a 5Y3.

The specs for the 6V6G say the Load Resistance 180V is 5500 Ohms and 250V is 5000 Ohms.

My plan is to replace the field coil with a resistor and wire the new output transformer in series after it, then run a permag speaker from that. I may need to increase the filter caps to counteract the hum.

I know I need to calculate the input/output impedance of the transformer to suit the 6V6G and the speaker when I get the transformer and determine the turns ratio.

Am I on the right track? What value resistor should I start with? The resistor won't have any reluctance (I think) so do I start with a 2k Ohm to match the impedance of the field coil?

Thanks for any help

A small choke would be better to replace the field coil.
Virtually any speaker transformer would work OK with a 6V6, 5000/3.5 ideal.
Do not over think the problem.
JJ

Haha... thanks Johnny, maybe I am overthinking it

Agreed a choke would be better but I don't have any and unsure where to get one or what value.

A stash of old wrecked radios is what you need for small parts.
For a small choke you could use the primary winding of any old surplus plugpack as supplied with most equipment these days.
As long as it has a transformer, and not switch mode.
JJ

Fully agree with using a choke.

The field coil in dynamic speakers often doubled up as the inductor of the Pi filter network in the B+ rail.

Sorry kept getting Error 500 trying to log in to this website. Thanks everyone, I have taken your suggestions onboard and will try and source a choke or transformer. A little experimenting is in order I think. Thanks for your suggestions!

Is this a table or a console? With a console there are ways of recycling the field coil.

Firefox & this site tend to be a marriage made in hell & its probably Firefox as IE seems not to have the same issue & I have had issues with Firefox on other sites.

Error 500 is a common thing with this site, and I can instigate it any time by clicking on a link as soon as the page appears, instead of waiting 5 seconds.

It sounds like you bought the correct transformer. Just hook it up to the permag speaker.

The choke is a good idea and it is much more efficient than a high-power resistor. It drops less volts and runs cooler.

Good luck !

I tried Chrome and IE with no luck yesterday. 5 second delay is interesting through.
Marcc... I was wondering if I could use the old field coil and was going to check it out when I got home. It's a console model, how do you suggest I tackle it?

Tippy, the 500 error isn't an issue with any browsers, it is a fault with the forum software that I am not able to diagnose. The site is due for replacement, when I can get the time to finish testing alternatives.

Thanks Brad, we appreciate what you do, hopefully an upgrade will fix the issue.

Marcc, I checked the field coil and it's pretty much mountable using the bracket that attachs it to the back of the speaker. I can probably mount it near the new speaker.

The entire speaker can either be left intact, if there is room and put elsewhere (floor of cabinet: Floor of chassis plate, perhaps under it?)

OR One removes the coil assy from the frame, put a piece of iron (flat) across its face and finds a convenient place to mount it. The VC secondary of the OP transformer is disconnected & transferred to the permanent magnet speaker.

Yep, got it, thanks Marrc

Wait... are you saying not to use the pole through the centre of the field coil. This one has a hat shaped bracket with the core or pole attached and the field coil fits onto it. I was going to put some flat iron across the bottom of the hat shape and attach it to the cabinet or somewhere suitable.

If you remove the field, it is the end where the voice coil was and the mounting bolts, that needs the flat iron plate across it

Great, thanks