very old amp w/non PM speaker. Customer wants PM speaker installed.

[tim]

Hey all,
I knew eventually I'd be facing this...I had a very old lap steel guitar amplifier (National Dobro Model 75) dropped off here at the shop today and the cusomer wants me to replace the speaker that is in it with another one. The original is an electromagnetic speaker and he wants it replaced with a more modern speaker. My experience many years ago with an old Philco Radio that had the same basic setup was not fruitful. Does anyone have any experience converting a non-PM speaker to a PM type?

[Chuck H]

It's not usually that hard to retrofit a standard speaker for a field coil speaker. But if the customer wants an exact replacement you'll need to find one, repair the old one, or turn him away.

What is wrong with the one thats in there?

Chuck

[Enzo]

Look at the schematic or trace the amp out yourself. The field coil of the speaker is typically either in placce of a choke in the B+ or it is across the B+. The current through the field couil takes the place of the magnet. Voice coils are voice coils.

To replace the old speaker with a permanent magnet, you need to replace the field coil function in the circuit. I restore old jukebox amps as well as working on guitar amps, and I used to face a lot of field coil amps. I have a large 5000 ohm resistor with clip wires to put in place of most field coils. It may not be exact, but it certainly works. You should be able to install a reasonable value resistor in place of the coil. Watch the power dissipation.

[tim]

Thanks Enzo...

I'll have to trace it out...I can't seem to find any schematics for this beast but haven't looked real hard yet. He just dropped it off last night. He's a good customer and I'd hate to disappoint him.

He uses it for playing harp through. He's afraid that the speaker may eventually lunch itself because it's so old and he'd rather have the original speaker in working condition should he ever decide to sell it. I didn't promise anything but that I'd take a look at it...he had deeper problems with the amp anyway.

He said it started humming pretty bad lately. I asked if anyone had ever worked on the amp and except for a grounded power cord installed a few years back, he said no one that he's aware of has ever been in it.

The amp was made, best estimate, in the late 30's, early 40's so it's high time the caps were replaced (at least)...

[6267]

Enzo's approach was the official approach of the Hammond organ company when replacing the speakers on the older hammonds. I didn't have a big 5k, so I used a big potted milspec choke from about the same era as the hammond. That worked too. I would check all the DC voltages downstream before and after the replacement, if you are particularly concerned. I don't know of anyone that repairs or recones the old field coil speakers, but there is probably someone out there. I think, but am not sure, that the electromagnet represented a fairly large permanent magnet, so I would overrate the speaker or pick one with lotsa bass to preserve tone, your mileage may vary.

[labeau]

Tim-

I have the same amp and wonder if you remember how this was performed!

Thanks,

Mike

[tim]

Hi Mike,

I never completed the job. The speaker that the customer wanted installed wouldn't fit (the amp chassis hit the frame) and he decided not to do the switch. Fortunately he had purchased the speaker, not me, so I wasn't stuck with A, the speaker and B, making a bad choice buying it, thinking it would fit. I was kinda hoping he would take the whole thing back but now he wants me to go through it, swap out all the tubes and do a cap/resistor job. I tried to hint diplomatically that it really wasn't worth it but he insisted...I guess if he has the $$$, who am I to complain...

[labeau]

Tim-

Well, I'll probably do a general post to see if someone knows this procedure. This amp has been sitting in my basement for 5 years and I finally pulled it out. It needs a cap job as well. Thanks for the quick reply!

Mike

[Gtr_tech]

I would measure the inductance of the field coil and use a choke of similar value that'll handle the current required. The field coil on the speaker acts as a choke in the pwr supply so keeping a choke there will keep it sounding the way it should.

[Prairie Dawg]

Originally Posted by Gtr_tech

I would measure the inductance of the field coil and use a choke of similar value that'll handle the current required. The field coil on the speaker acts as a choke in the pwr supply so keeping a choke there will keep it sounding the way it should.

Those old field coil speakers suck tone. More properly, the field coil winding does, and it does it by sucking power off the B+ rail. Measure the voltage into the field coil and what the voltage is at the plates of your power tubes and you'll see exactly what I mean.