Mailbag: Can An Antique Permanent Magnet Speaker Catch Fire?

Here’s a question I received recently that I thought could use a Mailbag post:

Can an antique permanent magnet speaker catch fire?


Interesting question. Short answer? Maybe. Longer answer? Well:

It really depends what the source of the fire is. Pretty much all speakers are made of some flammable parts like plastic or nylon wire lead insulation, a paper or plastic cone, and potentially some other fabric. Something would catch fire and burn if you held a match to it, but I doubt that’s the kind of failure you’re talking about.

A permanent magnet speaker lacks the high-current, high-voltage field coil sinking 50W+ of heat directly into the frame so there’s much less to go wrong. Such speakers are typically made of a metal frame, thin carefully wound voice coil, and the voice coil cover, cone, dust cap, and surround. Some models might also have an output transformer mounted to them if they’re very old. A failure in the output transformer might heat up and release some smoke, but the winding would likely go open and cut the power before anything really bad happened.

The voice coil itself, similarly, is unlikely to heat up enough to catch the cone and insulation on fire. Nearly impossible for most kinds of speakers you’re likely to encounter. The voice coil, while it does carry some current, needs to be light and flexible enough to respond quickly to audio and move the cone. Even in the event of a major failure that put a high voltage with lots of available current onto the voice coil,, such as a primary-to-secondary short in an output transformer or a shorted output transistor, it’s much more likely the voice coil would blow out and go open in a single loud “whomp” of the speaker before anything put off a lot of heat.

A vintage cabinet speaker with a crossover network is similarly unlikely to set a speaker driver on fire, although there are a few more possible failures in the crossover network itself. Poor ventilation, incorrect parts selection, or both can cause crossover components to heat up and potentially burn. It’s very unlikely to be a problem with any quality speakers, but speakers which haven’t been refurbished and have dead capacitors in the crossover network – or are being driven far over their power ratings – could fault in a way that releases some smoke and potentially ignite if the fault isn’t caught. It’s happened before. (Don’t leave your stereo playing if you’re not there to listen to it!)

So, in conclusion: an antique permanent magnet speaker is very unlikely to catch on fire, but you should follow safe handling practices for antique and vintage electronics anyway, as failures on older gear can be a lot more dramatic than they are today.

Speaking of vintage speakers, if you do need your crossovers repaired in a vintage speaker, Rain City Audio can take care of it for you.

1936 Grunow 566 Repair Finished! [Part 2]

I started this radio a few days ago in Part 1. I left off with the radio up on the bench ready for service. It had a lot of issues with the power supply – and as I discovered while working, some shipping damage: the volume control wires, and one wire from the power switch, had snapped. There was a fair amount of bad workmanship in the radio everywhere which contributed to that happening…you can’t expect much from a 50-year-old cold solder joint after all. I repaired it back to schematic along the way.

You can see some of the damage here where a 150 Ohm flex resistor broke. This could have been shipping or age but the combination sure didn’t help it.

Grunow 566 150R Broken Resistor

I went to the Mike and Key Electronics Show and Flea Market today down in Puyallup to replenish my stock of parts and hopefully find some new ones…actually, I’m pretty disappointed. I had a very long list of parts I was hoping to find, the sort of things that are in everyone with a real workshop’s stockpiles. Quite a few had some elsewhere, or didn’t think to bring any, but I ended up not finding anything. I did pretty well though getting four front end sets (6A7 6D6 75) and four of each #41 and #42 output tubes, as well as a bunch more of various numbers mostly from $1 working-pulls bins. I don’t have any photos inside, but here’s one of the line to get in.

Mike and Key 2012 Electronics Show and Flea Market Entry Line

I pulled out quite a bit from the radio.

Grunow 566 Removed Bad Components

And for the first power-up.

Grunow 566 First Powerup Photo - Outside of Case

Grunow 566 First Powerup - Broad View

I wish I could say it worked at this point, but it didn’t. I took the tubes out to test and found a 6A7 that tested just marginal but with no other issues. This really should not have been a show-stopping issue, but it seems it was as when I replaced it with one of the tubes I’d picked up earlier in the morning it fired right up.

Grunow 566 Repair 6A7 Bad

Another shot for fun of the bottom:

Grunow 566 Underside Chassis Shot - Finished

At one point in the radio’s past, the IF transformers were replaced with service parts – in this case with Meissner universal coils. They work just fine and I’d really like to know what happened to the radio to need both replaced. It’s the first radio I’ve worked on which was in the shop so many times for what looks like major surgery.

Grunow 566 Meissner IF Transformer Replacement

Time for an IF alignment. I decided to broadly tune these for a little better fidelity at the expense of some selectivity which was going to be bad anyway. I tuned the secondaries to 465kHz and the primaries to 455kHz. This should give about 10kHz of bandwidth to pass as much of the signal as possible.

Grunow 566 IF Alignment with Eico 324 Signal Generator

With that, I reassembled it and took some photos in front of a makeshift backdrop made out of cardboard boxes.

Grunow 566 Front Complete 1

Grunow 566 Complete Front 2

Grunow 566 Complete Front Top 3/4

This one came out with an interesting camera glitch, but it’s below the radio itself so I’m leaving it.

Grunow 566 Waterfall Side

Grunow 566 Rear Complete 1

This one needed some interesting troubleshooting to get it to work but most of the issues were things I spotted while looking it over initially, so I didn’t waste a lot of time on it. The radio has a really big speaker and is very powerful and sounds as good as it can sound so I’m glad I broadly tuned the IFs. This was a fun project with the bit of detective work involved in fixing mistakes.

Grunow 566 Finished








Next Project: 1936 Grunow 566 Radio Repair [Part 1]

I’m off to a good start for my March projects, fixing up the Grunow 588. Next up is my Grunow 566 from the year before, from 1936. I picked this one up from eBay in shockingly good original condition – it looks almost brand new. Digging into it, though, it turns out I have a bit of work ahead of me. Someone’s been in this one – several times – and has made several modifications of somewhat questionable workmanship, to put it nicely.

This one is an even simpler radio than the model 588; it’s still a superhet but this model has no AGC (automatic gain control) on the front end. The tube line-up is 6A7 6D6 76 41 80. Most of the other radios I’ve serviced have a #75 in the 3rd position (a double-diode + triode tube); the #76 is a single triode only. The lack of the two diodes accounts for the lack of AGC. This does make it a simpler circuit to work on, though, as there are several fewer capacitors and resistors that would otherwise be on the AGC line.

The chassis is very clean and both tubes that need shields have the originals present. Finding missing tube shields is a real pain. The chassis tags are in good shape too, this was the only model to use the chassis 5-S:

Pulling the chassis out, it’s easy to see what I mean about it being messed with before. The electrolytic cap which usually clamps to the top of the chassis is missing to the left of the front coil:

There’s also something about that speaker…it’s been replaced! The original, correct, speaker for this model is an electrodynamic speaker with a field coil. Field coil speakers were used before materials science advanced to the point of giving us reliable and powerful permanent magnets, where the magnetic field for the speaker was powered by a high voltage passing through a large coil. Electrodynamic speakers required four wires to operate (two for the field coil, two for the voice coil). This one only has two!

Someone replaced the speaker with a rather large permanent magnet speaker. The original speaker had a hum-reduction coil which doesn’t exist on this speaker, so it’s been eliminated as well. Granted, this one might actually sound a little better than original, but it’s a big change. Swapping a field coil speaker for a permanent magnet speaker also leaves the issue of the field coil, which is a part of the power supply circuit and is necessary for the radio to work.

The previous owner replaced the field coil with what looks to be a much-too-small resistor in its place:

I’ll have to check the tube datasheets to see what kind of current they’re drawing, but that may need to be replaced for safety and reliability. Even worse than these mods, though, is what happened with the power supply. Parts failed a lot back then, it was just a fact of life. When the filter caps died the first time, the external filter was replaced with one under the chassis:

When this cap failed, it was replaced by two more caps – put in the circuit across the old caps, without removing them.

When one of those caps started failing, it was replaced by another also just tacked in across the failed point. And arriving with one end broken off.

I’m somewhat curious what “Semi-Polarized” means, but suspect it’s just a quirk of older manufacturing methods. Regardless, though: this radio which is supposed to have only two electrolytic filter capacitors currently has five installed. I’ll be cutting them all out and sanity checking the entire power supply.

The age of the parts used in these repairs leads me to believe it was modified like this sometime in the late 1940s up through the ’50s, which makes it somewhat authentic if not original. I’m going to fix it up as well as can be but don’t plan on reversing the speaker-swap and we’ll see how it goes. Stay tuned!

This is kind of novel… [Meta]

In addition to writing once in a while, I read a lot of blogs. They’re about a quarter political, a good bunch of news, about a quarter are photo feeds, and the rest are life interest and hobbies.

One of the more interesting ones is “Art of Manliness” which runs all manner of short and frequently humorous how-tos on topics like shaving with a straight razor, catching a horse, fixing things on your car, you get the picture. They recently put up a great article about re-purposing a broken antique radio into an external speaker for an MP3 player.

When I encounter a broken antique radio, my first instinct is to fix it up and add an input for the iPod but sometimes they’re just too far gone to save or aren’t valuable enough to spend a dozen or more hours repairing. In that case, tapping into the radio’s volume control and re-using its existing speaker is a good alternative and is usually a reversible modification. A lot of purists might complain about ruining an antique to make this repair, but it’s a reversible modification and let’s face it – fixing the radio up well enough to receive a signal and then using an AM transmitter isn’t going to sound nearly as good most of the time, anyway.

Around WW2, they changed how antique radio speakers work. Before then, speakers were electrodynamic using a field coil instead of a magnet. Since they have no magnetic field if they’re not fully powered by a very high voltage, they won’t play sound – you need a permanent magnet speaker. I mentioned this to the author and he updated the copy of the page to reflect this important information that might have resulted in a lot of disappointment for someone who used the wrong type without knowing:

Important Note: Commenter J.W. Koebel brought to our attention that if you want to use the radio’s original speaker like we do in this project , the speaker needs to be a permanent magnet speaker. Radios from about the mid-1940s and on should have permanent magnet speakers. Earlier radios used electrodynamic speakers. Our amp won’t work with electrodynamic speakers.

How do you know if your old-time radio has permanent magnet speakers? Check the back of the speaker. If it has 2 or 3 wires going to the speaker, it’s a permanent magnet speaker.


Better-known gadget blog Lifehacker picked up the story, and devoted about 1/4 of their summary article’s copy to that same warning.

Two caveats: Make sure your vintage radio is not terribly valuable before you take it apart and also make sure the speaker in the old radio is a permanent magnetic speaker and not an earlier electrodynamic speaker that won’t work with the new amp. If 2 or 3 wires are connected to the speaker, it’s a permanent magnetic speaker.


That’s pretty cool. I didn’t think much of it at the time, but I’m glad some of my advice will help fellow hobbyists have a successful project. (Also, this is my blog’s 100th post!)