Field Coil Speakers

[Gingertube]

I did a resurrection job on an old amp - triode strapped 6J7, to pentode mode 6J7 to single ended 6V6, 80 rectifier - see schematic below.
It was drop dead gorgeous and i want to build a clone.
My question is how much of the lovely compression of that amp might be attributable to the 10" field coil speaker.
I have collected 5 identical field coil speakers (Rola K) and one 12" field coil speaker. I also have a Jensen 10".

Should I "design in" the field coil speaker or just go with the Jensen?

Thanks,
Ian

Edit: A better copy of the schematic is here:
http://www.diyaudio.com/forums/instr...amplifier.html

[R.G.]

I don't know - but I'd very much like to. You're in a unique position to find out.

Would it be possible for you to measure the bulk inductance of a field coil speaker, then try the amp with the field coil and then with the non-field-coil and a similar inductance in the power supply where the field coil was?

If it were me doing it (and if I had the time and instrumentation... which is probably a problem for you as well), I'd get a representative recording to play into the input, then take .wav files of the speaker electrical output both ways, and microphone recordings of the audio output both ways. That would let you visually and audibly compare the effect of the inductance on the power supply operation (i.e. the speaker electrical output) and on the speaker's audio output to see if the magnetic effect changed the audio output at the speaker.

[mhuss]

I love the tone control set between the output tube anode and screen. The screen side is virtual ground; I wonder if this was done so they could use lower voltage caps? I'm not a fan of having a user control with n-hundred volts on it!

[pdf64]

I think that the resistance of the field coil may have a significant effect on the B+ sag.

[mhuss]

Agreed, but wouldn't that be easy to substitute with a like power iesistor?

[pdf64]

Sure, just thought to mention it, to avoid it being overlooked.
Perhaps not on this amp, but for class AB field coil amps, would heavy B+ current draw at high signal levels result in a stronger mag field on the speaker, possibly resulting in a slight expander effect?

[Enzo]

Probably not as much as you'd think.

This being a single ended amp, and running class A, there won;t be much of any sag.

I worked for years on old Seeburg tube jukebox amps, and earlier ones used a field coil speaker. I have a large power resistor with clip wires mounted to it in my drawer just to clip in as sub for the field coil when the amp was on the bench. Works just fine.

[Gingertube]

Comments on above.
Yes that tone control by feedback around the output tube is quite different - it worked wonderfully - BUT I had to add a 15K grid stop on the 6V6 (not shown on schematic) to keep it stable at all volume levels.

My thinking which prompted the original question is that some guys prefer alnico magnet speakers over ceramic magnets. The difference there is magnet strength, Alnico magnets are weaker than Ceramic, so was thinking that field coil speakers may be weaker again in the "magnet" department, imparting some of the compression I'm hearing.

Amp stuff is very slow at my place right now - doing a kitchen renovation on the house and have a Hiwatt400 to retube and a Fender SuperSonic to delete someone elses modification (that is, to put back to original) before I can tackle the new project.
I will report on Filed Coil Rola K vs the Jensen when done.

Cheers,
Ian

[J M Fahey]

Try it

And you tell us

Personally don't agree on the theory that "alnico compresses" but *alnicos were used in old , simpler amps" and old simpler amps:
1) in general were lower powered
2) alnicos have lower magnetic flux and are less efficient
3) so amps were used closer to the limit, so **power tubes** were used in the compression area way more than today.
Of course the simplistic (incomplete) observation is : "yesterday I played with an old amp, it compressed a lot, it was loaded with alnicos".
FWIW the observation might have been: "yesterday I played with an old amp, it compressed a lot, it was covered in Tweed".

And what kind of flux will these field coil have?
Who knows?
The better ones, think the heavier Jensens found in Hammonds and such, had incredibly high magnetic flux (and impressive efficiency), only found today in, say, JBL, Altec or EV speakers.
Now, a mid priced one?
Who knows? Only testing.

[Enzo]

yes, I have no reason to think field coil speakers are "weaker" than permanent magnets. They use electromagnet to pick up scrap metal in junkyards. A coil magnet is as strong as you want it to be.

[Mick Bailey]

I've seen a modern version of the field coil speaker with a variable DC supply to the electromagnet. That would seem to be a nice way of testing the effects of magnet strength on sound.

Edit: Fluxtone is the one that springs to mind

[J M Fahey]

yes, I have no reason to think field coil speakers are "weaker" than permanent magnets. They use electromagnet to pick up scrap metal in junkyards. A coil magnet is as strong as you want it to be.

You bet.
Some 25 years ago I was part of an (economically) sick venture.
4 or 5 friends got together to make 30" woofers, then rent/lease (not sell) those wall shakers to Discos and become rich.

No big enough magnet was available and even if it were, there was no way to magnetize it, so Plan B was to use huge electromagnets.

Yours truly designed it and the power supply, a cone maker made the dies (no 30" cones available) , the voice coil maker supplied the huge custom coils, another took care of the sand cast aluminum frame, plus necessary machining (sandcasting has from 2 to 5% metal contraction and it's not uniform) , etc.

Sound was truly wall shaking, but Disco owners are the worst customers in the World, period.

As bad as receiving me with a .38 Special over the table and asking me to pick which bullet would I prefer if I ever dared to set foot there again, and other niceties.
Keeping the speakers, of course.

And when I went to the Police station, the Chief told me: "so you are bothering MY partner? ... I own 50% of that Disco ... get out of my sight, you scum!!!!"

Wonder why I much prefer to deal with a drunk/stoned/annoying/"star" Musician customer any day of the week?

[pdf64]

Personally don't agree on the theory that "alnico compresses"

Generally yes, or if they do it's a very subtle effect, and it may be that it is designed not kick in until near or beyond the rated power.
But the Fane AXA12 that I tried (2 units) seem to overdrive at quite a low SPL, such that they just can't get very loud, even though they seem reasonably sensitive at low power levels.
Used singly with a Fender 75, it seems to occur at ~20 watts (guessimate). Swap to a Gold and the SPL was way louder (the Fender 75 seems to depend on Sylvania STR387 to achieve its rated power; with modern 6L6 it's maybe 50 watts, up to 60 watts with Winded=C=, so the 50 watt rating of the Gold wasn't going to be exceeded much).
All I can ascribe the seemingly extreme AXA12 SPL limiting to is that its Alnico magnet circuit saturates?

Back on topic, FluxTone's EVM12L field coil assembly (speaker rated at 100 watts) seems rather larger than that of their other models, even the 75 watt models, see http://fluxtonespeakers.com/product-list.html
My Model 10 is really good.

[Enzo]

EV made the 30W, I have seen exactly one in my career.

University Sound 30 INCH WOOFER 30" Electrovoice EV 30W PATRICIAN Woofer Cabinet


Disco stole the speaker? pay some kid to stomp a hole in the cone. Then he can see about fixing it.

I was foolish enough to extend credit to a customer who just HAD to have an amp repaired for tonight. He then refused to ever come back to pay me. Silly man, a year later, it needed some form of service, and he called me to fix it. I said "Sure, bring it right in." He did. Now he has the choice of paying for the first repair or replacing his amp, because I won't be handing it back unpaid again.

[Gingertube]

I tracked down the original of that circuit, It is a "PA-5" published in the June 1942 "Radio and Hobbies" magazine here in Oz.

Here is the original schematic - use the mic input for guitar and you can delete the "PU" Input. Was stunning with my Fender Tele. Highly recommend that tone control via feedback around the output tube, works "a treat".

Resistor Values with decimal points are Meg Ohms, Caps with decimal points are uF.

The 6V6G needs a grid stop, I used 15K.

Cheers,
Ian