field coil wiring

[GibsonLover]

I met an old-timer over the weekend with original Gibson service manuals with my schematic of the GA-75 and no reference to the field coil at all. I'm absolutely convinced the speaker and electronics are original, but the field coil did not appear to be a part of the B+ at all, because there was a selenium rectifier, dropping resistor (125 ohms) and filter cap (40uF) attached directly to the speaker frame. There would be no need for those parts if it were connected after the B+. I'm thinking the quote from Rhodesplyr on the current field coil thread may be original wiring directly after the fuse:
"...but my five minute power supply consisted of four diodes, a resistor, a capacitor, and 80VAC from the Variac. Worked like a charm."

I cannot verify the connection because someone had been in the amp before me and the wires were disconnected. Could this connection be accurate? (connected to line voltage after the fuse, speaker frame grounded) Anyone ever see one?

This would be an obvious candidate to drop in the P15N with no other changes, and perhaps I will after hearing the F15N, but I have a strange need to restore it since it is almost there, with some minor rebuilding.

P.S. loudthud,
I have a letter from Novenber 1947 from JBL himself originally specifying 13500 Gauss for the D series. Maybe the 11000 gauss that you mentioned is where the finished product wound up. Splitting hairs here, but it was nice to see something in print backed up by the original source.

[loudthud]

I have a pdf file of Theile Small Parameters dated Nov. 3, 2008 that I downloaded from JBL. It lists the D and K series at 1.2T and the E series at 1.35T. There is no entry for the D120 or D140. Some of the numbers look like mistakes or typos. Excluding those, you don't see any numbers higher than the E series.

The point I was trying to make was that a field coil speaker may be able to break new ground in as far as efficiency. But will it sound any good? I think the problem with early neo speakers was a high Qts, something greater than 1. But it could be as Steve says, the magnet was just too strong.

[octal]

Well, the thing is with a field coil speaker, if you feel the speaker is overdamped (too low a Qts leading to thin bass) you can simply reduce the current going to the field coil and adjust the strength of the magnetic field as needed. (Of course, the efficiency will go down as you dial down the strength of the magnet....)

Nathan

ps. Loudthud, Qts will go down as the strength of the magnetic field goes up. Your reply is a little confusing. Was the magnet too strong on early neo speakers? If so, that implies that Qts was too low and the bass was overdamped.

[GibsonLover]

I don't disagree at all, only making note of how in 1947 that represented cutting edge manufacturing capability for 'new' technology of the time. Indeed, they thought he was off his rocker at first.

Back to my question however...Aside from the inherent dangers of the field coil wiring, is it possible some old guitar amps were wired directly to the mains (post fuse)? Specifically Gibson of course, because every Gibson schematic I've seen is wired to the B+ and would require an inductor/resistor to accomodate the voltage drop if replaced with a PM speaker. There may not have been enough of those amps made for enough people to have had much experience with them today. I know some of the old Fender Pro's were wired B+ to ground, and that's another animal...

[loudthud]

Back in the late 80s I wrote a BASIC program to plot a woofers' response based on TS parameters. Although a low Qts generally means F3 is well above resonance (you kind of expect this from a high efficiency speaker), it (F3) is resonance dependant. So it can be misleading to only look at one parameter. A low Qts allows the speaker to be used in a small box without a mid bass bump. When Qts is in the range of 1.0 you almost can't build the box big enough to eliminate the mid bass hump.

I remember looking at the first generation of Neo speakers offered by Eminence and thinking Qts was too high to make a bass enclosure in a size that would be marketable. The next generation had a workable Qts and an Xmax that was unheard of in a speaker of that efficiency. This seemed to contradict Steve's comments in the other thread. It could be that what applies to bass speakers, is not true for guitar speakers.

Looking at the response curves published by Eminence, it appears changing the flux density does a good job of changing SPL except in the resonance area. This might turn out to be an advantage giving a little bass boost as you turn the volume down. If someone can iron out all the kinks, they might create a must have product.

Back to the subject of this thread, it seems to me that the amp was designed to accomodate a specific field coil speaker design. With different size wire being used, some FCS's worked better with a B+ to ground connection and some as a B+ choke. In the case of the choke connection, the reduction of B+ voltage was taken into account as any designer would do to optimize the design. With the B+ to ground connection, the rectifier, filtering and B+ winding had to be sized to allow for the extra load. The speaker significantly influenced the amp design unlike today.

[octal]

Sorry for the Qts thread jack.

No, connected to the mains will not work, as it's AC. This will superimpose loud hum on your speaker output.

EDIT: I re read your post about the rectifier and the filter cap. I think you may be right about direct connection to 120AC. You could use an isolation X-mer to bring this closer to modern safety standards.

The two possibilities are connected in series with the B+ (probably between the first and second nodes of the power supply) or from B+ to ground.

What does the resistance of the field coil in question measure? What is the B+?

You may be able to estimate the current and figure out which way seems more plausible. It's also possible that someone removed a series field coil speaker and just jumpered the two connections which went to the field coil. Removing a <200 ohm resistance (just guessing) wouldn't have that huge an effect on B+.

Nathan

[Steve Conner]

I remember looking at the first generation of Neo speakers offered by Eminence and thinking Qts was too high to make a bass enclosure in a size that would be marketable. The next generation had a workable Qts and an Xmax that was unheard of in a speaker of that efficiency. This seemed to contradict Steve's comments in the other thread.

Loudthud, that is interesting. As I understand it, the stronger the magnet, the lower Qes (the "Electrical Q") becomes. But you're looking at the total Q, Qts, which is a combination of the electrical and mechanical Q, using the same formula as resistors in parallel. A super-stiff suspension could bring the total Q up a bit, but it can never be more than the smaller of the two Q figures.

It follows that as you turn down the current in a field coil, Qts will go up, and the bottom end will get boomier. Maybe that is actually good in a Fletcher-Munson kind of way.

If you have a big, strong magnet, you can use it either to increase flux density in the gap, making the motor stronger (lower Qes, higher efficiency) or to produce the same flux density in a longer gap, which gives better Xmax. I'd expect that the designer of a guitar speaker would go for the first option, and the designer of a bass speaker the second.

I'd also guess that Eminence's first neo speakers went for the first option, and their next generation the second option, if that didn't contradict Loudthud's observation of the first generation having higher Qts than the newer ones.

[GibsonLover]

The field coil is 1000 ohms. Given the dropping resistor in series, it would see around 100mA, and the winding would take about 11W to operate. Connected from B+ to ground, it would see more than three times the current, and wouldn't need it's own rectifier and filter. As a B+ choke the current would be more comperable, but again, wouldn't need it's own rectifier and filter.
Bringing it to todays safety standards would be a good idea, but given the wiring practies of the day, I still don't doubt my suspicion that it was wired to the mains. I may have to variac it up and see what happens.