field coils...again...

[pvsage]

I've recently acquired an old Jensen field coil speaker (those of you joining us from SEWatt may remember this thread there) and have just successfully tested it with my Valve Junior, using the field coil as a choke in the power supply, as I know this to be the traditional method. Speaker sounds OK, but really light on treble response. Also extremely low volume compared to my WGS Veteran - I'd say efficiency is below 90dB.

I know this speaker came out of an M-series Hammond, and have tracked down a schematic that shows the field coil running off of a separate 100V power supply. This is functionally similar to the straight-from-the-mains power supply that Ted Weber has here:
http://www.webervst.com/speakers/fcpm.htm

Any opinions on which would be better? I've already worked out a way to use a switching jack (obviously *ISOLATED FROM THE CHASSIS* with plastic nut and bushing, as there is high DC voltage here) so that an amp can use either a field coil speaker or a standard PM speaker. The field coil is 710 ohm, and from what I've read this resistance means it can be powered either way...

[Chuck H]

and have just successfully tested it with my Valve Junior, using the field coil as a choke in the power supply, as I know this to be the traditional method.

That would be alot more voltage than stock then. I don't know alot of particulars about field coil speakers, but how do you know that isn't too much voltage for the coil?

Speaker sounds OK, but really light on treble response. Also extremely low volume compared to my WGS Veteran - I'd say efficiency is below 90dB.

Then do you really want to use this speaker?

I know this speaker came out of an M-series Hammond, and have tracked down a schematic that shows the field coil running off of a separate 100V power supply. This is functionally similar to the straight-from-the-mains power supply that Ted Weber has here:
http://www.webervst.com/speakers/fcpm.htm...
Any opinions on which would be better?

No, I cannot see the original schematic.

so that an amp can use either a field coil speaker or a standard PM speaker.

Why would you want to do that? Especially if the field coil speaker you have doesn't sound very good.

I'm not trying to be antagonistic, but I found this post confusing in a few ways. I don't understand your goal.

Chuck

[pvsage]

One reason I was interested in this kind of speaker was *because* of it's lower efficiency - 5W through an efficient speaker is way too loud for my purposes (bedroom practice). I wouldn't mind more agressive highs though...in retrospect, I'd have probably been better off with a current production speaker like a MOD 8-20 - smaller, lighter, *cheaper*.

The "which would be better" question was with respect to B+ choke vs. separate 100V-to-ground supply. As I understand it, the actual voltage through the field coil when used as a choke is fairly irrelevant - supposedly it draws enough current for proper operation from the voltage drop across the coil's resistance. But I really don't know much about field coil speakers either aside from what I've been able to Google.

[Enzo]

I am not sure what voltage problem we refer to here. Using the coil as a choke does not put the full B+ across the speaker, onlt the voltage dropped across it as a choke. In other words if B+ is 350v on one end and 320 on the other, there is only 30 volts across it.

SOme speakers are used as chokes, but many are driven across the B+ at some point, which a direct supply would represent.

[pvsage]

The "Don't show it to any anti-Ted's" power supply is fairly inexpensive, especially if I only use the non-doubled part - I guess I could just build it into the cab and run an extra power cord to the wall. Considering the low voltage drop across a 710 ohm load, I suspect the field coil in my F12N (rated for 17W) is quite under-powered when used as a choke.

[loudthud]

Just to experiment, I'd try a variable bench power supply. You should be able to adjust the sensitivity of the speaker to bedroom levels.

Why is it necessary to isolate the speaker jack from ground? Don't you need atleast one connection to ground? It would be awfully dangerous to have the outside shell of a phone jack at anything other than ground because metal shells are so popular.

Ted's power supply would be a lot safer if you used an isolation transformer. At 17 watts it wouldn't have to be very big.

[pvsage]

The isolated jack would have been for the field coil "choke", not the voice coil. As both send and return for the field coil would be above ground potential, this must be isolated from ground. I'd have preferred to use a different kind of connector - preferably something like the old IEC7/8 connectors - but I couldn't find a switched version online. This is now a moot point as I have decided to build a separate field coil supply into the speaker cab.

A variable power supply is a great idea; a bench supply not-so-much, as I don't have one on hand. Does this look like a good variable voltage regulator if I wanted to include a field coil strength control in the built-in power supply?
http://focus.ti.com/lit/ds/symlink/tl783.pdf

Yeah, Ted apparently gets a lot of flak for his line operated power supply suggestions, hence his disclaimer about showing it to other people.

Since I want to limit my *maximum* voltage to about 110V, I might be better off if I can find a transformer with a 75-80V secondary...or maybe a mismatched pair of wall warts to get me close to that ratio...maybe 9V and 12V connected back-to-back?

[loudthud]

The problem with the TI chip is the 125V breakdown. Hard to guarantee that the chip won't see more than that during turn on and the 10V or so minimum in to out voltage is another problem.

Back to back transformers can work but the losses can be more than expected. Wall wart transformers don't have very good voltage regulation. 36V transformers are pretty common so you could use two in series or a voltage doubler to get near 100V. Another solution is to find a transformer of the correct VA rating (allow 20% over to be conservative) with a nylon bobbin and re-wind the secondary.

[pvsage]

Ted told me that reducing the field coil supply reduces the voice coil's power capacity. He also said he de-rates vintage speakers' capacity by about 10% per decade. Assuming the N in F12N means the same as it does in Jensen's alnicos (50W voice coil), I'm not sure how much of an issue this would be with a piddly little SE 6V6 output stage...

EDIT: Ted just informed me that the F12N was a 16-18W speaker.