OT/field coil?

[shooter]

I have an OT from a Hammond M3, S/n matches up using a field-coil spkr.  The primary is 2 6V6's, secondary, schematic shows 8ohm speaker with 700ohm F.C.  Can I use that OT with 8ohm PM speaker?

[pompeiisneaks]

I just built an amp that called for a FC speaker (Gibson EH-185) and replaced the 750 Ohm rated FC with a 750 resistor, works fine.  You could also try a choke about that range as well, but it definitely works to replace the FC with either.  Or am I misunderstanding your question?

The transformer shouldn't care, it's just delivering the juice through either, and any device in that range should effectively be about the same 'load' to the circuit I believe.

~Phil

[HotBluePlates]

I have an OT ...  The primary is 2 6V6's, secondary, schematic shows 8ohm speaker ...  Can I use that OT with 8ohm PM speaker?

Based on the above, yes, absolutely.

If you were trying to re-use the PT & power supply, as well as output stage from that Hammond, then you'd be looking to a choke or something else to fill the role the field coil previously played in the power supply.

Yes, the field coil is to set up a magnetic field for the old speaker, but its shortcomings (high series resistance, needing to be placed in the power supply for continuous current-feed) were incorporated in the power supply where some use was made of them. When you ditch only the field coil speaker in such an amp, the power supply needs either a field replacement, or supply-design re-work.

Since you seem to be re-using only the OT, none of that applies to you.

[jojokeo]

I'm a little confused on the field coil speaker situation. Some older Sears & maybe Maggie's have a field coil speaker in them. If one would like to use a modern type of speaker how is it done without changing the circuit? Or would it simply be better to replace the OT? I think I need a circuit drawn or schematic to look at to see the hookups and impedances/resistances to get a mental image of what's being explained? Also what would a speaker's impedance be without the field coil part?

[sluckey]

The field coil is not part of the OT. It's just a coil of wire wrapped around a piece of iron to make an electro magnet. It has to be mounted on the speaker so it is an integral part of the speaker. On a schematic it is often just shown as a coil in the power supply (looks like a choke) and often serves double duty as a choke. Other times it's connected between B+ and ground.

Sometimes an old FC speaker will have the OT mounted on the speaker frame also. In this case B+ often connects to one side of the FC and the other side of the FC supplies voltage to the OT.

[shooter]

none of that applies to you

Ya, just using the OT in a totally different  PP 6V6 amp.

Based on the above, yes, absolutely.

Great, so to clear up my brain, F.C. windings are  a separate beast from speaker impedance?

[shooter]

The field coil is not part of the OT.

Thanks you answered it before I asked!

[jojokeo]

It's just a coil of wire wrapped around a piece of iron to make an electro magnet. It has to be mounted on the speaker so it is an integral part of the speaker.

So the electro magnet that's created via the voltage is how speakers were created before alnico & ceramic magnets? Can you simply replace the old field coil & speaker with a modern one after a slight re-wire or using a choke or power resistor? So how would a replacement/modern speaker be wired up? In series with the speaker? Does the field coil speaker have speaker terminals? Sorry, I still need a schematic or drawing to look at the wiring?

[sluckey]

Look at page 3 of this pdf to see a field coil in an amp I have. It's in the power supply. Then look at page 4 to see the resistor I used to replace the field coil.

     http://sluckeyamps.com/rocky/rocky.pdf

And here's a pic. The huge resistor on the left side of the chassis is the 5000Ω/30W resistor that I used to replace the field coil. This is a 1954 Rock Ola juke box amp that I converted for guitar use.

[jojokeo]

Thank you Steve

[TIMBO]

Hey shooter, A bit info here http://el34world.com/Forum/index.php?topic=19150.0
I could have replaced the F/C speaker (2ohm) but this old amp had to have one.
Sounds awesome.

[shooter]

Thanks Timbo, I'm just using the OT in a completely different amp with NO F.C. worries.

So am I in the right room saying the magnetic field sets up a static condition and the voice-coil provides the dynamic change to move the cone in/out?   

(now I'm curious to see what a speakers magnetic strength is and what the corresponding Lamaur(sp) freq is ) 

[PRR]

> Lamaur(sp) freq

Are you an MRI guy??

FYI: "Larmor frequency"

> what a speakers magnetic strength

5,000-10,000 Gauss (to 20KG in the heaviest PA horn drivers).

It is very hard for any iron-core electromagnet to go over 10KG, so this will generally be the limit for practical structures. (The 20KG drivers are under intense demand for efficiency and bandwidth, and the gap volume is not that big, so extreme measures are warranted.)

I don't know that that is in Tesla.

A voice-coil gap is too small for my recent back MRI. Couldn't get a hangnail in there.

It appears the Larmor frequency for most biologic isotopes will be far-far above the audio band.

[shooter]

Are you an MRI guy

yup, 28yrs as a field service *engineer*  been out for 3 years and all my tools are still REALLY magnetic   And the touch screens, I just *fly* my hand over the screen and all kinda things start poppin up!!!  I think *we* as a group might be in the Madame Currie realm, "hell, we never expected that!"

I was just curious if the math worked at that low a gauss level and what the freq would actually be.

[Paul1453]

So you have been permanently magnetized being around those magnetic fields?   

I recently noticed that all my tools seem to have become magnetized after I started working on tube amps.   

[pullshocks]

This thread reminded me I have one of those M3 amps on the shelf.  It is dated 1951.  When I pulled the amp I remember it had the field coil speaker.

I never pursued building anything with it because the power transformer is 385-385, too high for a 6v6 amp without the field coil.  According to the schematic the field coil is 700 ohms. 

From the above posts, it sounds like a 700 ohm resistor from the center tap to ground would do the trick?  Am I understanding this right?

I have some of the chassis mount wire wound resistors on hand, I have a 650 ohm 30 watt, or I could do 2 348 ohm 30 watt in series.  Is 30 watts adequate?

[sluckey]

The schematic shows 100V across the 700Ω field coil. 100²/700=14.3 watts. Double that for safety. So a 30 watt resistor will work nicely. Give it plenty of breathing room.

[PRR]

> resistor from the center tap to ground would do the trick?

No. Put it between 1st and 2nd filter caps, just like the original.

Resistor in the spike-wave off the CT acts very different than resistor in the near-smooth DC from 1st to 2nd cap. And also gives far better filtering.

[sluckey]

No. Put it between 1st and 2nd filter caps, just like the original.

In the original AO-29 the field coil is between the CT and ground. The pulsing -100vdc is filtered and dropped to provide -22v to the pedal amp and -19v for fixed bias at the 6V6 grids.

[pullshocks]

Here is a snip of the schematic.  Looks like it includes components to get a variety of voltage taps that are not needed in a guitar amp.

[pullshocks]

Oops, did not see Sluckey already posted the schematic.

[sluckey]

If the point of using the 700Ω resistor is solely to drop the B+, then putting it between the rectifier cathode and R93 would accomplish about the same thing and allow you to connect the CT directly to ground, simplifying the schematic a bit.

[shooter]

as long as we're dissecting the M3, I have a couple questions on the percussion section;

Is T5 t6 working like a splitter, combiner circuit?

could T5 or T6 be used for inter-stage PI?, My thought is somewhere in the pre, split the signal, add equal post PI gain, then drive the PA.

[PRR]

> field coil is between the CT and ground.

Odd, but my point is there. First Capacitor is on the CT, FC goes from there to ground.

FC current is filtered DC, as it must be.

A dummy resistor does not "need" DC, but if in an unfiltered path it must be a very different value.