Field coil questions

[Scrounger]

Hi guys! I'm new to the forum. I recently bout a Wurlitzer amp to use as a foundation for a 6L6 push-pull guitar amp. I have done some reading and found out that to replace/substitute for the field coil in the speaker system that I could install a 6k ohm, wire wound, high wattage, resister in series with B+ in the circuit.

As I understand it, the engineers used the field coil as a filter/choke, so it has some DC resistance that has to be accounted for if I want to use a permanent magnet speaker (which I do).

My questions are as follows:

If the wire wound resister in place of the field coil gets very hot and uses quite a bit of current, did the field coil do so also? If not, why not?

I read a post yesterday that a guy rebuilt a field speaker for an old 6L6 and when he unwound the wire in the field coil it was about 44-47 AWG. The current carrying ability of a wire that thin is very small, certainly alot smaller than the wire wound resister.

The field coil is seeing DC voltage right? If so how can it hold up to 500 volts over 6k ohms with such a thin wire?

What am I missing?

Thanks,
Scrounger

[Eli Duttman]

Scrounger,

You replace the field coil in the PSU filter with a choke/resistor combo in series. Measure the DCR of the field coil. Subtract the DCR of the choke you select to get the necessary resistance value.

BTW, there a caps. on both sides of the field coil or its replacement. Those caps. charge up and the potential difference across the "element" is only a few, not 500 V.

[bear]

E= I * R
500 = I * 6000
500/6000 = I = 0.083 amps or 83ma.

So the total wattage is P = I * E or P = I ^2 * R = ~42watts

Of course much of that power is turned into magnetic energy, not heat. Also it only takes the enamel on the wire to be able to withstand the voltage which it does just fine in things like transformers and TV deflection yokes (remember when they had CRTs?). The only other thing the enamel has to handle is the temperature, which in most cases it does perfectly well. Of course things can go wrong and overheat the wire causing breakdowns between windings or breaks in the windings...

Obviously though, the scheme works!

_-_-bear

[Scrounger]

Originally Posted by bear

E= I * R
500 = I * 6000
500/6000 = I = 0.083 amps or 83ma.

So the total wattage is P = I * E or P = I ^2 * R = ~42watts

Of course much of that power is turned into magnetic energy, not heat. Also it only takes the enamel on the wire to be able to withstand the voltage which it does just fine in things like transformers and TV deflection yokes (remember when they had CRTs?). The only other thing the enamel has to handle is the temperature, which in most cases it does perfectly well. Of course things can go wrong and overheat the wire causing breakdowns between windings or breaks in the windings...

Obviously though, the scheme works!

_-_-bear

Thanks,
Scrounger

[bear]

PS. some folks have gone to low Z field coils with heavy gauge wire... low voltage, high current.

[Scrounger]

Originally Posted by Eli Duttman

Scrounger,

You replace the field coil in the PSU filter with a choke/resistor combo in series. Measure the DCR of the field coil. Subtract the DCR of the choke you select to get the necessary resistance value.

BTW, there a caps. on both sides of the field coil or its replacement. Those caps. charge up and the potential difference across the "element" is only a few, not 500 V.

Thanks Eli. The "caps on either side" comment helped a great deal.

[Scrounger]

Originally Posted by Eli Duttman

Scrounger,

You replace the field coil in the PSU filter with a choke/resistor combo in series. Measure the DCR of the field coil. Subtract the DCR of the choke you select to get the necessary resistance value.

BTW, there a caps. on both sides of the field coil or its replacement. Those caps. charge up and the potential difference across the "element" is only a few, not 500 V.

Eli,
Could I just get a $7.00 spool of 44 AWG wire and wrap in around an iron bar and insert that in the circuit? That would give me the DC resistance perfectly if I used 3144 feet?

[kevinkr]

Originally Posted by Scrounger

Eli,
Could I just get a $7.00 spool of 44 AWG wire and wrap in around an iron bar and insert that in the circuit? That would give me the DC resistance perfectly if I used 3144 feet?

Why not just scrounge or purchase a choke with roughly the right characteristics, safer and likely to perform a whole lot better?

I'm not sure where you are getting the 6K resistance value from, it is very likely that the resistance of the field coil used with your amp was just a few hundred ohms to 1K at most, and the inductance a couple of henries or more. A schematic showing where this field coil resided in the power supply topology would be helpful - usually these were used as the first choke after the input capacitor and would have had the entire unit's B+ current flowing through it. Usually in such cases DCR and inductance were moderate. In some cases they provided power to circuitry downstream of the output stage and hence supplied little current and had more turns in order to get the required flux at the lower current. Rarer still would have been a shunt connected field coil IMLE, and that often had a dedicated winding & rectifier in the supply.

Very few field coils use more than 20W of power IMLE, and that would be what is required for a very large driver, many use far less. Field coils in radios typically consumed 5 - 10W or less. Are you sure your DCR isn't more like 600 ohms? Shunt connected high voltage field coils aren't unheard of, but they are comparatively rare compared to series connected field coils used as part of the PSU filtering in radios, organs, and small self contained PAs.. (Shunt connected field coils usually run on low voltages, and I've not seen any that ran higher than 120VDC IMVLE..)

[Scrounger]

Originally Posted by kevinkr

Why not just scrounge or purchase a choke with roughly the right characteristics, safer and likely to perform a whole lot better?

I'm not sure where you are getting the 6K resistance value from, it is very likely that the resistance of the field coil used with your amp was just a few hundred ohms to 1K at most, and the inductance a couple of henries or more. A schematic showing where this field coil resided in the power supply topology would be helpful - usually these were used as the first choke after the input capacitor and would have had the entire unit's B+ current flowing through it. Usually in such cases DCR and inductance were moderate. In some cases they provided power to circuitry downstream of the output stage and hence supplied little current and had more turns in order to get the required flux at the lower current. Rarer still would have been a shunt connected field coil IMLE, and that often had a dedicated winding & rectifier in the supply.

Very few field coils use more than 20W of power IMLE, and that would be what is required for a very large driver, many use far less. Field coils in radios typically consumed 5 - 10W or less. Are you sure your DCR isn't more like 600 ohms? Shunt connected high voltage field coils aren't unheard of, but they are comparatively rare compared to series connected field coils used as part of the PSU filtering in radios, organs, and small self contained PAs.. (Shunt connected field coils usually run on low voltages, and I've not seen any that ran higher than 120VDC IMVLE..)

6k ohm is what I've read about the field coil on the net. I don't have the speaker, I just bought the amp. I have seen field coils @ 2000 ohms. The schematics are on order..........thanks

[Scrounger]

Originally Posted by kevinkr

Why not just scrounge or purchase a choke with roughly the right characteristics, safer and likely to perform a whole lot better?

I'm not sure where you are getting the 6K resistance value from, it is very likely that the resistance of the field coil used with your amp was just a few hundred ohms to 1K at most, and the inductance a couple of henries or more. A schematic showing where this field coil resided in the power supply topology would be helpful - usually these were used as the first choke after the input capacitor and would have had the entire unit's B+ current flowing through it. Usually in such cases DCR and inductance were moderate. In some cases they provided power to circuitry downstream of the output stage and hence supplied little current and had more turns in order to get the required flux at the lower current. Rarer still would have been a shunt connected field coil IMLE, and that often had a dedicated winding & rectifier in the supply.

Very few field coils use more than 20W of power IMLE, and that would be what is required for a very large driver, many use far less. Field coils in radios typically consumed 5 - 10W or less. Are you sure your DCR isn't more like 600 ohms? Shunt connected high voltage field coils aren't unheard of, but they are comparatively rare compared to series connected field coils used as part of the PSU filtering in radios, organs, and small self contained PAs.. (Shunt connected field coils usually run on low voltages, and I've not seen any that ran higher than 120VDC IMVLE..)

Here's one that is similar:

WURLITZER 12 INCH FIELD COIL SPEAKER DRIVER WOOFER WRKS - eBay (item 150560192155 end time Mar-09-11 19:11:56 PST)

That one measures 5.8k 0hms.