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
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
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.
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.
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
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
PS. some folks have gone to low Z field coils with heavy gauge wire... low voltage, high current.
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..)
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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
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.
Interesting, seems like this would be a case where the field coil is providing filtering inductance to lower current circuitry after the power output stage. Is there another choke or is this amp pentode mode with the center tap of the OPT going right back to the first supply cap? Or is something else altogether going on here? I'm very curious and looking forward to seeing the schematic - should make things a lot clearer, but in this case I think the 6K is appropriate - I expect the current through it is not that large.
Kevin,
If you look at radios from the field coil speaker days, they used CLCLC filters with rather small caps. and the field coil in the 2nd L position. Back then, the high capacitance 'lytics we take for granted were not available.
If you look at radios from the field coil speaker days, they used CLCLC filters with rather small caps. and the field coil in the 2nd L position. Back then, the high capacitance 'lytics we take for granted were not available.
Back in my teenage years I got a bunch of 15 inch field coil speakers that were stripped from Hammond organ extension cabinets. I used them to make guitar amps. I powered the field coil with SS rectified and filtered wall outlet. This worked out quite well, and the resulting speaker was quite efficient. The coil got only slightly warm after an hour or so so the current draw had to be rather low.
Then you didn't follow your moniker. Maybe you should have tried a voltage doubler off the wall with a variac as a tone control. 🙂
dave
Yep, well aware.. My 1932 Atwater Kent 301/501 chassis has the field coil in the L1 position, guess that was at least as common as the other way. (The other couple of radios I've worked on were also L1 position) The electrolytics were 8uF wet slug types.. Completely dried out, but that was all that was wrong with the radio.
I blow stuff up so you don't have to. That should be my new motto, but I think the Mythbusters already use it.
It's easy to turn the knob to the right now and fry stuff for the fun of it. That's because most of the stuff that I make works.....when I want it to. Back in my younger years....you know when you are 15 and know EVERYTHING, stuff blew up all the time when I didn't want it too. Very few of my big ideas actually worked and once they did, I didn't mess with them any more. By 15 I could reliably make a screaming guitar amp, tube or SS, and many worked. My budget was zero, so I used whatever free stuff I could find. Free 15 inch speakers were rare so I didn't try to fry them. 6 X 9 inch car speakers from the junk yard were a different story. I blew lots of them. 100 watts of tube power from a Stromberg Carlson amp can set a car speaker on fire!
I hate to admit it but the guitar amp that I used most often in high school did use one of those wall outlet powered speakers, but there were no tubes inside. It used a preamp circuit that I copied from a Heathkit amp, and a Plastic Tiger SS power amp design that was published in Popular Electronics magazine. I wound the power transformer myself using the core and primary from a TV transformer. It kicked the butt of my buddies solid state Peavey and cost me $0.
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