Has anyone out there built their own field coil power supply? There are a few commercial designs out there but in the spirit of DIY those aren't to my liking. I have a pair of German field coils and Lowther basket assemblies.
I have read diy posts on batteries, and using antique PSUs. I'd like to build my own based on the following theme:
120V-> isolation transformer->variac->HV Bridge rectifier->HV filter caps-> field coil. (Fusing for safety assumed)
My field coils will work 0-120VDC and 0-3 amps. Should be ridiculously simple but as I have no experience with field coils I'd appreciate any advise from those with experience. I'd prefer not to get into a long discussion of "sound of " power supply components. Posts or PM appreciated.
I have read diy posts on batteries, and using antique PSUs. I'd like to build my own based on the following theme:
120V-> isolation transformer->variac->HV Bridge rectifier->HV filter caps-> field coil. (Fusing for safety assumed)
My field coils will work 0-120VDC and 0-3 amps. Should be ridiculously simple but as I have no experience with field coils I'd appreciate any advise from those with experience. I'd prefer not to get into a long discussion of "sound of " power supply components. Posts or PM appreciated.
Are you sure of your figures ? 120vdc and 3A will be 360w of heat that just
doesn't seem right. Could that be 12v at 3A or 120v at .3A ?
doesn't seem right. Could that be 12v at 3A or 120v at .3A ?
Yes, you are correct. My Field coil has a 450 ohm field coil. At 150 V (max) it should require a current of .3A. However, it can hand a lower voltage at higher amperage(3A max). The manufacturer states "up to 150VDC with no current limitation".
I picked up a used paper chromatography power supply that will do 0-200V and will supply up to 1.5 amps. I am going to give this a try to start. I have found a number of 5U4GB based tube supplies in a 1930's Radio Handbook. This appears to be what a few commercial units are doing. I really appreciate your reply. Doesn't seem to be much interest. 🙂
I picked up a used paper chromatography power supply that will do 0-200V and will supply up to 1.5 amps. I am going to give this a try to start. I have found a number of 5U4GB based tube supplies in a 1930's Radio Handbook. This appears to be what a few commercial units are doing. I really appreciate your reply. Doesn't seem to be much interest. 🙂
I've been working with a couple of Jensen field-coil speakers in older guitar amps lately, but those speakers are different than yours, as they were designed to use higher voltage.
(One of the amps used the common technique of putting the field coil into the PS where a choke would normally be; the other uses a voltage doubler from the 120V AC supply to power the field.).
The guideline I picked up somewhere for the Jensens was max of 12W dissipation. Yours are rated a lot above that.
(One of the amps used the common technique of putting the field coil into the PS where a choke would normally be; the other uses a voltage doubler from the 120V AC supply to power the field.).
The guideline I picked up somewhere for the Jensens was max of 12W dissipation. Yours are rated a lot above that.
Some kind of misunderstanding here. At a lower voltage the current will be less than 0.3A, e.g. 100V/450 ohms = 0.222A
Will a DC coil exhibit a saturation (in the VC gap) limit similar to an AC transformer that starts to pass excessive AC current as the flux maximum is reached?
How would that look for a DC coil if one were to plot DC current vs DC voltage?
How would that look for a DC coil if one were to plot DC current vs DC voltage?
Yes, of course: at some point, the B/H curve will begin to flatten, but it isn't particularly detrimental: the B value will be clipped (softly) against further increases in current, that's all. It is certainly not going to decrease
The DC current is affected solely by the copper resistance, magnetic parameters having no influence whatsoever (unless of course you look at very tiny higher order effects, totally irrelevant in this case)
I run vintage Graetz field coil speakers. I built my own PSU based upon the general principle that a Constant Current Supply delivered superior results. I didn't do anything fancy. My coils are rated at 100V 100mA or about 1Kohm.
My power supply is a 70V AC secondary (35-O-35 in series) power supply rectified with just a standard bridge rectifier. I applied about 1Kuf of filtering followed by a Microwave Oven transformer as the Choke (big and heavy but cheap). It delivers the required 100V DC with no ripple and the field coils are whisper quiet.
The principle on which a CCS or choke output power supply is based is that the field coil will be subjected to a substantial back EMF which will generate an inverse signal in the field coil which will distort the signal been fed by the main amp and possibly create ringing between the power supply and the speaker. The Choke resists this back EMF and offers a more stable current and voltage to the coil.
I am very happy and my power supply was cheap as chips to build with the parts I had lying around. The only thing I quite fancy trying is a simple MOSFET source follower voltage regulator before the choke to allow adjustment of the DC voltage. I have a spare small variac lying around which I could try - but it seems overkill considering the source follower will do just as well.
Shoog
My power supply is a 70V AC secondary (35-O-35 in series) power supply rectified with just a standard bridge rectifier. I applied about 1Kuf of filtering followed by a Microwave Oven transformer as the Choke (big and heavy but cheap). It delivers the required 100V DC with no ripple and the field coils are whisper quiet.
The principle on which a CCS or choke output power supply is based is that the field coil will be subjected to a substantial back EMF which will generate an inverse signal in the field coil which will distort the signal been fed by the main amp and possibly create ringing between the power supply and the speaker. The Choke resists this back EMF and offers a more stable current and voltage to the coil.
I am very happy and my power supply was cheap as chips to build with the parts I had lying around. The only thing I quite fancy trying is a simple MOSFET source follower voltage regulator before the choke to allow adjustment of the DC voltage. I have a spare small variac lying around which I could try - but it seems overkill considering the source follower will do just as well.
Shoog
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Thank you very much. I am contemplating the same or similar setup. Could you explain how you wired the microwave transformer. I understand the principle of the choke for back EMF but I am not clear on how you executed that with the transformer. I too am having a HV voltage regulator made for the same use.
thanks for taking the time to respond to my query.
David
thanks for taking the time to respond to my query.
David
MOT have the windings earthed to the core so you need to identify the one end of the thin wired winding which is riveted to the core. Disconnect this and then find the other end of thin wire winding. These two ends of the winding represents your choke. its not a terribly good choke (probably only 1H or less under load) and it has about 100R of resistance to account for - but it does fine if you have one lying around.
Finding a commercial choke capable of passing 300mA is not that straightforward and quite expensive in comparison.
Shoog
Finding a commercial choke capable of passing 300mA is not that straightforward and quite expensive in comparison.
Shoog
The standard microwave transformer uses EI all facing in the same direction, with the gaps welded together.
You can cut along the weld line to separate the Is from the Es.
Now dismantle the two windings and remove the slugs.
You are left with two coils, a thick wire one and a thin wire one.
You have a core that that be assembled with a gap between the I & E. You can change this gap with layers of paper to change the inductance.
You can use the thick wire winding alone, or the thin wire winding alone, or use both in series.
This gives you three inductances for each thickness of paper. You could add a third winding to use up the spare window area. Lot's to experiment with.
BTW, for one that I have dismantled, the thick wire gives an air core of 6.9mH DCR 2r7, the thin wire gives air core of 317mH DCR 99r
You can cut along the weld line to separate the Is from the Es.
Now dismantle the two windings and remove the slugs.
You are left with two coils, a thick wire one and a thin wire one.
You have a core that that be assembled with a gap between the I & E. You can change this gap with layers of paper to change the inductance.
You can use the thick wire winding alone, or the thin wire winding alone, or use both in series.
This gives you three inductances for each thickness of paper. You could add a third winding to use up the spare window area. Lot's to experiment with.
BTW, for one that I have dismantled, the thick wire gives an air core of 6.9mH DCR 2r7, the thin wire gives air core of 317mH DCR 99r
dc resistance of the coil and applied dc voltage determines current(ohm's law) and flux density of the magnetic circuit, which required a certain ampere turns....
so define your magnetic circuit, know how many ampere turns is required and you can design the field coils based on that requirement...
what does your speaker magnetic circuit look like?
so define your magnetic circuit, know how many ampere turns is required and you can design the field coils based on that requirement...
what does your speaker magnetic circuit look like?
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