Field Coil Power

[Ax1]

I have just bought up about 6 old field coil speakers 30s/40s, and want to use 1 in a custom guitar amp. Going to base it off an old Valco amp design with a 5y3 rectifier and 6v6 in push-pull...

The schematic I am looking at is here: http://music-electronics-forum.com/attachments/27667d1393246245-supremewtonecontrol.jpg

Any idea what the 6v6 plate voltage would be on this one, I am guessing its meant to be about 250?

Main question is, how should I power the field coil?
Using it as a choke in the main power supply seems like it might be asking for a noisy amp, but then that might be where the magic is!

Should I add some additional filtering before the coil, or would it be better to give it a separate power supply, or just keep it as the main filter choke?

[Geek]

Hi,

Field coils were used in place of chokes. Unlike chokes, one way would make the hum worse, the other would practically eliminate it. If the windings are still in good shape (which they almost never are), you could use it that way again.

You can use a lower DC voltage across them, but we'd need to know the current draw of the amp. Being my day off and first cuppa tea in hand, I'm too fuzzy yet to figure that out

As for 6V6 voltages, under 350 is the nom. 250-350 is safe. Don't want to play safe? RCA black plates will take 460V on them

[DeathRex]

You'll have to find the right field coil, most would be for SE, and use less current than PP. Just a little too much current and you'll burn it up. Less AC on it would be a good thing. I'm thinking you won't quite get guitar fidelity out of a field coil speaker. A lot of FC speakers had very big fields, like 20-50 henries. The last one I worked on 2 months ago, a Emerson, had a 1200 ohm field, and just 45ma through it.

[Geek]

Along that thought line and looking at the schematic again, it'll probably draw 80mA for PP class-A with losses. Probably too much for a field with a 1K ohm resistance. Fields for that level of current are usually under 250 ohms.

[Ax1]

The field coil speakers I have all seem to be between 1-2k ohms, not been able to find one at 250 ohm yet.

Any idea how I can best workout their power ratings, as some don't have any markings on, no power, voltage or current?

Along that thought line and looking at the schematic again, it'll probably draw 80mA for PP class-A with losses. Probably too much for a field with a 1K ohm resistance. Fields for that level of current are usually under 250 ohms.

I might be in luck, one of the speakers I ordered has got a current rating of just over 80ma at 1K ohm

[battradio]

Unless it was used in a high end radio or an organ , 15 tubes or more they will all be 1, 000 to 2,500 ohm , run about 40 ma of B+ through it . Use it in parallel with the power supply with a 4 k or so 20 watt resistor .

My Zenith 15U270 speakers has a 240 ohm field coil

[Ax1]

I have come up with this circuit to power one of the speakers, can anyone have a look and tell me if there is anything wrong with it?

The field coil has the markings written on it: 79V/76mA - and then "SP . 72" ending with what looks like an upside down italic letter 'A'

I will use a dummy load first so I can measure the actual values. I wasn't sure about the voltage after rectification, wouldn't that actually be higher than the input voltage as the frequency doubles? Anyway I have put it in to a simulation program and these are the values I got:

[battradio]

120 Volts X 1.414 to get the DC peak voltage or basicly 170 volts DC at the first cap , measure the DC coil resistance of it cold and figure it will go up about 7 to 10% hot for the current calculation .

I would also use an Isolation transformer from the line .

[Ax1]

ok, so assuming about 170v at the first capacitor, I have changed R1 and R2 for 430 ohm and R3 and R4 to 4k, and assuming the resistance of the coil can increase by 10% it seems to be giving appropriate values for across the field coil.

I would also use an Isolation transformer from the line .

Yes, just not shown because I have no idea how to setup the transformers in the simulation program. I have a step down transformer 240v to 120v so will be using that.

Just a thought. As the speaker is quite old, I would like to add a warning light and some protection if the coil either short circuits or becomes open circuit. What would be the best way to do this? I think I could probably use an 80mA fuse in series with the field coil in case it short circuits.

But what should I do in case the field coil becomes open circuit, as the LED with then draw too much current. I was thinking I could possibly add a relay to up the resistance of R3 and turn on a warning light, but not sure that is the best solution.

[battradio]

Put a neon light in parallel with the field coil , that wont fire ( light ) unless the field coils opens and the voltage rises to the striking point of the lamp .A cheep voltage panel meter would work also .

http://www.mouser.com/ProductDetail/VCC ... YyVqklb04W

[Ax1]

Put a neon light in parallel with the field coil , that wont fire ( light ) unless the field coils opens and the voltage rises to the striking point of the lamp .A cheep voltage panel meter would work also .

http://www.mouser.com/ProductDetail/VCC ... YyVqklb04W

Thanks, I will try that and see what happens.


Back to the original schematic, http://music-electronics-forum.com/attachments/27667d1393246245-supremewtonecontrol.jpg
Any idea what values the capacitors are? I am assuming the 40/450 means 40uF at 450V, but not sure about the one marked 10-10/450, is that just saying both are 10uF at 450v? - I think its a "V ." but at first I saw it as a "K" which confused me.

[DeathRex]

Yep the 10/10 450 is a 2-10uf section cap. I would increase those to at least 22/22 up to 47/47. It would also be good to replace the phase splitter with a real LTP, probably using a LM334Z. And increase the coupling capacitors to about 0.1 to 0.3. Those 0.02uf caps are going to have an impedance of 398089ohms at 20Hz, and 398ohms at 20,000Hz, if my math is corect.

[Ax1]

Ok, so does this look ok? I believe my grounding is correct? As I want to have a look at the waveform directly after the rectifier on my oscilloscope, I assume that is safe to do (my scope is rated at 300Vrms), providing I connect the black lead of all probes to the earth connection. To measure across the load I would need to use a second channel inverting it and adding to the first channel?

Yep the 10/10 450 is a 2-10uf section cap. I would increase those to at least 22/22 up to 47/47. It would also be good to replace the phase splitter with a real LTP, probably using a LM334Z. And increase the coupling capacitors to about 0.1 to 0.3. Those 0.02uf caps are going to have an impedance of 398089ohms at 20Hz, and 398ohms at 20,000Hz, if my math is corect.

Excuse my ignorance, but what is an LTP?

[DeathRex]

A LTP is a long tail pair, a type of phase splitter, I believe Mullard made them popular. Look at Shannon's Poseidon schematic for a LPT with a LM334Z.

[Ax1]

I have tried the circuit, and the 500mA fuse blows. It is actually located on the hot wire of the transformer secondary. The LED lights for a second and then the fuse goes. I am assuming this is just due to a high inrush current? Should I try a slow-blow fuse?

Also, is there a good schematic for an inrush limiting circuit. The transformer I am using is a toroid step up/down (230-120 / 120-230) transformer rated at 1000VA.