Adding choke in place of field coil speaker

[yobigman]

I am planning a scratch build based on a Supro Supreme schematic (1950) that includes a field coil speaker.  I am planning on using a modern speaker, and I figured I would simply include a choke in the circuit in place of the field coil winding.  The schematic identifies the field coil as 1K Ohm.  What would be a roughly equivalent choke value in Henries?  Will matching this value really make much of a difference?  Thanks!  --Matt

[HotBluePlates]

I assume this is the schematic for that amp...

It has no info on supply voltages. Let's guess the voltage will be similar to a 5C3 Deluxe.

12w / 350v = 34mA x 2 6V6's = ~70mA

If you want a choke in the same position as the Supro, it will carry the current of the whole amp. The "Large Fender Choke" (125C1A) sold by Hoffman is rated for 4H @ 90mA, and can handle this entire amp. So that seems a good fit.

Supply voltage may actually be lower than 350v, as the 200Ω cathode resistor for the 6V6's seems small (unless plate/screen voltage is low enough that less bias will hold the tube in check).


I'm guessing you'll want a B+ of 320vdc at the screens and plates. Some slop is allowable, as I haven't figured for the reduction of tube voltage due to the drop across the cathode resistor.

The G.E. 6V6GT data sheet says the triode amplification factor (mu) is 9.8.

For a 250v supply, idle current for class A is 12w / 320v = 37.5mA per tube, or 75mA total. Plates and screens for the Supro are at the same voltage.

Cutoff bias for the 6V6 will be roughly screen volts / mu, or 320v / 9.8 = 32.7v.

We see from the data sheet class A 250v condition that bias is set at almost exactly 1/2 the cutoff bias (0.5 * (250v/9.8)) = 12.75; published bias is -12.5v). So we can assume bias for 320v B+ is 1/2 the cutoff bias figured earlier, or 16.3v.

0.075A * 200Ω = 15v of bias across the cathode resistor. Another 6.5mA would land right on the calculated bias, and the screens may draw that much at idle.

A 5Y3 and 75mA (plus screens and preamp) suggests you'll want a PT which outputs 300-0-300v or a bit less. The 125P1B is a 320-0-320v @ 70mA, and seems to have too much voltage, too little current. That said, you could just bump up the cathode resistor to 250Ω or more.

[yobigman]

Yes, that's the schematic.  Thanks very much for all the info!!  I really appreciate the help.

[sluckey]

If you check the inductor calculations, you should be able to back calculate the Henries using 120 Hz as your frequency, as you are using a center tapped secondary.  I have to do some research, to verify what I just posted.

Please do. I'd love to see your math that solves for inductance when you only know that the DCR is 1KΩ.

[sluckey]

Thank you. But I was hoping to see some math for deriving inductance from DCR.

[PRR]

> reactance is high enough to ignore the DCR

But we *only* know the DCR, *not* the reactance.

On a "good" *choke* we might assume 120Hz reactance is much more than DCR. Perhaps 10 times more.

But a field coil is not made to be a choke. The air gap is much bigger than it would be in a choke.

We are practical men. *In General*, when a field coil must be retired, we can find its DCR and use a power resistor the same value. Ripple reduction is a little less. Capacitors are very cheap today, up-size them until ripple is acceptable.

[smackoj]

The 'Supro Supreme' is the same amp as the Valco 'Supreme' because they were all built by Valco. If you look in Doug's schemo library at the Valco 510 1-B you find the same amp. The guy who has built more of these than maybe even Valco, David Barnes at Vintage 47 Amps, helped me build one and he uses a power resistor in that slot and offers several different speaker combinations. I can't remember the exact value and I don't have my notes with me a work, but I will check tonite and revise this post so you have it if you decide to use a resistor.

jacko   

[sluckey]

The stated DCR is 1000Ω, not 300Ω.

So what do you do? You ignore that fact and grab some numbers from Stancor  (Using Stancor data for coils of 1.3 to 3 Henries, DCR = 300 ohms). Then you do some circle math and end up right back at the assumed 1.3 to 3H you started with.

Amazing!

[PRR]

> not sure if the air gap is critical in the operation of a dynamic speaker

In a Field-Coil, the air-gap is the *point*. We need a strong magnetic field for the moving voice-coil. In an "air gap" so the voice coil can move freely. Today we'd use a Permanent Magnet and some iron to bring most of the magnetic flux around to the air-gap. But until the 1930s, good strong large permanent magnets were a real problem. OTOH, electromagnets were sorta-easy: iron and wire. The need for heavy DC power is a problem, but a radio (or PA amp or geetar amp) also needs heavy DC so the problems can be merged.

The air-gap for a DC choke is often near 0.005". An air gap big enough that the voice coil will fit with some leeway so it does not rub is 0.05" or more. The cross-section of a choke is about the same area everywhere. The cross-section of a field coil core is usually designed to concentrate the flux density in the gap; the gap-area is smaller than the coil-core area. Both differences will tend to make the field coil a "lousy choke", with a poor ratio of inductance to resistance.

> In a field coil application, the purpose of the coil is to saturate core

Not necessarily "saturated". Common iron saturates near 10K to 15K Gauss. You can build many good speakers with 5K to 10K Gauss (however a few need 15K or even a heroic 19K Gauss). In PM design there is a tendency to take the iron into saturation because iron varies less than magnets do, an iron-limited design is more consistent than a magnet-limited design. OTOH magnet costs more than iron so you don't want to jam 5 pounds of magnet in a 1 pound iron structure.

I won't argue with 5H as a good general value for a tube amp's C-L-C filter. As a direct replacement, the pure choke has lower DC resistance, thus the output DC voltage is likely to be higher than stock. This can be balanced-out by adding a resistor in series with the choke. However I believe that in many cases you get equivalent performance (proper voltage with low ripple) with just a resistor (no choke) and perhaps a few extra uFd.

[smackoj]

In case you decide to use a resistor instead of an inductor, my build records show that the recommended value is 1350 ohms which is worked out by 2-2.7k five watt Rs in parallel.  works great and simulates as close as poss. the resistance of the inductor in the power rail.

smilin' jack  

[PRR]

> a 5H choke, using a DCR of 300, (Stancor 100ma choke).  
> the 5H choke provides slightly greater resistance than a field coil of 4H with a DCR of 1000.

So 300 ohms is greater than 1000 ohms?

The *impedance at 120Hz* is greater, not the *resistance*.

Since we expect significant DC current and thus significant DC voltage drop, purely due to DCR, I can't accept that 300 ohms DCR is greater than 1000 ohms DCR.

> Assume that DCR is 10% of Z, at 120Hz

Why?

> the optimum air gap calculations, the optimum space is a least a degree of magnitude tighter than the gap found if you remove the voice coil.

That's what I guestimated.

> Lets say the gap was at the optimum value,...

Optimum for which? Pure choke, or field coil?

> ...wouldn't we be looking at swing choke?

Insufficient data.

[billcreller]

I've built half a dozen of the Valco/Supro/National models, using just a 1 K resistor in place of the field coil.
 My clones are quiet compared to the originals with field coils.  But of course I don't know enough to understand why !

[HotBluePlates]

In this case the schematic shows the resistance of the field coil is 1000 ohms. ...

I make no apologies for taking the 1000 ohm reading on the field coil as a nominal resistance and using it for the Z value.  ... I took the stated resistance of 1000 ohms and used it as the inductance i.e., nominal resistance, ignored the DCR. ...

Sluckey asked for calcs based on 1000 ohm DCR.

Assume that DCR is 10% of Z ...

It can be shown that Z2 will equal 10,000,000, or Z is about equal to 3200 ohms rearranging ...

... Assume that DCR is 10% of the total impedance, just a handy number to assure that if did the calculations with 951 or 1049 ohms, calculations using 1000 ohms are close enough.  ...

First, you acknowledge the DCR is 1kΩ. Then you say you're using 1kΩ as the total impedance of the field coil (not just the DCR). Then you say without basis that DCR is 10% of the impedance of a field coil, but show math where the 1kΩ DCR is 10% of √(10,000,000) = 3162Ω (my calculator says that's 31.6%).

At the risk of being the Forum A-Hole, you are wrong in more ways than I can count, but are too busy arguing to let anyone help you.

The 1kΩ is DCR, not Z. It's not a choke, so the B.S. rules you made up by looking at choke data are useless. It is a speaker field coil, and you have no data for any typical relation of DCR to X_L in that application.

For the love of God, why don't you listen to anyone?? I had decided to simply ignore all your posts, but my concern is that others here will take get the impression that the mass of figures implies knowledge, and will be lead astray by your sophistry. This stuff is really much easier to grasp than all that.

I for one am done debating with you from now on, because you refuse to let info sink in (which could only benefit you). From now on, if I'm inclined to refute any future bad info you present, I will save myself time by linking back to this post. I regret only that I've wasted my time in trying to help you where help is obviously not wanted.