Field coil work-around

[Luthierwnc]

Hi All,

I'm in the sketching stage of a close Gibson BR-1 variant. I had the pleasure of recapping and tuning an example from 1947 15 or 20 years ago and enjoyed the sound. Unfortunately, the owner wanted it back but I kept the tone in my head.

One thing I don't plan to replicate is the field coil speaker. They aren't terribly safe. It also gets rid of the 20k bleed resistor on B+2. On paper I have an L/C filter power section like a mid-50's Fender going to a P12N. The donor has good iron but will need a choke.

My questions are these: is there any special mojo about field coil speakers where I should add/subtract capacitance or resistance? and there is an unusual feature where 270k resistors go from the power tube plates to the inverter plates in addition to 100k resistors from further down the rail. I haven't seen that before and am not sure of the purpose/necessity. In the attached schematic I've circled one of them in red. There are a couple other small changes (like dropping one of the accordion jacks) but nothing substantial.

Thanks for any comments, Skip

[Cliff Schecht]

Field coil speakers are interesting beasts. The trick to using them is to place a constant DC bias on the field coil thus forming an "artificial" magnetic field (as opposed to using a real magnet). In a lot of older amps you see the field coil used as a choke as well which actually has a dual purpose of placing said bias on the speakers field coil as well as cancelling out some buzz in SE amps (I think..).

Another neat trick with a field coil speaker is you can vary the sensitivity of the speaker depending on the bias. You can underbias the FC for a less efficient speaker or go over a bit to increase the sensitivity. The overall response can actually change when you do this as well. I've never played with this but have wanted to, I just don't have a good FC speaker to do this with. I'm sure I'll find a nice Jensen FC12R (instead of a P12R) someday to play around with..

With all of that said, I don't think they are too safe either and really, they offer no benefit to a permanent magnet speaker when used as they typically are. I'd just stick a choke in there and call it a day myself, you can find a nice vintage P12R's to stick in there.

[Aurora]

FWIW - modern field coil speakers are among the rather expensive ones in the HiFi market, and ranked veryvhigh for sound quality.....
I have no personal experience with neither these or the "ancient" ones....

[martin manning]

Hmm... Are they rather expensive because they're ranked high for sound quality, or are they ranked high for sound quality because they're rather expensive? :^)

[Aurora]

Sound quality is the main point, albeit these are all fullrangers with generally very high efficiency, AFAIK.....
PHY, Supravox and more are in the market.......
a pair of 8"PHYs is prob'ly around 5000$ a pair, or more

[Structo]

I know the Hammond organ amps of past used the field coil as a choke in the power supply, so it was dual function.

So you have to consider the inductance and resistance of the coil in the plan.

[wyatt]

I know the Hammond organ amps of past used the field coil as a choke in the power supply, so it was dual function.

So you have to consider the inductance and resistance of the coil in the plan.

+1

It works the same in guitar amps. The coil doubles as a filter choke. I've seen people add a choke when converting from field coil and I've seen just as many techs ignore it since it's a guitar amp and "lo fi".

But that coil resistance had a big effect on B+ voltage, and some of those amps could benefit from more B+.

[Luthierwnc]

I hacked it out of the posted schematic but this is a combination choke and coil driver. The chassis will be more tweed style because on the original, the PT was the only iron not bolted to the speaker frame. This amp is being built around one of two 1962 Jensen P12Ns I got on Ebay as Baldwin organ pulls. One needed some tea bag patches and the other was clean. One good part justifies a build, right?

Any bites on the circular rail? I don't know if that is part of the field coil experience or something else. For sure the ground ties for both nodes should be together. My power-supply chops aren't good enough to recognize how 270k from B+1 meeting 100k from B+2 effects the inverter plate voltage. That may be one thing the designers got rid of when they went to permanent magnets. I was planning to use pin 6 as a tie point for the 270k to run to the .05/100k junctions.

Keep those cards and letters coming in! Thanks, sh

[Firestorm]

Any bites on the circular rail? I don't know if that is part of the field coil experience or something else. For sure the ground ties for both nodes should be together. My power-supply chops aren't good enough to recognize how 270k from B+1 meeting 100k from B+2 effects the inverter plate voltage.

That's not strictly part of the power supply. It's feedback, probably to linearize 6L6s running screens at higher potential than plates and to improve push-pull balance with that style phase inverter. I expect playing with that 270K value could really let you manipulate the character of the power amp.

[Luthierwnc]

Thanks Firestorm,

At first I was wondering if it was a variation of a conjuctive filter. The temptation is to just throw a couple of 470R screen resistors on and have done with it. I'll go with the schematic but order some extra parts just in case.

It seems like field coils would be the mother of sag. Energizing dead iron must take more juice than material that is already gaussed. I'll bet a big transient could brown out the whole rail. sh