Field coil as choke

[g1]

Looking at an old Gibson EH-185. There are 2 schematic versions, one has the speaker's field coil in series in the B+ line, one has the field coil in series on the ground side. I've never seen it done like that before. I'm wondering how it affects the filtering, and if there are any other pros or cons of doing it in the ground line.

[Enzo]

It is in series with your load current either way, isn't it?



Voltages with respect to chassis are lower.

[Leo_Gnardo]

What Enzo said ^^^. Some of the hi fi builders over at Audio Asylum suggested this a few years back as a solution for filter chokes that have a limited hi voltage rating usually 400 - 500V. I guess it's not a new idea.

[52 Bill]

Boy, I've become so used to seeing the typical application where the choke comes after the OT plate supply, that the Gibson schematic just looks wrong. And there are no screen grid resistors either. I wonder if this was a Valco made amp. I think that they did that too.

[Jazz P Bass]

Yeah, that is odd.

As Enzo stated, it doesn't matter where in the B+ circuit it is installed.

I was asked at one time whether the current limiting resistor for an LED should be placed on the anode or the cathode.

It doesn't matter which of the two.

[g1]

Thanks everyone. I thought it looked "wrong" too, but I realized it could work either way, my concern was the LC filtering. So if that is also the same it makes sense to keep it in the ground side. Safer too as you are not carrying high voltage to the speaker. This thing has the chassis removable with about a 20 foot cable so you can have your controls handy and the cab elsewhere.
The other thing I just noticed is there are only 3 wires to the speaker, so the ground is common to the speaker and field coil. Not that in the era they would have been opposed to having the speaker coil floating on the B+ I'm fairly sure .

[Silvertone Jockey]

It's common practice to put the choke in the negative ground return in high voltage RF amps for all the reasons cited above. Chances of the voltage alone breaking down the insulation are next to none when wired that way

[g1]

Ok, as far as the insulation goes, I guess the chassis reference would be via the frame of the OT (chassis mounted) ?

Fudging in a multi-can for the first filter stages, with a common ground, would not be good for the tones, yes? (it really is a field coil and not just a choke)