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Does anybody know the impedance of the field coil on the Victor Electrodynamic driver?

rlb955 wrote:Does anybody know the impedance of the field coil on the Victor Electrodynamic driver?

The impedance of the field coil is of no consequence for the field coil, only the DC resistance and maybe the
inductance.


Chuck

A common 3 or 4 Henry choke of 100ma will work beautifully. Such a unit may be had for $10 to $20 from AES. They may be salvaged from 1950s electronics for nothing. I recall success using the primary winding of a television vertical output transformer in this application some years ago.

The core permeability of the modern choke is so much greater than that of those used in the 1920s that the DC resistance of a given value is much lower.

I was afraid mine was open it was reading about 500k ohms on my Fluke. I have never seen a coil read that high. Is it possible that it is ok?

rlb955 wrote:I was afraid mine was open it was reading about 500k ohms on my Fluke. I have never seen a coil read that high. Is it possible that it is ok?

It's open.

DC resistance should be about 1000 ohms

FYI the inductance around 35 Henries


Chuck

Hmm. That is some inductance.

The choke coil used in the Victor combinations which use the balanced armature driver measured just under 5Hy, but then hum is not as noticable with that sort of speaker.

Bob,

I should clarify the answer. I didn't have a 10-51 driver so I measured a 104 speaker, which appears to be similar in electrical characteristics. I measured the inductance with an HP 4262A LCR meter at 120Hz.


Chuck

Your measurement is doubtless far more accurate than mine. I measured that choke using a ninety-year-old General Radio Impedance Bridge with a 400CPS clock tuning fork as the power source, for I do not have a 120 CPS interrupter.

Have a look on the schematic to ensure that the field choke coil is not in the negative leg of the power supply. The common design in those days was to use the dc resistance to generate a negative voltage drop which was used to bias the output tubes. If the DC resistance is too low you may end up with insufficient bias which will stress the output tubes.

As far as I have ever seen the "Tomcat" amps all had the speaker field coil or filter choke on the negative
side of the power supply. The bias for the output tube (210) was developed by a tap on one of the
power resistors, usually 270 ohms. It seems that their amplifier designs ran the output tube hard in almost every amplifier.


Chuck