Valco speaker field coil windings

[MzunguSteve]

I have a Valco 51 amplifier for repair. Thanks to the forum I've correctly identified the amp., but this is the first time I've come across this use of a field coil on the speaker.
One of the issues is a dominant low level hum which remains present when the volume is turned right down.
I'm beginning to suspect the circuit involving the field coil.
One side of the voice coil is shown on the schematic to be connected to earth, but this is absent on the amp.
The connections to the field coil are clear to understand but there is also a pair of wires (another winding?) coming from the top of the field coil and connected only on one side to the voice coil.
There appears to have been work done replacing caps, on this amp, and some resoldering of the speaker and field coil connections at the amp, so I don't know if there is a mis-wire?

[Enzo]

Schematic?

[Jazz P Bass]

A few schematics that may help.





Valco51.pdf

[MzunguSteve]

Thanks. I have the schematic for the 510 with 6J7 input valve.
Its the top winding on the speaker.
I need to know how it should be connected.

[mozz]

It's a sort of humbucking coil. I have a model 50 prewar (schematic doesn't quite match) here and have the same setup. I can see how this is wired, i may have redrawn the schematic.

[g1]

Can you post more pics of all the wires coming out of the speaker assembly and where they are going?
And the little terminal board on the speaker with the three lugs, what connects to the back side of the far right lug where the temflex 105 wire is going?

[MzunguSteve]

Can you post more pics of all the wires coming out of the speaker assembly and where they are going?
And the little terminal board on the speaker with the three lugs, what connects to the back side of the far right lug where the temflex 105 wire is going?

The right hand black wire is not connected..
This photo may help explain things. It’s from a Gretsch amp of the same period and pretty much the same circuit layout apart from the input valve.
You’ll see the coil in question is in series with the voice coil in this case.

[Jazz P Bass]

Really all you need is 4 wires.
Two for the voice coil & two for the field coil.

If there are six wires then you have a 'humbucking' coil.
This goes in series with the voice coil.
The polarity of the hum bucking winding is important... in one direction it will reduce the hum... if reversed it will increase the hum. Make sure you have the voice coil and hum bucking coils in series, and wired for the least amount of hum! It is easy to check by ear.

dynspkr.pdf

[g1]

The right hand black wire is not connected..

You mean that lug does not connect to anything on the back side? So where does that wire go at it's other end?

[g1]

Never mind, I think the black OT wire that connects to the second lug should be connected to the third lug like in the gretch amp pic you posted.

[MzunguSteve]

Really all you need is 4 wires.
Two for the voice coil & two for the field coil.

If there are six wires then you have a 'humbucking' coil.
This goes in series with the voice coil.
The polarity of the hum bucking winding is important... in one direction it will reduce the hum... if reversed it will increase the hum. Make sure you have the voice coil and hum bucking coils in series, and wired for the least amount of hum! It is easy to check by ear.

[ATTACH]47747[/ATTACH]

[ATTACH=CONFIG]47748[/ATTACH]

Many thanks for the exploded diagram. I was wondering if humbucking was its purpose.
The Valco 51 has the upper, humbucking, coil.
From its appearance some resoldering has been done and the humbucker coil is not connected.
Will for continuity first.

[MzunguSteve]

Never mind, I think the black OT wire that connects to the second lug should be connected to the third lug like in the gretch amp pic you posted.

Agreed, as Jazz P Bass suggests. Exploded diagram shows its purpose.
Many thanks.

[J M Fahey]

[MzunguSteve]

Thanks for this helpful diagram.

[g1]

We'll be interested to hear the results of connecting it, and if possible, connecting it both ways (phases). I'd expect one way will introduce even more hum than what is there when it is not connected, but I'm only speculating.