Hammond BC

[Drawbar Dave]

Can anyone tell me what this gadget is for? I’m just curious!

[David Anderson]

It's an inductor/choke that filters the bass pedal signal to the first pedal drawbar along with a 10Ω resistor.

The pedal harmonic mixing circuits became more complex over time. I think there are three different versions even within the BC model run. See Service Manual page 2-21 for details. They reached the final version mid-way through the CV production run, and that stayed the same through the end of 3-series production.

[Drawbar Dave]

Thanks David. I’ll read that part of the manual now I know what it is!

[Drawbar Dave]

I had a look in the manual and in the restored pdf version on page 97 the various versions are shown. It looks like mine is the earliest version as my resistor panel is missing. I just have a small phenolic board below the pedal draw bars with two sets of windings and no resistors.

[David Anderson]

The windings may be resistance wires, similar to those used in the manuals. Low value CC resistors tend not to be particularly accurate or stable. It's where you hit the limits of the technology. Wirewounds are much more accurate and stable in this range.

If you have a panel with two windings, you may have the second version with two mixing resistors.

[Drawbar Dave]

The windings may be resistance wires, similar to those used in the manuals. Low value CC resistors tend not to be particularly accurate or stable. It's where you hit the limits of the technology. Wirewounds are much more accurate and stable in this range.

If you have a panel with two windings, you may have the second version with two mixing resistors.

I found a picture of it that was one that I took for Enor when I first got the organ to show the modified swell rod (it had an AO29 fitted).

[David Anderson]

Yes, those are resistance wires, and I think they remained resistance wires throughout console organ production, unless someone has seen something different. It's not something we think about a lot since they never cause any problems.

Can you cross your BC's serial number to the diagrams?

[Drawbar Dave]

I don’t have a serial number as the rheostat box was missing when I got it. I have now replaced it with one that Enor kindly swapped me. The only indication of age is “Fred 25/10/37” written on both manual chassis below the left hand cheek blocks. I was expecting to see carbon resistors rather than resistance wire hence my confusion!

[David Anderson]

I was expecting to see carbon resistors rather than resistance wire hence my confusion!

This kind of thing falls under the category of details that you would not be expected to know without asking. :-)

[Drawbar Dave]

Thanks for the reassurance 👍