1972 Dan Armstrong London Bass Pickup

[Telemachos]

Hi everybody,
I'm about repairing/ rewinding a friend's 1972 Dan Armstrong London Bass Pickup. It is using a 38 gauge wire I guess as the wire is fairly thick. Does anybody know what the correct DC resistance for that pickup is? Again, it's a bass not a guitar pickup.
Any information is welcomed and much appreciated

[big_teee]

Bump!
Anyone have any experience with the Dan Armstrong London Bass Pickup?
I would love to help, but not something I've worked or beat on.
Maybe someone here has?
T

[Telemachos]

@big_tee , thanks bunches for bumping me up. I much appreciate it. However meanwhile I learned that these pickups are of low impedance variety. The customer had taken them to some luthier before he finally brought them in to my place. He said that the luthier told him that the pick was dead and needed a rewinding job. So when I got my hands on them I took numbers and indeed the DC reading was oddly at 0.12 DC on each coil. So that made me believe the coils were dead. After taking the pickup apart and inspecting the coils, unwinding one coil and double checking readings it still went to the same DC resistance. Conclusion after I read this article on Premier Guitar was that the pickup wasn't dead at all but very different from what I had been experiencing with pickups previously.
So, now I'm wiring back the orig. wire and will put thing back into place as it was when delivered. I assume that the transformer or some other wire connection must be malfunctioning. I don't hve the bass on the bench, just the pickup. Yeah , learning by failure.

Quote// "The low-impedance pickups use a transformer housed in the guitar’s control cavity to adapt the signal to high impedance for use with an amp. To keep the pickups lean and mean, the Armstrongs created thin pickups by using 38-gauge wire.//

https://www.premierguitar.com/1972-dan-armstrong-london

[Helmholtz]

What is 0.12 DC?
Do you mean 120 Ohm?

[bbsailor]

@big_tee , thanks bunches for bumping me up. I much appreciate it. However meanwhile I learned that these pickups are of low impedance variety. The customer had taken them to some luthier before he finally brought them in to my place. He said that the luthier told him that the pick was dead and needed a rewinding job. So when I got my hands on them I took numbers and indeed the DC reading was oddly at 0.12 DC on each coil. So that made me believe the coils were dead. After taking the uts. apart and inspecting the coils, unwinding one coil and double checking readings it still went to the same DC resistance. Conclusion after I read this article on Premier Guitar was that the pickup wasn't dead at all but very different from what I had been experiencing with pickups previously.
So, now I'm wiring back the orig. wire and will put thing back into place as it was when delivered. I assume that the transformer or some other wire connection must be malfunctioning. I don't hve the bass on the bench, just the pickup. Yeah , learning by failure.

Quote// "The low-impedance pickups use a transformer housed in the guitar’s control cavity to adapt the signal to high impedance for use with an amp. To keep the pickups lean and mean, the Armstrongs created thin pickups by using 38-gauge wire.//

https://www.premierguitar.com/1972-dan-armstrong-london

You need to do a little reverse engineering to get more information about the coil turns. With the transformer unloaded, measure the primary and secondary inductance at about 100 Hz. Divide the lower inductance into the higher inductance and calculate the square root of this number to get the approximate transformer turns ratio. Assume it is 10 then you will only need a pickup with about one tenth the turns of a high Z pickup using thinner AWG42 wire with no transformer.

When you drop three AWG wire sizes you will have about one half the resistance, so AWG 38 is four sizes thicker so it will have about 65 percent less resistance than AWG 42 per foot of wire. But it’s larger wire size will occupy more space per turn.

I suspect your pickup is in the approximate 100 to 200 Ohm range. For more information, look up how low impedance microphone matching transformers work to boost 150 ohm low Z mics to high Z inputs.

I hope this helps.

Joseph J. Rogowski

[Telemachos]

You need to do a little reverse engineering to get more information about the coil turns. With the transformer unloaded, measure the primary and secondary inductance at about 100 Hz. Divide the lower inductance into the higher inductance and calculate the square root of this number to get the approximate transformer turns ratio. Assume it is 10 then you will only need a pickup with about one tenth the turns of a high Z pickup using thinner AWG42 wire with no transformer.

When you drop three AWG wire sizes you will have about one half the resistance, so AWG 38 is four sizes thicker so it will have about 65 percent less resistance than AWG 42 per foot of wire. But it’s larger wire size will occupy more space per turn.

I suspect your pickup is in the approximate 100 to 200 Ohm range. For more information, look up how low impedance microphone matching transformers work to boost 150 ohm low Z mics to high Z inputs.

I hope this helps.

Joseph J. Rogowski

Hi,
thank you for sharing your invaluable insight on this particular problem. Fortunately I saved the orig. wire and rewound the bobbin. I get 660 turns @ 125 Ohms. I previously lost almost exactly 20 turns during the unwinding process. So the orig. count of windings is at about 680 turns on a bobbin appx. the size of a mini HB bobbin, although the mini HB bobbin is a bit taller. The other bobbin that I did not touch reads 132 Ohms. I have finally put everything back together so it's ready to go back on the bass.
I don't have the bass at my place. It is still at some other repairman work shop. So I can't comment wether or not anything went wrong in the control cavity. But my only conclusion is that s.th. must be malfunctioning on the other end of the chain, not the pickup.