2 sets of wires going into speaker


2015-12-15 1:52 am
This is an old tube amp from the 40's. It's got the OT mounted on the speaker, with 2 wires going into the speaker. Also, it has a set of wires going into the speaker magnet, then into the driver, as well as a ground wire attached to the frame. Does anyone know anything about this? As far as I know it's not a field coil speaker, just a regular alnico but I could be wrong. Thanks!


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Note, there are all sorts of ways to turn down a loud amp, yet preserve most of its tone. The attenuator (power brake, power soak....) and VVR are the most common. Despite the concept, there will always be some effect on tone.

There is a third way that has not been successfully commercially explored yet (yes, I know that there have been a few attempts). Put a variable power supply on the field coil and adjust the magnet strength. The tone remains fairly constant over a fairly broad range of loudness. I did this maybe 35 years ago with a variac feeding an isolation transformer feeding a solid state bridge and a filter cap wired directly to the field coil. Also note most of the field coil speakers I found do not live long behind one of my amps! Field coil OK, voice coil, not happy! The spiders on these old speakers were quite loose, smacking the strings will bottom out the coil, resulting in a loud pop.....for a few times.

For some really unusual effects build a second small SE guitar amp, and wire the field coil in place of the OPT. Yes, two amps feeding the same speaker. Try both polarities of the field coil and be prepared for weird sounds.....especially with effects on the amp feeding the field coil.
Interestingly, back in the 70's, a friend of mine who was an electrician was involved in rewiring an old cinema, which was been converted to another use. The original PA system was been thrown out, so he said I could have it if I collected it - so I did.

The amplifier was a six foot high rack system with one LARGE knob on the front, labelled 'Volume' :D Checking the valves in it, it was only rated around 5W output.

It was no use to me, so I gave it to someone else - I've no idea who it was now? - so no idea what became of it.

However, the speakers consisted of two BTH 18 inch field coil speakers, and a single pressure unit and wooden horn for the treble - a beautifully made latticed horn, some skill involved in making accurate joints on the curve of a horn.

Around this time we were involved in setting up a small music club, following on from a mobile disco we're run for some time.

We'd no idea what the spec. on the speakers was, but we made two boxes for the bass units, used a transformer we happened to have to power them, and used them for the PA fed from a TUAC 125W amplifier module.

Despite originally only been fed 5W, they performed quite happily with 125W, and sounded pretty damn good :D

The speakers were still there when the club finished, no idea what happened to them :(