1953 Gibson BR9-GA9 hybrid model Low output volume

[davohilts]

Hi,
I've been working on an unusual Gibson BR9 that has a field coil speaker but isn't like the BR9 schematic that uses a 6SN7 preamp tube, it is really a GA9 that doesn't mount on the top of the box like a GA9 but on the bottom of the cabinet like the BR9 and uses a 6SJ7 preamp tube. The 6v6 pair is in parallel and the amp doesn't have the transformer phase inverter. The circuit it an exact match to the GA9.
That said, the amp was received in non-working condition. I have replaced the filter caps and old paper caps, a few out of tolerance resistors and have the amp up and running but with very low output volume. Injecting a signal into the input and following it thru the amp to pin 3 of the output tubes I get signal all the way through and pin 3 of the 6V6's is very loud and strong.
Does this mean that the OT is bad? I get about .5 ohms across the secondaries of the OT and 820 ohms across the field coil (schematic shows 1000 ohms).
Measuring the bias I get 38.7 mA with 285 volts on the plates.
I get 84 volts on the plate of the 6SJ7, 304 volts on Pin 3 of the 6V6s, 390V on the input into the field coil, 317V on the output of the field coil 267 volts at the last 10mfd 450v filter cap.

Does anyone have any thoughts as to what I could do to further trouble shoot??
Thanks,
Dave

[EFK]

I would temporarily wire up a 1K or more 5W resistor across the leads to the field coil (after disconnecting them from the speaker, of course) and then temporarily wire up a known good 'normal' speaker. See what happens. If you are seeing good signal all the way through to the output tubes, and the output tubes are known to be good, then the next things to look at are the OT and the speaker. The speaker seems easiest to test first.

[davohilts]

I didn't disconnect the field coil or use a resistor but aligator clipped onto the OT leads and connected them to a 12" speaker and got good volume. Does this mean the field coil and OT are working but the speaker is bad? When I test the primaries of the field coil with my multimeter I get 870 ohms or so, the schematic calls out 1000 ohms on the field coil.
If I install a 1K resistor where do I put it in the circuit, does it go on the leads from the field coil on the speaker?
Thanks,
Dave

[davohilts]

Just for more information, when the amp is running there is no magnetic pull at all from the field coil.

[52 Bill]

A field coil speaker is just a speaker that uses an electromagnet instead of an Alnico or ceramic one. So the problem could be in the field coil or in the voice coil or in the output transformer that is usually riveted to the speaker frame.

Your tests reveal that the OT is working so you are down to the field coil or the voice coil. If you are reading a resistance across the field coil, then you know that the wire is intact, but the low reading could be either tolerance or shorted turns. If there really is no magnetic field, I guess that that could be caused by shorted turns.

The field coil in you amp is probably being used as a filter choke in the power supply. If you want to remove the field coil, remove the coil from the circuit and install a 1K power resistor or a equivalent choke in its place.

To test the voice coil you will need to unsolder one of the output transformer secondary wires and then read the resistance of the voice coil. It will read just like one on any common permanent magnet speaker. It will normally read slightly less dc resistance than the rated impedance.

[davohilts]

Well, looks like my field coil in shot so I'm going to look into having the speaker and coil rebuilt by The Speaker Shop.