National Dobro Model 75- replace field coil speaker?

[labeau]

Hello, first post here. I have a 40's national dobro model 75 that needs a new speaker. It used to have a field coil speaker that is now gone. A standard magnet speaker sits in its place, and is hooked up to the transformer by 2 wires out of the available 4 that were attached to the original speaker. I'm pretty sure this is not right. In addition, the transformer that was mounted on the original speaker is now mounted to the amp chassis (see pics) Does anyone have experience replacing a field coil with a standard magnet speaker?

Pics are here:
national dobro model 75 pictures by malabeau - Photobucket

(yes I know it needs a recap )

Thanks for any help!

Mike

[52 Bill]

Hello, first post here. I have a 40's national dobro model 75 that needs a new speaker.

Is there a problem with the replaced speaker, or are you looking to make this original?

A standard magnet speaker sits in its place, and is hooked up to the transformer by 2 wires out of the available 4 that were attached to the original speaker. I'm pretty sure this is not right.

The original 4 wires that went to the speaker were 2 wires from the output transformer secondary to the voice coil (these are hooked up to the new speaker), and 2 wires that went to the field coil (basically just an electromagnet that replaces a permanent magnet).

Now this is where there may or not be something wrong with the speaker replacement: Was the original field coil used as a choke in the power supply or not?

In general terms, there are 2 types of field coil speakers that can be considered either high or low resistance. Low resistance types will measure in somewhere around 100 ohms and are wired in place of a power supply choke. If the original speaker was one of this type, then the power supply would either need a new choke or a power resistor to replace the missing field coil.

The high resistance types will measure in the 1000's of ohms and will be wired across the power supply from the high voltage to ground. If the original field coil was one of this type, then the wires can just be unhooked and insulated, with no need of replacement.

So, where do the additional wires on your amp go?

[labeau]

The original speaker is long gone (but the transformer from the original speaker is there- mounted to the amp).

Heres the wire layout:

5 wires in all go to the speaker plug. 3 of these run to the rectifier tube, and the remaining 2 go to one each of the power tubes.

yellow and black wires run from rectifier tube directly to plug, (and not into speaker transformer) are cut and taped off. (I believe these used to run straight to the old speaker)

brown wire running from rectifier tube runs to the plug and continues to the transformer.

One wire from each of the power tubes (blue and green) run to the plug and then into to the transformer.

There are two wires leaving the transformer, which are not color coded, that are soldered to wires that lead to the new PM speaker.

[52 Bill]

yellow and black wires run from rectifier tube directly to plug, (and not into speaker transformer) are cut and taped off. (I believe these used to run straight to the old speaker)

These are the wires that originally went to the field coil on the old speaker. What is the rectifier tube in the amp? And what pins do these wires go to?

brown wire running from rectifier tube runs to the plug and continues to the transformer.

One wire from each of the power tubes (blue and green) run to the plug and then into to the transformer.

These are the output transformer primary winding leads. The center tap will go to the B+ and each end of the winding will go to one power output tube.

There are two wires leaving the transformer, which are not color coded, that are soldered to wires that lead to the new PM speaker.

These are the output transformer secondary leads. They are wired correctly to the new speaker.

[labeau]

The rectifier is a 5v4G. If I am doing my pinout right, the yellow wire goes from speaker transformer to pin 1 of the 5v4g, black to pin 3 (no pin on tube) -which is jumpered to pin 8 with a large hollow ancient resistor.

~pin 8 of the rectifier has 3 other wires attached: one is connected to pin 6 of the first power tube (6L6) with a large blue resistor between; brown wire to speaker transformer, and one wire to a huge 20/20/40 can cap.

*you can see all this in the pic "guts" in the link I originally posted below- one question- the resistors in the pic (the ones that look like caps but are pink, purple and orange- lined up just above large silver can capacitor) are these standard type resistors, do I need to just pull them and measure, or does someone know what value they may be?

Thanks!

[52 Bill]

The rectifier is a 5v4G. If I am doing my pinout right, the yellow wire goes from speaker transformer to pin 1 of the 5v4g, black to pin 3 (no pin on tube) -which is jumpered to pin 8 with a large hollow ancient resistor.

Pin 1 and pin 3 of the rectifier are not used by the tube, so they are just using these socket pins as tie points. Pin 8 is the high voltage output from the rectifier. Is there anything else connected to pin 1?

~pin 8 of the rectifier has 3 other wires attached: one is connected to pin 6 of the first power tube (6L6) with a large blue resistor between; brown wire to speaker transformer, and one wire to a huge 20/20/40 can cap.

Pin 6 of the power tube is not used by the tube, so again the socket pin is being used as a tie point.

*you can see all this in the pic "guts" in the link I originally posted below- one question- the resistors in the pic (the ones that look like caps but are pink, purple and orange- lined up just above large silver can capacitor) are these standard type resistors, do I need to just pull them and measure, or does someone know what value they may be?

The resistors should either be marked with a printed value or color coded with a value. Google resistor color codes and see what turns up. I can't remember, but the body color and the end colors and the color dots were all part of the code scheme.

By the way, what's wrong with the amp?

[labeau]

The resistors should either be marked with a printed value or color coded with a value.

These are solid colors, and have no writing. One is hollow. The paint on them is so old its cracked off or in bad shape, they do not have color code stripes. The other resistors in the amp look normal (brown, ID stripes etc). The electrolytic caps all have values printed on them.

By the way, what's wrong with the amp?

I just want to make sure the PM speaker is hooked up right, or if it needs an additional cap or different transformer to work correctly; before potentially damaging the other parts of the amp- before recap, retweed, etc.

Thanks for your help!

[labeau]

Here's some close ups of the resistors. I guess they could have color coding, but pretty crude:

Resistors pictures by malabeau - Photobucket

[52 Bill]

The paint on them is so old its cracked off or in bad shape, they do not have color code stripes.

The old color code didn't use stripes, the main body color was the first digit, the end color was the second digit and the dot in the middle was the multiplier.

The orange resistor in your photo has color dots on it. The others look so heat browned, that color codes will be difficult to see.