Need help with 40's lap steel field coil wiring...

[guttersnipe]

Hi,

I am new to this forum... I recently purchased a 1940's lap steel amp...
I had the field coil re-coned and they removed my newbie flags so I am
trying to figure out the correct wiring... So here goes:

There are two leads coming off the speaker itself and three leads coming off the transformer on the speaker.

One of the leads coming off the speaker (field coil?) itself goes to pin 5 of power tube A (6V6), which is jumpered to pin 5 of power tube B... Which has a lead (see lead #2 below) that should be going to the speaker transformer.

The other goes to pin 4 of the 5Z4 metal Sylvania rectifier tube. This lead is green..

What I am having trouble with though is the wiring (3 wires) going to the speaker transformer.... Here is where the wires are coming from:

lead #1 is coming from pin 6 of power tube A (6V6)
lead #2 is coming from pin 5 of power tube B (6V6)
lead #3 is coming from pin 6 of power tube B

all these leads are yellow...

though the transformer has 2 yellow (left and center) and one green lead (right)...

Here's my theory... as deduced from my flag lettering (the amp stayed with me so those flags remain)... I labeled them A, B & C and believe that I labeled them left to right as is the western convention... lead #1 above is A, lead #2 is C, lead #3 is B... So if my guess is correct lead #1 or A would go to the left lead coming out of the speaker transformer... lead # 3 or B would go to the center wire coming out of the speaker transformer... and lead #2 or C would go to the right hand lead coming out of the speaker transformer (this is the green wire)...

Am I risking blowing something up if this isn't the correct configuration?

I look forward to your responses...

I can post pics (if need be where would be best?)... It is black metal the size of lunchbox with an 8" field coil speaker... No on/off switch or tone knob... Tubes are all Sylvania (preamp is a 6N7-G, Power tubes are 6V6-G and 6V6-GT (they are different shape and sizes but both 6V6), Recifier Tube is a 5Z4 (metal)... the amp worked before I pulled the speaker to have it re-coned because of dry rot...

Thanks to all...

[speaker dave]

I'm looking at a base diagram for a 6V6 and there is no connection to pin 6. I'm guessing you are counting the wrong way around and your 6 is really 3, the plate? If so your 5 is actually 4 the screen? Using your number, then 5 would be the B+ supply and common to both output tubes. The 6s would be the plates of each 6V6.

The 3 taps on the transformer on the speaker are the output transformer primary. It is two outer output taps and one center tap. The center tap goes to 5 (your #5), the outer taps to the 6s and the order doesn't matter for the 6s. All you have to do is confirm which is the transformer center tap.

Best to measure DC resistance between the 3 transformer taps. Just guessing but say you measure all 3 pair combinations, you might see 100 ohms, 110 ohms and 210 ohms. 210 would be with the wires across the total transformer (and go the the pin 6 connections) and the common wire of the 110 and 100 pairs would be the number 5 pin.

Can somebody else confirm that?

Hope that all makes sense.

David S.

[guttersnipe]

You are correct.... I counted the pins wrong (6 should be 3 and 5 should be 4 in my description).... Thanks for the help!

Scott

[guttersnipe]

Originally Posted by speaker dave

I'm looking at a base diagram for a 6V6 and there is no connection to pin 6. I'm guessing you are counting the wrong way around and your 6 is really 3, the plate? If so your 5 is actually 4 the screen? Using your number, then 5 would be the B+ supply and common to both output tubes. The 6s would be the plates of each 6V6.

The 3 taps on the transformer on the speaker are the output transformer primary. It is two outer output taps and one center tap. The center tap goes to 5 (your #5), the outer taps to the 6s and the order doesn't matter for the 6s. All you have to do is confirm which is the transformer center tap.

Best to measure DC resistance between the 3 transformer taps. Just guessing but say you measure all 3 pair combinations, you might see 100 ohms, 110 ohms and 210 ohms. 210 would be with the wires across the total transformer (and go the the pin 6 connections) and the common wire of the 110 and 100 pairs would be the number 5 pin.

Can somebody else confirm that?

Hope that all makes sense.

David S.

I got a multimeter and measured the resisantce between the transformer taps.... With mulitmeter set to the 20K range I got 1.46 Kohm across the yellow wires coming off the speaker transformer taps and 7.5 and 8.9 Kohm across the each yellow to greenish/yellow tap.... Does this sound right? Does this confirm that the greenish/yellow is indeed the center tap?

Thanks,

Scott

[speaker dave]

Originally Posted by guttersnipe

I got a multimeter and measured the resisantce between the transformer taps.... With mulitmeter set to the 20K range I got 1.46 Kohm across the yellow wires coming off the speaker transformer taps and 7.5 and 8.9 Kohm across the each yellow to greenish/yellow tap.... Does this sound right? Does this confirm that the greenish/yellow is indeed the center tap?

Thanks,

Scott

I was guessing that the transformer DC resistances would be lower, but no matter.

Let me guess again, do you mean that you got 1.46k along with .75k and .89k? The two primary halves need to add up to (approx.) the value of the primary whole.

Either way, I didn't realize you had the color code you mentioned. If two wires are yellow and one is yellow/green, then very safe to say that yellow/green is the center of the primary and goes to the common #4 (actual #4 that you had called #5, the common 6V6/5Z4 screen conection).

Hope I haven't confused you.

David S.