1935 National Dobro Amp - Seeking audio transformer substitute.

[tubeamptech]

Hey!

This came in the shop, powered up, but no sound. Of course all the caps were dead in the water, and replaced them. Attached is the schematic.

I now have signal right up to the 2A3, well almost. Between the 56 and the 2A3's is an audio transformer. I found on the secondary side, that one leg is open. On the good side, I measure about 3K DC resistance, referenced to ground. The primary winding measures about 1.6KOhms. Sorry, all I can do is measure DC resistance, if anyone has a better way.

This caused one side of the power amp section to float, and get very hot. Audio completely garbled, and crap.

I need to find a sub for this transformer. ie. the W3173. Does anyone know of a suitable sub for this transformer? I unsoldered the leads on the grid side of the 2A3's, and no cold solder. It looks like an open internally.

national-dobro_6107.pdf

Anything that gets close should work. There will be no luck finding this particular one off the shelf.

[Enzo]

Hammond Manufacturing - Transformers - 6W68HF - MAX DC 30MA 10K CTOHMS SEC IMPED : PRI 10K OHMS INTERSTAGE UDIO TRANSFORMER - Allied Electronics

I bet that would work



Or maybe this series from Mouser, with higher impedance for your grids:
http://www.mouser.com/ds/2/177/5C_124_126-32259.pdf

A tiny one:
http://www.mouser.com/ds/2/410/TY-141P-223974.pdf

[g1]

Is this a customer unit?
I could be wrong but I think it may have enough vintage value to warrant having the original drive transformer repaired/rewound.
Not to disagree with Enzo's suggestions, just mentioning in case vintage value is an issue.

[Enzo]

G1 makes a good point. Is the vintage-ness or originality a major part of the value or does the owner just want a good working amp?

[tubeamptech]

G1 makes a good point. Is the vintage-ness or originality a major part of the value or does the owner just want a good working amp?

The store owner picked this amp up, along with a National lap steel. He just wants to get it working, so he can sell it. The winding I mentioned is about the size of a Hammond choke, just slightly bigger. Enzo, I will check out your suggestion. Not sure if this tx is unity gain, or stepped up or down. The main thing was matching the primary and secondary impedances based on the info I gave you. (If that is a huge concern as mentioned). It's transferring the drive power to the grids of the 2A3's which is important.

[52 Bill]

The store owner picked this amp up, along with a National lap steel. He just wants to get it working, so he can sell it. The winding I mentioned is about the size of a Hammond choke, just slightly bigger. Enzo, I will check out your suggestion. Not sure if this tx is unity gain, or stepped up or down. The main thing was matching the primary and secondary impedances based on the info I gave you. (If that is a huge concern as mentioned). It's transferring the drive power to the grids of the 2A3's which is important.

If the transformer is open on one secondary leg only, you can still apply a signal to the primary and read the output on the secondary to figure out the in/out ratio. And if the transformer is toast, I would open it up and see if the problem is just a bad connection at the output lead.

[tubeamptech]

I injected .668V on the primaries to get one volt out on the one half secondary leg. I am assuming if the other leg was working, I would measure 2 volts from end to end. Now I have to figure out which transformer would give me similar results.

[tubeamptech]

Ok, going back to the original transformer dc resistance specs. Pri = 1.6KOhms. Secondary good side = 3KOhms (CT to good end). Multiply by 2, assuming measuring across winding end to end. Divide 1.6 by 6 and you get an in/out ratio of .27. Comparing specs to the Hammond 124A, the primary is 403 DC ohms, and the secondary is 1524 DC ohms. Dividing input resistance by output resistance yields a ratio of .26. I have no idea what the original transformer is impedance wise, and how the impedance relates to and affects the circuit using the 124A. Attached is the pdf.

Am I on the right track with my assumptions?124A.pdf

[jmaf]

You have a 1:3 turns ratio so it's a 1:9 impedance ratio.

The 124A seems right.

[tubeamptech]

Thank you Jmaf. How did you calculate the impendance ratio from the given dc resistance ratio? Much appreciated.

[jmaf]

Thank you Jmaf. How did you calculate the impendance ratio from the given dc resistance ratio? Much appreciated.

I derived it from the voltages you provided, not from DC resistance.

You provided a 0.668 voltage for the primary. The secondary is 2 equal halfs and you got one volt. You concluded correctly it'd be 2v end to end.

And 0.668:2 is 1:3 which squared is 1:9

[Enzo]

You cannot rely on resistance alone. If the two windings are different gauge wire, then the same number of turns will have different resistance. Also if the secondary is wound over the primary, the same number of turns will be a much longer piece fo wire, and again, the resistance will be very different.

The DC resistance of a winding - which is what your meter measures - has very little to do with the impedance. A transformer has no inherent impedance. It may be optimized for particular impedances, but a transformer is like a set of gears, the input and output have a ration relationship. But a 10k to 16 ohm transformer only shows 10k to the tubes if a 16 ohm load is on the secondary.

jmaf came up with a 9:1 ratio, and the 124 series transformers I linked with a 10k to 90k ratio look about the same.