Re-Use an old Power Transformer ?

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Timinator01

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Joined 2011
I've been looking into building a Tube amp for some time and the only thing that has prevented me from doing so is the cost of the components, mainly the transformers. A few years ago my high school was throwing out an old Hammond organ and I managed to get some of the electronics out of it before it made it to the dumpster. From that i got a few tubes (nothing special) and a power transformer that i have been unable to identify. I've attached a few pictures of it and other than what you can see the insulation on the wires has hardened. So my questions are...

1. Is there a easy way to find out if this transformer would even work for a new amp project?

2. Is it safe to re-use or should I just buy a new one?

Thanks for your help. :)
https://drive.google.com/file/d/0B15bRfKHtSluVmMya0VUYjlmNW8/edit?usp=sharing
https://drive.google.com/file/d/0B15bRfKHtSluTDNGRDhiNVVCSFE/edit?usp=sharing
 
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crispycircuit

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Joined 2007
That looks to be out of an AO-29 amp that was a mono 6v6 amp. The transformer should have numbers on it. My xmer from that amp has stamped into the bell cover AO-24110-1. I have it mounted on a chassis with 2 Hammond organ 6bq5 output xmers. Hoping it will power 2 6v6pp channels. I haven't started wiring yet so I DON'T KNOW YET. I want to try a 6sl7 input driver circuit that looks promising.
 
dchisholm

dchisholm

Member
Joined 2011
Timinator01 said:
. . . 1. Is there a easy way to find out if this transformer would even work for a new amp project? . . .
Click to expand...
If the secondary voltages are similar - say, within 10%, or even 20% - to what the design calls for, and the transformer's power rating is at least as much as what the design needs, the transformer is probably usable.

When measuring voltages, keep in mind that secondary voltages from an unloaded power transformer will be higher than the rated value, perhaps by as much as 10%.

You can gauge the transformer's power rating by its physical dimensions and weight. There are threads on this Forum telling how to do that. The main value to watch is the transformer's TOTAL power rating. As a first approximation, the power which is NOT used by one winding (such as an unused 5V filament winding for a rectifier tube that has been replaced by a solid-state rectifier) can be supplied by another winding.

. . . 2. Is it safe to re-use or should I just buy a new one? . . .
Click to expand...
Transformers like that usually die from insulation breakdown. If you don't see signs of overheating, and it doesn't smell scorched, the transformer was probably good when it was last used. It's hard to tell what has happened since, especially if it's been many years since it was last powered up. Some kinds of transformer insulation deteriorate over time even if the transformer is unpowered; some are said to deteriorate even faster when unpowered.

Start with an ohmmeter check of the windings. You shouldn't find any conductance between any of the windings. Of course, none of the windings should read open. You may want to record the winding resistances for future reference.

Be careful when you apply power. Depending on what resources you have . . .

  • Use a variable transformer (Variac, Powerstat, etc) to bring the primary voltage up slowly. Pause with only 1/4 to 1/2 of the rated primary voltage; verify that the secondary voltages are what you expect with the reduced primary value; and let the transformer sit unloaded for a while - perhaps as long as a few hours. Pay attention for any signs of overheating or internal sparking. Repeat with the full primary voltage.
  • Power up your transformer using a low voltage from another transformer - say, 12V from a filament transformer, 24V from a bell transformer, etc. (Since your transformer under test is unloaded, the transformer producing the low voltage doesn't need to provide more than a few watts.) Follow the procedure outlined above.
  • Wire an incandescent light bulb (40 or 60 watt) in series with the transformer primary and a line cord. Stand at a safe distance when you plug it in or operate the switch. The light bulb should NOT light! (For larger power transformers the bulb may glow very dimly.) The secondary voltages should be essentially what you expect. Let the transformer operate for a while; watch for overheating or breakdown.
After these tests, repeat the ohmmeter tests. The windings should still be isolated from each other. A turn-to-turn or layer-to-layer short within a winding should have caused overheating during the test but may show up as a change in winding resistance.



Dale
 
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