Dagnall Transformer ID?

[Lonely Raven]

Long story short, I had some 80's vintage amps stolen and recovered, but I didn't get them back the way I found them. It looked like someone was stripping them for parts or something.

So have this 100 watt Laney chassis and a few Dagnall transformers. The chassis I might reuse to build a 50 watt something for myself, but I have like 3 Output transformers and a working 100 watt power transformer and choke I'm not sure what to do with.

One OT is a Dagnall C2668, looks almost new.

One OT looks exactly like the C2668 but doesn't have a sticker saying that's what it is.

Last OT looks like it might be the 50 watt OT of the same vintage amp. The 50 watt amp wasn't recovered unfortunately...which is too bad because that's the one I actually used.

Is there any way I can test these to make sure they are OK before I sell them? I hate the idea of selling them to someone not knowing they are 100% good.

[Lonely Raven]

No way of testing Output transformers for health besides putting them in circuit? Otherwise I'm just going to eBay these as is and take a hit on them I'm sure...

[Phil_S]

Raven: Sorry to hear of your troubles. It's painful to hear, particularly since whoever did it actually understood how bad this is.

The best test is in circuit, but there is a lot you can do. Transformers are basically about turns ratios, along with voltage and current carrying capability.

First, for the two that you think are alike, I think you can determine if they are fairly quickly. The first thing is to measure the DCR on each winding. If the Ohms are the same (or close) there is a good chance they are the same. Next, use a low voltage AC supply to test the turns ratio. I use the filament supply from a PT. 10VAC is ideal because it simplifies the math, but we don't often get such luxuries -- that's why we have calculators. Apply 5VAC or 6.3VAC to the secondary and check what appears on the primary (outer leads, not center tap). If you get the same results on both, chances are very good they are the same. Make sure to meter both sides while hot and under test so that you get the loaded voltage on the source side (secondary). Note, you should not apply high voltage to the secondary. If you only have a high voltage source, you might try it on the primary, but I would be hesitant to do so.

Now that you know the turns ratio, you can pretty well assure yourself the OT is right or not. If you know the secondary rating, let's say 8 Ohms, then you can infer the primary with a little math. Divide the primary voltage by the secondary voltage (Vp/Vs) and that is the turns ratio. The square of the turns ratio tells you the impedance ratio. Using that you can make a judgment to determine if the primary DCR seems appropriate for your output tubes.

I'll give an easy example, in case this isn't clear. 5VAC into the secondary gives 158VAC on the primary. The turns ratio is 158/5 = 31.6. The turns ratio is 31.6:1. The square, 31.6^2 = 1000 (rounded). If your speaker connection is 8 Ohms, then 8*1000=8000 Ohms for the primary. This would be suitable for a pair of EL84 into 8 Ohms and if the result describes your amp, then I think your OT is OK.

For an OT, this is probably enough to tell you if it's good. I'd be disinclined to stress test an OT. Hope this helps. Maybe someone can confirm what the primary and secondary ratings should be. I have no idea.

[Lonely Raven]

That helps greatly, and is very logical. Thanks so much for that writeup! I'll see what I can do to do some testing...hopefully these are both good, because I'd like to part with them to buy the parts I'd need to rebuild this chassis as a 50 watt of some sort. Or just sell everything I have here and go buy myself a built 50 watt 800 or Plexi or something fun. I've never owned a real Marshall before...