How to discover what voltages etc are on a Power Transformer primary and secondary

[Resonator Guy]

I have yet another future project on my shelves and want to know more about power transformers. I bought this beauty cabinet with a view to building something in it. Someone had taken an old guitar amp and home-built an even older style radio in it. I thought I could salvage the chassis and go from there. Looking at what I had when it arrived I found a number of cool old tubes, 76, 78, maybe 42 and I can't tell what the rectifier is. It also has a 6" Utah field coil speaker (never saw a fc that small.) So I started wondering if the power transformer might be usable for something else. Trouble is I can't tell what is what in here. The chassis had been partially stripped and one secondary is taped off. The mains cord was a crumbled mess. They used terminal strips for a lot of connections so I have no idea what was hooked up where.
Is there something I can read to discover how to test various sections of the PT to find what they do, or am I better off buying some new Hammond parts and starting from scratch?
If this PT is salvageable it would save me about $100, that's with shipping, and that is about 3 times what I paid so it is worth a shot, but I must be safe.
Any suggestions on what to read or how to test would be appreciated.

[Tom Phillips]

The process to determine voltages and current ratings are fairly straight forward but the writing it would take to give a complete and accurate description is long. Attached are two image pages from an old article about determining the ratings of an unknown power transformer. I don't remember where I got it and the scan is a little off. There are other methods but I think the article covers some basics. Hope it helps you and maybe others can contribute additional pre-writen information or link to other discussions on this topic.
Cheers,
Tom

[Resonator Guy]

Thanks, Tom. I checked and the pages seem readable. I will try and digest them tonight and see what I can understand. I am hoping this PT will work for me.

[Resonator Guy]

I read the article Tom attached as well as one by Tino Zottola covering much the same topic. I checked the voltages on this transformer very cautiously (testing with no current showed which were the mains leads) and found the following:
One pair that seem to be the same colour (dark dirty brown) read 7.1 vac so this should be the 6.3 sec.
One pair (dirty yellow brown) read 5.6 vac so this should be the 5 sec.
One pair (smaller yellow) gave a flashing overload, as my meter only goes to 500 vac, however from the yellow/red lead(centre tap??) to either yellow lead gives 346 vac and 340 vac.
I suspect, with a better meter I would get a reading of approx 680 vac from these two. There is also a lead taped off that may be the centre tap for the 6.3 sec.
Does this suggest I should be able to use this transformer in a single ended guitar amp?
What else should I check short of getting a new meter which won't happen today.

[Mick Bailey]

I'd check the physical condition of the leads where they pass through the end-bells. If there's any doubt you can sleeve the leads with heat shrink and push it as far as it will go inside the bell. Unless you have an insulation tester you won't be able to check the integrity between each winding and the laminations, but you may want to do a resistance check off your meter between each single lead and the laminations. It should read OC on every one. Those leads look a little dry to me. Sometimes they're waxed to prevent moisture absorption. Effectively you're relying on an air-gap with fabric insulation and anything you can do to improve matters on an old transformer is good. Sometimes if the transformer is in-situ I'll use potting wax (1 part beeswax melted with 4 parts paraffin wax) and brush this on while keeping the wiring warm with hot air, but if the transformer is disconnected heat shrink is the way to go.

Incidentally, if your meter over-ranges you can use a 470k+470k voltage divider, then double your meter reading.

Should be fine for an SE amp. You'll have to work out how much heater current you can supply (look at what it powers right now for a clue). You may be able to get away with a single 6L6, but you'd have to work out the HT current rating of the transformer, too. A 6V6 would be a safe bet if you're using a tube rectifier.

[Resonator Guy]

Thanks, Mick. Those leads are a little crusty, and a couple were quite bad. I think I will undertake an exercise clipping them, one at a time near the end-bells, then splicing properly coloured leads using heat shrink. All the leads look good at the bell-ends. It may end up a waste of some time but if it works it will be worth it and I need to get better at this stuff.

[Tom Phillips]

Some of the leads on your transformer appear to be solid wire. If so then they are most likely an extension of the actual coil winding. You can slide off the whole piece of insulation and replace it with piece of spaghetti tubing or insulation salvaged from other wire. It's some effort but you seem to be having fun with the whole process. No actual cut & splice required. It's not that hard to remove the end bell and do the joints out of sight. If you do that then please post pics of the guts.

[Resonator Guy]

Tom, I had already trimmed two leads before I saw your post. I may take the end bell off just to make it all easier (?) You are right, I am having fun with this, as you must keep your mind active when retired. Always liked the sound of tube amps so now I try to build them in old empty cabs.

[Tom Phillips]

David,
Your transformer has not been dipped in tar and doesn't look like it has a heavy coat of transformer varnish so the end bells will come off easy. Even if you splice wires, you can make the heat shrink joints inside the bell and end up with with a nice clean result. Glad you are having a good time. When we are young and when we are retired we have the time to do these type of things which are not practical commercially but which really add to the learning experience.
Have fun,
Tom