Tube power transformer wires

[s0b9r]

Hello everyone, this is not pedal related but maybe someone has knowledge on this topic.
I got this power transformer from an old german tube radio. I'd like to built a tube amp with it if it's possible, but the colors of the wires don't help much to understand which is which.
     
         

I think the right side is the primary, some multimeter tests show 394ohms between the red wires, and about half of that between the white and each of the reds. Between the green ones there's 1.3 ohms but the meter also beeps in continuity mode. There's also these two thicker white wires coming out of the two coils that also have continuity between them. On the left side, there's continuity between all the wires, but I checked the resistance anyway and there's from 3 to 12 ohms between various pairs. Unfortunately there's no visible coding on the yransformer to check for an old datasheet or something. So I was thinking of building a current limiter and try to figure this out but I don't know where to start..If someone could help that'd be great! Thanks in advance

[duck_arse]

if you can search up and link us to a circuit diagram for the particular radio you pulled this from, you will be well on the way to identifying connections you really don't want to get wrong.

[s0b9r]

Ok I'll try that and report back!

[s0b9r]

I think this is it.

[tubegeek]

Find a wall wart that puts out 12VAC (or anything like that, but 12V will make the math easy.)

Connect the 12 V to one pair of the leads that are part of the large group that are all connected, using the two leads with the largest resistance between them. This will be the 240V primary connection.

Measure AC Volts across each pair of your secondaries that match up. Multiply each answer by 20. Your whites are probably 5V rectifier and will measure about .25, your greens are probably 6.3V center tapped (.315), and your reds are probably the high voltage center tapped. (this will measure somewhere around 30V give or take, translating to 600V center tapped = 300-0-300.)

All measurements will be too high (no load) and very very ballpark. But you can certainly verify which is which, safely.

The transformer is marked NT35 on the schematic....

[tubegeek]

Also - did you salvage the 600 ohm speakers?

[s0b9r]

Nice, that seems doable. Let me see if I got it right:
So you're saying that the primary side is the left one with all the differenet coloured wires right?
The other side has 2 reds(and a white which I think is the center tap, it measures about half the resistance from each red) and two greens. Is there a center tap for the green wires?
Also a thicker solid core white-yellow wire and another one on the other side, which both come out from the outer coil's ends. I mean, there's that exterior coil you see in the pictures and each of it's ends come out with that white-yellow insulation around them. That should be the rectifier winding?

I don't have them with me now cause the radio is at my brother's house but I could easily get them. You think they're worth it?
Thanks a lot for your help by the way!

[Phoenix]

For future reference, try and note down where the leads of the tranformer are going before removing it from the original equipment so that you know their function in the circuit.
You can also take this opportunity to note the topology of the circuit to estimate the current draw of the circuit, or better yet, if the circuit is working, use the opportunity to measure the current draw of the circuit by disconnecting one lead of a secondary and inserting a multimeter in series to measure the current while its operating, then resolder, move onto another secondary and disconnect one of it's leads, measure it's current, resolder, repeat until you've completed each of the secondaries, before desoldering all the leads for removal. This way you can get an indication of the current ratings of each secondary winding.
To make sure you're measuring the circuit at full power, you may need to run a test signal into the input, or if it's a radio you may need to inject a test signal into the circuit to make sure the power amp is drawing full current. Probably best if you have a dummy load so you don't need to listen to a loud test signal though.
You may be able to slightly exceed what the original circuit was drawing as the original design may have been conservative, but it may also have squeezed every last drop of available current out of the transformer it could - I tend to assume the latter and avoid drawing more current that the original design.

If you're not removing it yourself, Mr Arse's recommendation of trying to find a schematic allows you to do much the same as I've recommended above - estimate the current draw of the original circuit. Many schematics won't label the voltage of the transformer windings, and even fewer will list their current capabilities, so in that case you'll need to follow Tubegeeks very good advice - although I'd recommend also including a light bulb current limiter to power the low voltage transformer/wallwart with a low wattage bulb (25W or thereabouts). This will help prevent damage to the low voltage transformer/wallwart and also the tube transformer if you make any mistakes in connecting the two together like accidentally connecting 12VAC to the 5VAC tube rectifier filament winding by mistake.

Of course, the usual advice to be careful - work with one hand in your pocket, make sure caps are discharged applies.

[s0b9r]

Yes that would've helped. I don't think I should be too worried about current draw because I intend to build a really small amp. One 12AX7 preamp and one EL84 for the power tube. I'd also like to use diodes for rectification and not another tube. Would regular 1n4007 diodes work?

[amptramp]

Yes that would've helped. I don't think I should be too worried about current draw because I intend to build a really small amp. One 12AX7 preamp and one EL84 for the power tube. I'd also like to use diodes for rectification and not another tube. Would regular 1n4007 diodes work?

Yes, 1N4007 diodes will work.  Note that you will need at least 450 volt rated filter capacitors because the voltage will be sitting there for about 10 seconds while the tubes warm up.  With the tube rectifier with the separate cathode and heater, everything warmed up at the same rate so you only needed the RMS rating on the capacitors (unless something went wrong like a failed output tube).

[s0b9r]

I'm thinking of using the caps I salvaged from the radio. Old caps and resistors look so cool haha. I 'll have to test them though.
The schematic I found is pretty similar to this: http://www.ax84.com/static/p1/AX84_P1_101004.pdf

[bancika]

I wouldn't use any electrolytic caps, they are probably long gone. As for the foil caps, I would but only after tested for capacitance and leakage.

[tubegeek]

Nice, that seems doable. Let me see if I got it right:
So you're saying that the primary side is the left one with all the differenet coloured wires right?

Most likely. Each of those measures some resistance to all the others? Then that's the line voltage selection section of the schematic, on the primary side of the transformer.

The other side has 2 reds(and a white which I think is the center tap, it measures about half the resistance from each red)

Almost certainly. Verify with that low voltage check described above.

and two greens. Is there a center tap for the green wires?

Usually color coded yellow/green in the US. Sometimes you have it, most often you don't. Do you find a (very low) resistance reading between either green and any other wire?

Also a thicker solid core white-yellow wire and another one on the other side, which both come out from the outer coil's ends. I mean, there's that exterior coil you see in the pictures and each of it's ends come out with that white-yellow insulation around them. That should be the rectifier winding?

Almost certainly.

I don't have them with me now cause the radio is at my brother's house but I could easily get them. You think they're worth it?
Thanks a lot for your help by the way!

My pleasure.
Good 600 ohm speakers are very hard to come by these days and very beloved by some tube maniacs. If they carry a brand name (Philips, Norelco) check eBay and you tell me if you think that's worth your time. (Some models draw big $.)

Don't re-use the old electrolytics. It's by far the most likely point of failure and when they fail they can take your sweet power trans with them.

[s0b9r]

First of all thanks to everyone for your help!
Things are busy so I'm going a bit slow.
Here's a crude drawing showing the resistances between the wires of the transformer.


From the schematic it seems that for 220V, which I need to use, the connection should either be red to black or blue to grey.
Any idea which one of these is the right one?
On the secondary, red (and the white center tap), green and white wires are all separated to each other. No resistance reading on the multimeter, not even a high one.
Any ideas about the secondary side? If the thicker white wires are the rectifier winding shouldn't they have a center tap?
It seems like the red ones have the tap instead.
I don't have a low voltage ac adaptor around so I can't really test it yet.

Thanks for the tip on the 600 ohm speakers!

[PRR]

Rectifier windings are rarely CT.

> I don't have a low voltage ac adaptor

Get one! Don't be the carpenter who shows up without a hammer.

Either the 1,9 or the 1,S winding is 6.3V. We can not guess which. But they are so close we can apply 6VAC to one, and see if the other comes out 5V or 8V. Having found the 6V and 5V windings, the others should be clear. Do not get shocked!!

[s0b9r]

Hello again after a long time

I found a 19 volt ac source and I was able to do some tests on the power transformer.
I hooked up the 19v between the blue and red wires on the primary. These read the higher resistance between them and maybe are the 240v winding?

Results: 43.4v netween the red wires and half of that between red and thinner white wire. These are surely the high voltage winding with the center tap.
0.4v between the two thick white wires but also between the green ones. This doesn't help much but could we assume that the thicker winding is the rectifier winding?
The green ones are the same diameter as the red ones.

Then, any advice on how to determine which is the 220v winding(220v is what we use in Greece) on the primary?
Again, thanks a lot for your help and advice.

[PRR]

You have 220V:502VCT (252V each side), and two windings nearer 5VAC.

Get a Lamp Limiter, be VERY CAREFUL, and connect red/blue to your 220V. VERY CAREFULLY (use clip-leads) see what comes out the other leads.

[s0b9r]

I did some experiments with the mains 220v voltage(very carefully of course), hooked in the primary. I added a full wave rectifier in the high voltage winding and measured from that node to the center tap. The heaters and rectifier winding always had the same voltage. I'm thinking the red is the main wire and each other one is for choosing a different voltage winding.

So I got:                      Primary in                        Secondary out (rectified)                Heaters/Rect.
                                      red-blue                                 226v Dc                                     6,2v Ac
                                      red-black                               242v Dc                                      6,6v Ac
                                      red-yellow                              293v Dc                                     8,1v Ac             

From the old radio schematic I posted above, it seems that for 220v mains voltage they connect the first wire (probably red) and the one before the last (black). Also in that setup(red-bl), the secondary voltages seems pretty reasonable right?

This is the amp schematic:


It shows 264v DC right after rectification. Would it be much of a problem that I'm getting about 20v less?
I'll definitely build a lamp limiter. I'll buy a 100w lamp tomorrow. It has to be a filamnent lamp right? Not like these new economy ones that have the little circuit inside them?

[tubegeek]

It shows 264v DC right after rectification. Would it be much of a problem that I'm getting about 20v less?
I'll definitely build a lamp limiter. I'll buy a 100w lamp tomorrow. It has to be a filamnent lamp right? Not like these new economy ones that have the little circuit inside them?

+/- 10% is OK almost always in tube designs.

Filament lamp is correct, not the more recent type you asked about.

[PRR]

> Secondary out (rectified)           226v Dc

It appears you tried this without a first filter cap. Do again with a cap, should be 1.41 times higher, around 350V. This and the 6.6VAC will come down when fully loaded.

I would start on the Red-Blue primary to get lower voltages. Safer than coming up with higher voltages. When fully assembled you can re-consider this based on tests.