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Old 17th Dec 2021, 4:48 pm   #1
Richardgr
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Default Identifying primary and secondary on Transformer

I have bought a second hand transformer for a tube project, but I'm having difficulty working out which is the primary and the secondary. There are some handwritten markings on the cover of the windings from the previous owner. He was German, and there is a marking 'Si' for where the blue/green wires come out, which I don't understand.

On one side of the bobbin there are 4 wires, red green blue white. It looks like two windings, red/green and blue/white, and they are marked as 220Vac each one.

On the other side of the bobbin there are two windings. One has two black wires, heavy duty, and is 6.98v, so the heater winding. The other winding has two blue wires, and is marked 245Vac.

I am presuming the primary is the 245Vac winding (blue leads), and there are two secondaries that can be combined as 440VCT (or less, since I think the voltages recorded on the transformer are unloaded).

If this is an unsafe assumption I would be interested to know!

Also, what is the likely VA of a transformer on an EI84 core? (84mm x 70mm x 30mm laminations)

There is some printing on the bobbin two:
NT-BV 513
Lag.Nr.22-414.
That does appear to point to a Swiss transformer supplier page, but could not match the model number.
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Old 17th Dec 2021, 9:21 pm   #2
Ed_Dinning
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Default Re: Identifying primary and secondary on Transformer

Hi Richard, if you have a known voltage winding (6.98V) feed something close to that into that winding and measure the other ones to find out the ratings (not that precise, but close enough).

EI 84 could be up to 100VA depending on duty of the transformer and the type of lams used.
To be more precise try loading up the windings and see how warm it gets over 1/2 hour

Ed
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Old 17th Dec 2021, 9:32 pm   #3
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Default Re: Identifying primary and secondary on Transformer

NT-BV 513, is the same number as the mains transformer that's used in the Imperial 796 chassis, two 110 Volt primary windings, a full wave bridge HT winding and a 6.3 Volt (nominal) heater winding, the two 110 Volt primary windings being connected in parallel for 110 VAC operation in the schematic, so I would think connect in series for 220 VAC operation in the schematic:

https://nvhrbiblio.nl/schema/Imperial_769.pdf

Lawrence.

Last edited by ms660; 17th Dec 2021 at 10:00 pm. Reason: extra info
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Old 17th Dec 2021, 10:28 pm   #4
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Default Re: Identifying primary and secondary on Transformer

Also used in the Imperial 766 Q chassis:

https://nvhrbiblio.nl/schema/Quelle_ST700.pdf

Lawrence.
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Old 18th Dec 2021, 5:14 pm   #5
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Default Re: Identifying primary and secondary on Transformer

Thanks both of you! I'll check the links.

Edit: yes, that is probably it! That is great, thanks! I can use it with reasonable confidence. I'm looking at 4 X 7C5 and two ECC81, which should be doable.

Last edited by Richardgr; 18th Dec 2021 at 5:42 pm.
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Old 18th Dec 2021, 5:18 pm   #6
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Default Re: Identifying primary and secondary on Transformer

Si could be sicherung which is fuse? And Lag Nr could be stock number as Lager is warehouse.
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Old 18th Dec 2021, 10:10 pm   #7
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Default Re: Identifying primary and secondary on Transformer

100 VA is going to be on the light side for four 7C5's. In class AB1 they will use about 80 mA per pair at full tilt. You will also get something like 18 watts RMS.

Joe
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Old 19th Dec 2021, 9:53 am   #8
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Default Re: Identifying primary and secondary on Transformer

Taking DC resistance readings of all the windings helps identify what's what. The higher the resistance, the higher the voltage. The primary is usually the highest resistance. If you have a varaic and a lamp limiter, power the tfmr from that, if the lamp lights with no load you have connected it wrong.

In your case rd, grn, blu, white sounds like your primary, blue wires HT. With UK mains being 240v + in a lot of places you might be pushing that tfmr, depends on how well wound it is. 245v AC will give you about 330v to 340v HT. Try a 3k4 (3k3) resistor to load it for 100mA. That's 34w, a 50w would be nice, on a heatsink. At a push use the biggest resistor you have in a cup of oil or water, be very careful you don't have a cup of T on the bench and pick up the wrong cup!

If the cup of water sounds scary use the biggest R you have, test quickly monitoring secondary voltage. Reduce the value of the load until Vsec falls by over 10%. If the tfmr under test starts buzzing or it gets too hot to touch it's overloaded.

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Old 19th Dec 2021, 12:31 pm   #9
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Default Re: Identifying primary and secondary on Transformer

Quote:
Originally Posted by Diabolical Artificer View Post
Taking DC resistance readings of all the windings helps identify what's what. The higher the resistance, the higher the voltage. The primary is usually the highest resistance.
?

Lawrence.
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Old 19th Dec 2021, 2:45 pm   #10
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Default Re: Identifying primary and secondary on Transformer

Hi Andy, not a reliable rule or thumb.
If it has a 240v pri and a 240v sec as well as other secs, the 240v sec will have the highest resistance. If it is a 350-0-350v sec that will have a much higher resistance

Ed
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Old 19th Dec 2021, 4:04 pm   #11
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Default Re: Identifying primary and secondary on Transformer

Could the 'Si' be an abreviation of sicherheit (securtiy) and possibly refer to an inter-winding screen? If so it would have no electrical connection to the windings.
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Old 19th Dec 2021, 5:53 pm   #12
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Default Re: Identifying primary and secondary on Transformer

I am pretty sure Lawrence correctly IDed the transformer, and Si is Sicherung.

I realise now that it is not be powerful enough for 4 7C5s, so I'm thinking along the lines of ECL86 PP, shooting for 9W per channel.

I inherited a few other transformers in a job lot, so now I feel a bit more confident they can be put back to use in some way.
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Old 19th Dec 2021, 8:19 pm   #13
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Default Re: Identifying primary and secondary on Transformer

Quote:
Originally Posted by lesmw0sec View Post
Could the 'Si' be an abreviation of sicherheit (securtiy) and possibly refer to an inter-winding screen? If so it would have no electrical connection to the windings.
Sicehrung (fuse) more like than sicherheit (security / safety) but have the same root.
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Old 19th Dec 2021, 9:39 pm   #14
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Default Re: Identifying primary and secondary on Transformer

Here is an ECL86 amplifier design of very good quality.
http://www.r-type.org/articles/art-003f.htm

Note the rating for the power transformer!! It asks for 140 mA. I still think that traffo is a bit light on for a stereo push pull amplifier. It looks to be very nicely made and it would be a shame to toast it.

It is however, imminently suitable for a mono push pull amp. It would be quite comfortable pushing a pair of 7C5 OR ECL86 or as I know them 6GW8.

Joe
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Old 20th Dec 2021, 7:26 am   #15
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Default Re: Identifying primary and secondary on Transformer

Quote:
The primary is usually the highest resistance
I should have qualified that by saying on your average everyday mains tfmr, valve mains tfmr's being uncommon in the scheme of things.

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Old 22nd Dec 2021, 11:53 pm   #16
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Default Re: Identifying primary and secondary on Transformer

Quote:
Originally Posted by joebog1 View Post
Here is an ECL86 amplifier design of very good quality.
http://www.r-type.org/articles/art-003f.htm

Note the rating for the power transformer!! It asks for 140 mA. I still think that traffo is a bit light on for a stereo push pull amplifier. It looks to be very nicely made and it would be a shame to toast it.
....

Joe
I'm coming round to the same opinion. I suppose with a valve rectifier and an input stage then there may 40mA going there. I aim to have it in the chassis, and if it gets warm or voltages sag then I will get a toroid, and use it for something else.

Thanks for the suggestion. My plan is to build the Dissident Audio ECL86 amp, which was the basis of the Baby Huey from DiyAudio
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Old 24th Jan 2022, 11:34 pm   #17
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Default Re: Identifying primary and secondary on Transformer

I compared the current needs of the ELL800's in the first schematic posted by Lawrence, and it is clear it does not have the capacity for a quad of ECL86's, which require around 38mA each, compared to 28mA for the ELL80's. It could be marginal, but it is spoiling the ship for a ha'porth of tar.

I have another transformer, but I have a dilemma with that one. It is 85mm square as opposed to 70mmx85mm, and it is weightier. It's marked as ...
109-0-109 150mA
and 8-0-8 1.6A

I am thinking of a silicon bridge, so omitting the CT I reckon on around 270Vdc for the B+, which would be OK.

But - how to adapt the other secondary for 4 x 6.3V heaters? It is almost crying out for PCL86s instead, and taking 16Vac from the secondary. Is there a solution for 6.3V that does not involve power sapping resistors? The heater of one ECL86 is 700mA, so it is manageable in terms of power consumption.
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Old 24th Jan 2022, 11:46 pm   #18
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Default Re: Identifying primary and secondary on Transformer

My quick calculations will fetch something closer to 300 volts HT.
BUT 1.6 amps for the heaters
6GW8 needs .7 amps for each valve, 4 X .7 = 2.8 amps
7C5 needs .45 A per valve 4 X .45 = 1.8 amps.

I think you will need to search a bit further for a suitable transformer.

Regards

Joe
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Old 25th Jan 2022, 12:58 am   #19
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Default Re: Identifying primary and secondary on Transformer

Further to my last post.
Can you separate the centre tap of the 8-0-8 winding?. If that is possible you can wire the two 8 volt windings in parallel which will yeild 3.2 amps @ 8 volts. Thats still a tad high for 6.3 volt valves, but it might sag sufficiently to come down to about 7 volts. 7 volts is on the high side, but it will only require a very small resistor to bring that down to a happy 6.3 volts.

Joe
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Old 25th Jan 2022, 12:59 am   #20
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Default Re: Identifying primary and secondary on Transformer

150 mA for the secondary is still a bit on the low side. I would be looking for 200 mA at least, and in fact a bit more wont hurt at all.

Joe
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