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Old 8th Jan 2022, 9:59 am   #1
Gabe001
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Default Transformer - very odd readings

Just wondering if I could pick your brains

I am currently building an amplifier and have a transformer rated 5v, 6.3v and 250-0-250v which came with the chassis. It's perfect for my needs. It has 3 voltage settings
200/215
220/230
240/250

The voltage is selected using a selector wander plug type dual connector. I don't know if this has a proper technical name. You can see the transformer in the picture.

I took some readings to check the voltage output with a 5v4gt rectifier in situ, but no other valves as the majority of the wiring isn't done yet.

240/250v transformer setting:
Rectifier filament 5.8v AC under load (2A) - should be 5v
Instead of 500v AC between the 250-0-250 I'm getting 620v, but this is with no load present.
DC resistance of primary 22 ohms.
Readings confirmed with 2 different multimeters
Mains AC 242v

200/215v transformer setting
Rectifier filament spot on 5v AC under 2a load
500v AC spot on between 250-0-250 transformer output (no load)
DC resistance of primary 28 ohms

220/230v setting
Rectifier filament 5.4v AC under 2a load (ie in between the above two)

Based upon these readings, am I correct in my assumption that the transformer voltage labels are in reverse? i.e the 200/215 setting is actually the 240/250 and vice versa. I've never come across this before.

Thanks
Gabriel
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Old 8th Jan 2022, 10:09 am   #2
Ed_Dinning
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Default Re: Transformer - very odd readings

Hi Gabriel, it does sound a bit like it. To confirm, with 240v (measure) applied to the 240/250 setting, measure the mains outputs on the other taps and see if they are greater or less than the inputs.
To avoid any saturation effects it might be better to do this test at 200v if you have a variac; radios between the tapping will not alter.
As a double check try using an analogue meter (Avo etc) as some DVM's can misread, especially on resistance ranges and distorted AC waveforms on inductive devices

Ed
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Old 8th Jan 2022, 10:38 am   #3
joebog1
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Default Re: Transformer - very odd readings

As Ed has already stated.
BUT with only a single loading of the rectifier valve your readings " are very " close to what I expect for a transformer of that type.
Build your amplfier NEATLY and then measure your voltages. I will bet they come within a few percent of requirements.
REMEMBER::: Voltages within +/- 10 % are perfect for valves. read ANY valave manual!! ALL of them state quote:
Voltages within plus or minus 10% is perfectly normal Unquote.

Joe
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Old 8th Jan 2022, 10:49 am   #4
Gabe001
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Default Re: Transformer - very odd readings

Hi ed, thanks for the prompt reply.

I've tried both the digital and analogue multimeters as I don't trust the digital one especially at low AC readings. The readings are consistent

So, with the connector between common and 240/250v, the voltage readings are as follows
278v on the 220/230 setting
300v on the 200/215 setting

This confirms that the voltage labels are reversed right? The transformer was humming a bit more than I expected which prompted me to check - running it on the other voltage settings cures this.

Gabriel
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Old 8th Jan 2022, 11:09 am   #5
Gabe001
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Default Re: Transformer - very odd readings

Quote:
Originally Posted by joebog1 View Post
As Ed has already stated.
BUT with only a single loading of the rectifier valve your readings " are very " close to what I expect for a transformer of that type.
Build your amplfier NEATLY and then measure your voltages. I will bet they come within a few percent of requirements.
REMEMBER::: Voltages within +/- 10 % are perfect for valves. read ANY valave manual!! ALL of them state quote:
Voltages within plus or minus 10% is perfectly normal Unquote.

Joe
Hi Joe thanks for your reply. I am not really worried about the voltages, although 5.8v does make me a bit uncomfortable. What prompted this was the fact that the transformer was noisy on the 240/250 tab. Not dramatically, not a delamination buzz, just a bit of a hum (a bit like when the heating boiler turns on). As soon as I switched voltage settings and it's working to spec, it went quiet, as expected.

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Old 8th Jan 2022, 1:45 pm   #6
Lucien Nunes
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Default Re: Transformer - very odd readings

Quote:
This confirms that the voltage labels are reversed right?
Yes, if you are getting a higher voltage between common and the low tap than between common and the middle tap. Presumably the leads were just soldered to the wrong tags. The interesting question is whether it has been like this since new, or whether someone fiddled with it after it was removed from its original application.

I had a similar anomaly where a transformer fitted to a motor controller was wired with the two different secondaries interchanged, so one rectified voltage was too low and the other was too high. No idea how long it had been like that, I only opened it because oil had leaked into it and discovered the electrolytics on the rail that was too high looking very distressed.
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Old 8th Jan 2022, 1:54 pm   #7
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Default Re: Transformer - very odd readings

I would have thought that a simple resistance reading across the mains primary would confirm if the voltage taps have been wired incorrectly. J.
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Old 8th Jan 2022, 3:54 pm   #8
Ed_Dinning
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Default Re: Transformer - very odd readings

Hi, to absolutely confirm reversal of voltage tappings, and in the absence of a variac, check resistances with an analogue meter. the should progressively increase as you go up wards.
OR
With a lamp limiter connected measure the voltages on all the voltage taps, with mains applied between 0 and 200V. (voltages will be LOW, but this protects the windings).
If the connections are reversed the voltages will now gow down as you go up the tappings

Ed
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Old 8th Jan 2022, 3:55 pm   #9
Gabe001
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Default Re: Transformer - very odd readings

Lucien, I suspect it was like this from new. It doesn't appear like it's been meddled with
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Old 12th Jan 2022, 4:01 pm   #10
Graham G3ZVT
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Default Re: Transformer - very odd readings

Could you please post the resistance measured across the pins of the mains plug corresponding to the three positions of the mains selector?
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Old 12th Jan 2022, 6:05 pm   #11
Maarten
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Default Re: Transformer - very odd readings

While +-10% is perfectly normal, Philips tube manuals usually state that for an optimal life, the valve heater voltages (currents in case of series heater strings) should be kept within 5% of their nominal value.
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Old 12th Jan 2022, 8:29 pm   #12
Gabe001
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Default Re: Transformer - very odd readings

Graham, I did post resistances in my first post.

On the highest voltage setting, 240/250, resistance 22 ohms

On the lowest voltage setting, 200/215 resistance 28 ohms

The middle voltage reading gives a resistance someone in between the 2, but I can't remember what it was.

The labels are definitely wrong. The voltage readings also confirm this. I wonder if anyone has come across it before?. Easy to miss if you just measure HT, especially if the rectifier is past it's best.
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Old 13th Jan 2022, 1:06 pm   #13
Graham G3ZVT
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Default Re: Transformer - very odd readings

Sorry for not reading your initial post properly.

I can imagine a scenario where the paxolin was put into whatever held it for screen printing the wrong way round, and this went unnoticed because of the symmetry.
I wonder how many others were made?
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Old 15th Jan 2022, 12:13 pm   #14
Jon_G4MDC
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Default Re: Transformer - very odd readings

Wouldn't it more likely be an error by the wireman?
I would think it should be possible to lift the end cheek and swap over the two wires. The middle one is already in the right place!
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