WEM Control ER15 Output transformer turns ratio question

[marlinster]

That "Secondary: 0.8 Ohm to unknown(8?) lug and 0.4 Ohm to 15 Ohm" is suspicious. It suggests that the unknown lug has more secondary turns than the 15 Ohm lug and that therefore it is not an 8 Ohm lug. (Assuming that the 15 ohm lug is really 15 ohms)

Yes, I measured it again and the readings are correct.

Now if the measurements are correct, the only explanation I can think of would be shorted turns at the secondary as Chuck mentioned.
This will show in low output power and/or overheating with a primary test voltage of say 100V.
You may want to do the neon bulb transformer test.
https://www.premierguitar.com/articl...ormer-tester-1

Ok so I tested that super secret method, thanks for the link. The neon bulb flashes if I connect the bulb to the primary side and battery to primary. But not the other way around...

I tried this method on a output transformer I know is working and the same result, working when bulb connected to the secondary but not the other way around...? Is this correct? I tested with a 9V battery.

[Helmholtz]

Yes, I measured it again and the readings are correct.

What is the DCR between the "8" Ohm lug and the 15 Ohm lug?

The strange thing is that the voltage at the 15 Ohm lug was higher but the DCR to the ground lug is lower than at the "8" Ohm lug.

Here is a modified neon tester that sometimes gives more positive results (neons have some variation regarding breakdown voltages):
https://www.vintage-radio.net/forum/...ad.php?t=14317

[Steve25]

The resistance test of the secondary should be close to this.
4 ohm tap,.5 ohm.

8 ohm tap, .71 ohm

16 ohm tap, 1 ohm.

Please note that to get the correct readings for these low ohm test you can not hold both metal tips of your meter other wise the meter will read thru you.

[marlinster]

What is the DCR between the "8" Ohm lug and the 15 Ohm lug?

The strange thing is that the voltage at the 15 Ohm lug was higher but the DCR to the ground lug is lower than at the "8" Ohm lug.

Between the 16 Ohm lug and the "8 Ohm" lug it measures 0.6 Ohm.

When I search for pictures of other WEM's I see that sometimes the ground lead seem to go to the in my case 16 Ohm lug, and sometimes all three lugs are used. This is very strange.

[Helmholtz]

Please note that to get the correct readings for these low ohm test you can not hold both metal tips of your meter other wise the meter will read thru you.

No problem at low Ohms. The body resistance is in the 100k range and thus won't change the reading.

[marlinster]

What is the DCR between the "8" Ohm lug and the 15 Ohm lug?

The strange thing is that the voltage at the 15 Ohm lug was higher but the DCR to the ground lug is lower than at the "8" Ohm lug.

Here is a modified neon tester that sometimes gives more positive results (neons have some variation regarding breakdown voltages):
https://www.vintage-radio.net/forum/...ad.php?t=14317

Ok, I tested with that Improved method, 47k resistor and a 9V battery.

When applying this to the primary winding the neon flashes.

When applying to the secondary the neon does not flash.

I tested this on another otput transformer and it flashes for both primary and secondary.

So maybe there is a short...?

Update - It flashes for both windings! Only not so bright for the secondary winding. I had to shut down some lights

[marlinster]

Ok, I tested with that Improved method, 47k resistor and a 9V battery.

When applying this to the primary winding the neon flashes.

When applying to the secondary the neon does not flash.

I tested this on another otput transformer and it flashes for both primary and secondary.

So maybe there is a short...?

Update - It flashes for both windings! Only not so bright for the secondary winding. I had to shut down some lights

So then perhaps it is not short? But what can it then be??

[Helmholtz]

There can be different "degrees" of short (number of turns shorted, contact resistance) in an OT. All of them mean that it should be replaced (or in rare cases repaired).

But if it delivers rated output power in the amp (and your voltage and DCR measurements were wrong) and shows no signs of overheating, it still may be O.K.

[marlinster]

There can be different "degrees" of short (number of turns shorted, contact resistance) in an OT. All of them mean that it should be replaced (or in rare cases repaired).

But if it delivers rated output power in the amp (and your voltage and DCR measurements were wrong) and shows no signs of overheating, it still may be O.K.

True..

Does anyone know a transformer rewinding service in Germany or England?

[pdf64]

If the amp is working ok there's no need to do anything; any of the faults mentioned would surely completely hobble the amp's output?
No reason to fix something that isn't broke.
Re-winding will inevitably be more expensive than replacing it.

[marlinster]

If the amp is working ok there's no need to do anything; any of the faults mentioned would surely completely hobble the amp's output?
No reason to fix something that isn't broke.
Re-winding will inevitably be more expensive than replacing it.

I did some more testing today and actually the output is not really what I would expect. Also the resistors by the OT had been replaced and I suspect it's getting hot. I will rewind it, I wanna keep it in original state as much as possible. Thanks

Many thanks to everyone with all the tips, very helpful.

[pdf64]

The output power of tube amps is usually less than expected, eg 10 watts from a '15' watt amp,
If the OT was bad, eg shorted turn, it would be crippled, a tiny fraction of the intended power, as the primary inductance would collapse.
The stock snubber resistors were 1/2 watt and they overheat when the amp gets overdriven, so by now they will look burnt or be replacements.

[Chuck H]

THIS^^^^^^^^^^^^^^^^^^^^^^^

I have a 2xel84 design that uses a snubber across the primary. Though not two snubbers each to the CT like this design. My design is snubbing at a much higher frequency too. Anyway... I took great pains selecting the components for the snubber circuit to avoid the problem Pete is talking about. No such care has been taken in this amp. So I would expect the snubber parts to fail occasionally.

Also in keeping with Pete's post... I have two of these amps that are schematically identical. Both are working perfectly using the same PT, the same circuit and the same tubes. One puts out 13W and the other puts out 17W. The only difference between the two is the layout and the output transformers. The output transformers are both 8k primary. One amp has an off the shelf Hammond (13W) and the other is using a proprietary Heyboer model (17W). I have a hard time reconciling that the output transformer construction could be the cause for the wattage difference, but I haven't swapped the OT's to eliminate that possibility. More importantly, both amps sound pretty much the same and I don't recognize a significant difference in volume.

Maybe there'll be an "AHA!" moment some day on the difference in wattage. It's just not a problem such as to prioritize it right now. So... How many watts is YOUR amp making? Your opinion of "output is not really what I would expect" isn't something we can sink our teeth into.

[eschertron]

You've used each meter on each test point (swapped meters) and get the same voltage?