Question about toroidal power transformers

[catalin gramada]

Heard is more like the core was coiled and formed.Most small workshops made it on regular lathe machine without a straightening, tension regulator and forgot to pass a thermal conditioning from economy reasons (...deh, energy cost special when keep an electrical owen on for hours just for few cores)
. Anyhow the electrical loses can be compensate by overdesign, the leakages not. But for quarter of price...It.s a far way between what is writes in books and what really found on street market.

[Pedro Vecino]

Thank you very much everyone for so many good information
It´s the predisposition of the amp to radiate noise. But 1 meter away with the best guitar orientation (I always use a 5ft/1.5m. guitar cable for any test) the difference with the other two is very remarkable. And same difference exists - at the same volume - compared to any standard Fender amp (Princeton, D.R., etc.)
Could it be that I´m taking an excessively short reference measure? I have also seen similar effects (prone to noise although not so obvious) with some Laney GH/VH100 that use toroidal power transformers. Of course I cared less because I did not build them.
I'll do the same tests with a longer guitar cable, but being very small amplifiers for studio is an inconvenience. If I do not get good results I will ask for a transformer equal to the previous two to test.
This is a comparative pic of both with good resolution in case it were possible to deduce something:

http://www.pedrovecino.com/transf.JPG

[catalin gramada]

Btw. If prefer to put a steel cage over it,as Helmoltz states (which is very tempting as time can get easy chromed or stainless kitchen pot at any size), it is a general advice, a good one, to not make contact between central bolt and chassis as will act like a turn in short circuit.

[Helmholtz]

Btw. If prefer to put a steel cage over it,as Helmoltz states (which is very tempting as time can get easy chromed or stainless kitchen pot at any size), it is a general advice, a good one, to not make contact between central bolt and chassis as will act like a turn in short circuit.

There are "magnetic" and "non-magnetic" stainless steels (can be identified with a permanent magnet). The non-magnetic type will not give effective magnetic shielding.

[Helmholtz]

To add still another seasoning to the sauce, we may also have an improperly designed or cheaply made PT.

I mentioned poor transformer design and distorted mains current in posts #3 and #5.

[catalin gramada]

There are "magnetic" and "non-magnetic" stainless steels (can be identified with a permanent magnet). The non-magnetic type will not give effective magnetic shielding.

Actually this is a very pertinent observation as time the kitchen pots are made from food grade stainless steel with reduced percent of Fe which makes the shielding more decorative than effective, at least for the top of the range. So, don't buy german made pots but chinese instead.

[R.G.]

Do these amps use solid state rectifiers? It is possible that you're getting radiated rectifier hash more from one to the other.

[J M Fahey]

I mentioned poor transformer design and distorted mains current in posts #3 and #5.

So????

Did you also "mention"

and "ugly waveform" , I mean a very distorted *current* waveform (although mains voltage will look fine) which in due turn will induce an ugly waveform magnetic field.

Full of harmonics which will be easily picked up by a single coil pickup,not as deep smooth 50/60Hz hum but as its buzzy cousin.

Please link your Intellectual Property Registration Form (duly stamped by Public Notary) so nobody infringes on your Copyright

[J M Fahey]

Thank you very much everyone for so many good information
It´s the predisposition of the amp to radiate noise. But 1 meter away with the best guitar orientation (I always use a 5ft/1.5m. guitar cable for any test) the difference with the other two is very remarkable. And same difference exists - at the same volume - compared to any standard Fender amp (Princeton, D.R., etc.)
Could it be that I´m taking an excessively short reference measure? I have also seen similar effects (prone to noise although not so obvious) with some Laney GH/VH100 that use toroidal power transformers. Of course I cared less because I did not build them.
I'll do the same tests with a longer guitar cable, but being very small amplifiers for studio is an inconvenience. If I do not get good results I will ask for a transformer equal to the previous two to test.
This is a comparative pic of both with good resolution in case it were possible to deduce something:

Cool, but which is the noisy one?
The one on the right probably weighs near twice what the one on the left does.
Of course the one on the left is rated around 30VA ; the one on the right about 50VA.

I am amazed at the counter intuitive difference in OT size, the one on the right is *tiny*

[Helmholtz]

From post #5:

especially with "economical" PTs that already operate close to the saturation "edge" (as can be seen by the waveshape of the no-load mains current)

All fine, I like agreement and confirmation.....and harmony

[Pedro Vecino]

I do not remember if I said it before: the chassis are made of aluminum.
The rectification is through 4x1N4007 diodes in bridge (I'll keep that in mind to check them), and the problematic amp is the one with a 6V6 (larger transformer). That's right, Juan Manuel: 30VA and 50VA.
The amount of information and possible details is incredible. Thank you very much to all.

[R.G.]

The rectification is through 4x1N4007 diodes in bridge (I'll keep that in mind to check them)

If you have some high voltage (500V? 1000V) ceramic disk capacitors of about 1000pF/1nF, you could try soldering one of them across each of the four 1N4007s. This will suppress some of the turn off noise from the diodes. If this changes things at all, you may want to get four UF4007 diodes, which are like 1N4007, but have a soft turn off and don't broadcast as much noise.

[Pedro Vecino]

Thank you very much to all. I went on vacation and after that I had a lot of work. I hope to find a moment to do those tests.