Singing transformer

[jeff]

 I did some tests on my amp today. I wanted to use my oscilloscope so I connected the speaker jack to a 4 ohm dummy load and used a signal generator. This really threw me because I wasn't expecting it but I could hear the note....no speaker!!! As I dialed the generator I could hear the note going up and down. At first I thought it was the tube but I put a tube up to my ear(not a vacuum tube, the kind tin foil is wrapped around) like a sonic telescope and it seemed to be comming from the output transformer.

Is this normal?

Also this amp when in a combo cab with the speaker, rings out at certian pitches at high volumes. Could the transformer be causing this? If the note is causing the transformer to vibrate maybe vibrations fron the cab are causing a note???

By the way I disconnected the negative feedback so that's out of the picture.

For what it's worth the amp is a SE 6V6 champ clone.

[jeff]

 Time for the stupid question of the week. Could/should I wax pot the OT?

[Dave]

I can't really address the question much, but I am about to urinate in anticipation of someone giving some good insight on this. It is a very interesting situation.

I will say, though, that I have seen lots of power transformers that vibrate audibly when under stress.

Dave

[stingray_65]

Hi Jeff,

Is the OT new?

The varnish you see on the E I laminates is there to stop this very symptom.

As for the ringing out, is the speaker centered in the baffle?

it can cause a resonance at very specific notes and its harmonics.

Ray

[PRR]

> Is this normal?

Yes.

Is it a problem? I assume it is MUCH-MUCH weaker than the speaker, and that you only hear it above 500Hz, maybe only 2KHz-8KHz.

> The varnish you see on the E I laminates is there to stop this very symptom.

The primary function of lamation varnish is to break eddy-current paths. Yes, it also hides mechanical looseness and rattle/buzz. It would not be real effective at 3KHz-6KHz where OTs typically sing because the vibrations are so microscopic.

[jeff]

Is the OT new?

Yes the transformer is new. Weber's champ transformer.

> Is this normal?

yes

OK Just didn't expect that

Is it a problem? I assume it is MUCH-MUCH weaker than the speaker, and that you only hear it above 500Hz, maybe only 2KHz-8KHz.

Yes much. I'm not to concerned about the sound it makes, yes the speaker is louder, I'm worried if it's a problem, a symptom that there is something wrong with the transformer.

[jeff]

As for the ringing out, is the speaker centered in the baffle?

it can cause a resonance at very specific notes and its harmonics.

Yes

which lead goes to B+, blue? https://taweber.powweb.com/store/022905msch.jpg

[sluckey]

which lead goes to B+, blue?

Red typically goes to B+.

[billcreller]

So.....what key is it singing in?

[jeff]

 I assume whatever key the input signal is in. As I turn the signal generator up or down I can hear the transformer vibration go up or down. I just assume that this noise is the same pitch as the input signal.

I just read the thread on caps, that even though they're non polarized there is a right way to wire them.
This made me think. Is there a "right" way to hook up a transformer.
I've always heard that you can reverse the primaries or secondaries and it would be the same thing. It'd be much easier to reverse the leads on the speaker jack to test if the transformer/feedback loop is in phase(because the lengths of the wires)

which lead goes to B+, blue?

Red typically goes to B+.

Does the black dot on the diagram indicate phase and therefore one of these red is B+ and the other blue is B+?

[sluckey]

The black dots do indicate phase. They mean that the phase of the signal at the primary dot is the same as the phase at the secondary dot. It has nothing to do with which color wire connects where.

Typically, red goes to B+ and blue goes to plate. It's never that cut and dried though. In an amp such as yours that uses NF, transformer phasing becomes more important and you may have to swap the primary leads to get the properly phased signal for the feedback. Wrong phase may just turn your amp into an oscillator.

Transformer manufacturers can't predict what circuit your tranny will go in, so sometimes the colors may work right in one amp but will have to be reversed in another amp. It's usually a 50/50 guess in a homebrew amp. Now, if you were building thousands of the same amp, you could list phasing and wire colors in the specs and all trannies built to that spec could be wired by colors and would all be properly phased for that amp.

[jojokeo]

Another way of expressing "tranny phases" is that those dots are there to tell you where the beginning and ending of the wiring turns are. In the same way as pick-ups are wound.

With negative feedback, in a pp amp you will get LOUD squeal/oscillations when the pri wires are incorrectly wired but not really anything immediately noticeable in SE amp. BUT, you will notice if you unsolder the feedback wire or resistor at either end and touching it back again a couple of times... do you hear an increase or decrease in sound and response? You'll know it's wrong if you hear an increase. This is positive feedback. Switch the two pri wires and then it should decrease slightly and is why it's called "negative" feedback from the cancellations you get.

[HotBluePlates]

Are you 100% sure the transformer is making the noise? Did you hear noise when using it normally? Did you feel physical vibration from the transformer?

I ask because I have maybe 5 old Hewlett-Packard tube signal generators. If I turn any of them up to maybe 1-5kHz, I can hear the whining pitch from the oscillator itself. Dunno if it's a tube vibrating or the plates of the tuning cap, or what.

Just wanting to make sure you're not hearing the noise from the oscillator.

I also hear the oscillator in older CRT televisions. Don't know nuthin' about a T.V. circuit, but I'd guess it's 10-15kHz judging by the pitch. It's pretty well up there...

[PRR]

H-sweep is 15.75KC.

I used to be able to tell, from the other end of the house, if the TV were on.

Haven't heard that in decades. (I thought you'd been through weapons-training? They make you wear earplugs now??)

[jeff]

Are you 100% sure the transformer is making the noise? Did you hear noise when using it normally? Did you feel physical vibration from the transformer?

Pretty sure. Like I siad I held a cardboard tube up to my ear and it sounded loudest when it was pointed at the transformer. Not 100% sure.

Is there any reason why you want to reverse the primaries instead of the secondaries if it is out of phase? Wouldn't reversing either side reverse the outputs phase? Or is there something about how the wires are wound/construction of the OT that you want to keep the black wire of the secondary grounded/negative?

[sluckey]

Is there any reason why you want to reverse the primaries instead of the secondaries if it is out of phase? Wouldn't reversing either side reverse the outputs phase? Or is there something about how the wires are wound/construction of the OT that you want to keep the black wire of the secondary grounded/negative?

Reversing either side accomplishes the same thing. However, if you have a multi-tapped secondary it's much simpler to reverse the primary.

Have you tried tightening the screws on the OT. I think singing OTs is probably common and have never been concerned with it.

[octal]

Also this amp when in a combo cab with the speaker, rings out at certian pitches at high volumes. Could the transformer be causing this? If the note is causing the transformer to vibrate maybe vibrations fron the cab are causing a note???

It doesn't seem like anyone has addressed this part of your problem- or perhaps I've overlooked it. You might have a microphonic tube if the "ringing" does not occur when the amp is not mounted in the combo cab. You can experiment by lightly tapping tubes or just swap them out if you have a spare set.

Nathan