Hello all,
I just finished my first tube amp! It is basically the Mullard 5-20 circuit and it works and sounds good. The problem is that at around 3.6kHz the transformer seems to be squealing at that frequency. Into a dummy load at 10W, the sound is really quite loud! I'm pretty sure it is the transformer having tried two sets of output tubes from different manufacturers and the frequency and level is the same.
Is this normal or does it indicate some problem in the transformer? The transformer is an open one without bells (just whistles 😉 ) if that makes a difference.
Thanks for your help!
I just finished my first tube amp! It is basically the Mullard 5-20 circuit and it works and sounds good. The problem is that at around 3.6kHz the transformer seems to be squealing at that frequency. Into a dummy load at 10W, the sound is really quite loud! I'm pretty sure it is the transformer having tried two sets of output tubes from different manufacturers and the frequency and level is the same.
Is this normal or does it indicate some problem in the transformer? The transformer is an open one without bells (just whistles 😉 ) if that makes a difference.
Thanks for your help!
Attachments
Output transformers often sing due to magnetostriction. Most noticable when on the dummy load. music doesnt contain continuous high power 3.6Khz tones. I wouldnt worry too much about it unless you notice it whilst listening to music.
cheers
matt
cheers
matt
Singing transformers are pretty common. Often given two identical transformers, one will be louder than the other and they will peak at different frequencies. This is normal. Open frame transformers tend to be the loudest.
OPTs do tend to sing. This is normal. Mechanical resonance of the coils at some point.
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Hey thank you all, Matt, Tubelab, 50AE, for your quick answers! That eases my mind. I really didn't want to source another transformer.
And to grovergardener, yeah I've checked the amp pretty thoroughly and it's really stable. Even without NFB. For a first tube amp build, I got lucky, I think.
And to grovergardener, yeah I've checked the amp pretty thoroughly and it's really stable. Even without NFB. For a first tube amp build, I got lucky, I think.
And to grovergardener, yeah I've checked the amp pretty thoroughly and it's really stable. Even without NFB.
Amplifiers are more likely to be unstable when NFB is applied.🙂
Amplifiers are more likely to be unstable when NFB is applied.🙂
Looks like the output transformer is not impregnated. In such case you should do that and will probably fix your problems. Also holding the laminations tighter might help. Those small bars around the laminations probably are not rigid enough and they are only making some pressure around the screws and (slightly) bent elsewhere so that the laminations still have room for small vibrations.
@jhstewart9
Thank you. You prompted me to do some more reading and so I learned more. In my prototype (a mess of dangling parts and wires) it was unstabile without NFB. But that was obviously from other issues.
@45
Playing music, the amp sounds good with no noticeable "singing". What would impregnating the transformer entail? Could it be simple like painting it with resin? Or actually potting it?
Thanks for your responses!
Thank you. You prompted me to do some more reading and so I learned more. In my prototype (a mess of dangling parts and wires) it was unstabile without NFB. But that was obviously from other issues.
@45
Playing music, the amp sounds good with no noticeable "singing". What would impregnating the transformer entail? Could it be simple like painting it with resin? Or actually potting it?
Thanks for your responses!
You can pot it or something like this:
Transformer Impregnation At L/C Magnetics - YouTube
You can buy the material and it's not expansive. Then you only need to warm it up, dip the transformer in and let it dry.
You can find easily info in the web.
Or you can ask some local manufacturer if can do it for you.
Transformer Impregnation At L/C Magnetics - YouTube
You can buy the material and it's not expansive. Then you only need to warm it up, dip the transformer in and let it dry.
You can find easily info in the web.
Or you can ask some local manufacturer if can do it for you.
It might be worth just dipping the opt in a bath of lacquer. But unless you use vacuum it is almost impossible to cure transformer microphonics because only vacuum can suck all the air out of the inner portion of the opt. I pulled a vacuum pump from an old refridgerator. Bought some glass jars with plastic lids at ikea. Works great.
When testing amp into resistor load you often hear the tone even from the resistors in some cases. Caps can sing and some ceramics can be quite loud, but c0g/np0 ceramics are not microphonic.
When testing amp into resistor load you often hear the tone even from the resistors in some cases. Caps can sing and some ceramics can be quite loud, but c0g/np0 ceramics are not microphonic.
Since the amp performs well and I don't hear any "singing" under normal use, I'll probably just let it be. I do have a second transformer (the amp is actually to be a part of a single-channel system) and may experiment with dipping that one since the bobbin is slightly loose in the core.
Thanks again for your replies.
Thanks again for your replies.