Amp is built! Whats this weird buzz?

[furrnornos]

With the help of this forum, I put together my record player guitar amp!

I ended up ditching all the old components except for:
1) The switched 750k knob
2) The output transformer
3) The power tube (there was only 1)

I think it sounds awesome (esp for $25), but there is a weird buzz when I'm playing.

I think it might be microphonics in the output transformer. I've read about this phenomenon, but never listened to it so I can't be sure.

Here is a youtube video of the amp - if you listen in the background there is a buzz that cuts off before the note ends (i.e. I'm holding my finger on the fret and the buzz kicks out before the sustain does). You may have to turn it up to hear it.

I tried it with the 5" driver in the amp and with a 10" spare to make sure it wasnt the speaker.

The last part of the video is the inside of the amp.

Anyone know whats causing the buzzing?

ps don't make judge me on how many transformers I have - it was done with salvaged parts so besides the 1:1 isolation for the tubes, I needed a 25V and 6V (pulled out of wall-warts), plus the output xform.

-Jordan

[Reeltarded]

Sounds like something a little loose in a joint and the harmonics are vibrating the amp when it's making the most of that range. It is note dependant to an extent, isn't it?

Neat amp! hah! Fun,

[furrnornos]

I've been through it and nothing is vibrating I think.

Is it possible that the speaker I'm using can't handle a Class A power amp? I pulled it out of an old sansui hifi speaker.

If the quiescent current running through the voice coil is non-zero, then the centerpoint of the mechanical vibration will move compared to an output stage where the quiescent current is zero (i.e. class B), right?

[Structo]

Make sure it isn't a sympathetic vibration, although it doesn't really sound like one because it does it on low notes as well as high.

What are those transformer looking things to the right of the tubes?

Take a glove or hot mitt and hold the power tube and preamp tube (carefully!) while striking a note to see if it is tube related.

Finally, it may be a parasitic oscillation which can be difficult to trace.
Do you have a scope?

That would show if it is a oscillation riding on top of the note.