Noise.

[engels]

I'm taking care of an old amp, looks like a DIY build from 50 years ago. It was converted to guitar head, I've made some changes in the circuitry, genreally speaking it's 6L6 PP with 12AX7 input and 12DW7 inverter. Rectifier is EZ81.

It has a constant noise and I cannot figure out the source of it. The noise is always there and the level is constant. It's not the preamp and not the PI - I went through the signal chain grounding all stages and even when I'm grounding the output tubes' grids the noise is not changing. Is it something in the OT?

I've recorded the noise, maybe it sounds familiar to some of you?
The video was recorded with the amp volume down, at some stage I turn it up (you hear scratchy pot), the guitar starts to feed back and I turn it back down. The audio compressor in the camera makes it sound like the noise goes down after the feedback, but it's just the compression.

https://youtu.be/d_9zWiZDmO8

 

[rayma]

I'm taking care of an old amp, looks like a DIY build from 50 years ago.
It was converted to guitar head, I've made some changes in the circuitry.

Can you post the current schematic? It could be oscillating.
Has your circuit ever worked without the noise?

 

[hooman]

for detect the source of noise.....first turn on amp without output tubes .

 

[JonSnell Electronic]

An EZ81 won't supply enough power for two 6L6s! An EL81 is a pentode output valve.
Check the anode load resistors and coupling capacitors. Maybe a good idea to gently tap each valve with a Biro.

 

[engels]

for detect the source of noise.....first turn on amp without output tubes .

Am I supposed to hear something? 😱