Ampeg GU-12 on the bench

[TubeGeek]

I have an ampeg gu-12 on the bench for a checkup.  Came in saying it has pops and crackles.


The first thing I noticed was a high noise floor, 120Hz hum.  It was too much so I began looking into the issue.  I decided to install discrete filter caps instead of a cap can, and separate the preamp from power supply grounds like in my other builds. 


There is one node (B) where I cannot separate the ground from preamp to power section...the screen grids are connected to (B) as well as a couple plates in the preamp tremolo circuit and part of the 6U10 preamp tube. See annotated schematic in green.


With installing the discrete filter caps, I have significantly reduced the noise floor to much lower than before.  It is now tolerable but I wonder if I can get the hum even lower by having a separate filter cap for the screen grids.  Thoughts?
Would this be a source for hum?

If I do experiment with this idea, I will have to scrape the pcb trace in order to install the new filter cap node, i think.

[sluckey]

It's common to see the trem oscillator powered from the screen node. That circuit needs a strong tube and high B+ to operate reliably. The trem tube is not in the signal path so it should not be contributing to hum.

Powering the reverb driver from the screen node is also very common.

I don't think you'll gain anything by adding another B+ node. Just connect another 20 or 40µF cap across the node B cap. Does that lower the hum enough to be worthwhile?

[TubeGeek]

It's common to see the trem oscillator powered from the screen node. That circuit needs a strong tube and high B+ to operate reliably. The trem tube is not in the signal path so it should not be contributing to hum.

Powering the reverb driver from the screen node is also very common.

I don't think you'll gain anything by adding another B+ node. Just connect another 20 or 40µF cap across the node B cap. Does that lower the hum enough to be worthwhile?

Thanks for the clarification.


Adding another 50uF cap to the B node did get rid of almost all the hum. It is certainly in the far background now, when it came to me the hum was overbearing.


This amp is a tone machine! It has a new production Fane speaker in it, which is a nice compliment.