Hum at Zero volume - decreases at mid-point, then more hum...

[Mandopicker]

Thanks in advance for any insight you'all can provide me. I think the title of the thread sorta of says it all. Essentially, I built an all Octal guitar amp, base on a Super Reverb but without vibrato. The amp itself really sounds spectacular, but there is this issue when adjusting the volume. I can't help but think that it has to do with the number of Triode Stages being used. Could it be a phase or cross over distortion issue? The symptom is negated as the volume is increased (hum starts to decreases at the arbitrary "3" position of the volume control), but returns once the volume control resistance level is increased further. (Hum returns at the arbitrary "7" position on the volume control.)

Again thanks in advance for any thoughts and ideas.

[Enzo]

Yes, you have more than one source of hum.

One hum is coming through the amplifier signal path and is controlled by that volume control. The other hum is after that point somewhere. For whatever reason the two sources of hum are out of phase - reverse polarity if you prefer. So with volume on zero, you hear only the second hum, and as you increase the volume control, the early hum increases, but is out of phase with the later hum. They cancel. At 3 or 5 or 7 or wherever, the two reach the same level and cancel out entirely. As you continue to increase the volume, the earlier hum takes over as the louder. and increases.

Solve the hum at zero volume first. Once that is silent, then your other hum should be a straight louder as you turn up, and you can address that.

I hope my clumsy description works for you.

[Randall]

Your description sure enough works for me Enzo. It's brilliant, straight to an issue that would have flummoxed me. You sir are a Ninja.

[Enzo]

Yes, I magically turned your single problem into two problems.

[Mandopicker]

Thank you very much Enzo for the clear explanation. Honestly, I sort of thought this was the case and have been chasing it down for a while. I've tried to pay attention to grounding - what and where - so to speak, but surely am missing something. So, time to focus on Hum #1. More to follow. And of course, anyone following, please do feel free to throw out any suggestions as I dive in...once again! I'll try anything.

[Mandopicker]

Yes, I magically turned your single problem into two problems.

First, HA!

Secondly and thinking a bit more about what you said, "For whatever reason the two sources of hum are out of phase - reverse polarity if you prefer."

From your experience,
Would adding another triode stage - making it a total of "5" in my amp change the characteristics of the hum?

Or would adding a separate stage of filtering specifically for the reverb section in an effort to isolate these triode stages from the first be useful?

Not exactly sure why I am hooked on the idea of stages and how it might affect signal (and hum) phase, but I can't help but feel like there is a connection.

Again thanks for your individual time...

[Enzo]

Sure, and if my car had a flat tire, I could add a fifth wheel to compensate for it.

If you added an inverting stage, then yes, the relative phase relation between the two source of hum would change, and then you'd have hum that simply increased from zero evenly, or was present all the time. In other words adding a stage wouldn;t cure the problem.


And how did we decide the reverb needed anything?

[Mandopicker]

And how did we decide the reverb needed anything?

I guess it's me over thinking it - and me assuming that there is some interaction. But thinking it through a bit since earlier posts I'm gonna spend some time focusing on the area before and around the volume control.

I do appreciate your thoughts...

Keep you'll posted.

Best,

[Enzo]

Turn the volume control to zero, some hum remains, right? Solve that first.

[Mandopicker]

Turn the volume control to zero, some hum remains, right? Solve that first.

Correct and heading right for it.