What kind of noise is this?

[psychepool]

I had previously asked noise related questions about DR style amplifiers.
https://tubeamparchive.com/viewtopic.php?f=6&t=31289

I asked that question because I know that the noise I am having trouble with is background noise that unrelated with turn the gain(preamp volume) knob.
But there was no change with ground wiring in any way.

I know that the tone of the noise differs according to the cause of noise. But I do not know how to distinguish the type.
So I would like to add a sound sample to ask what type of noise is this.
File attached.

The sound that appears in the middle of the sample and disappears is that the preamp volume is turned up and down again.
The noise that I want to eliminate is the background noise.

As you can hear, compared to the noise generated by the volume knob operation, this is not really such a serious noise.
I am using a THD hot plate.
The background noise is much less in -4dB, hard to hear at -8dB, almost completely disappears at -12dB.
And when PPIMV is lowered, it disappears. If decreasing it to the level of home use almost completely disappeared.

Nonetheless, the background noise is larger than the other non-master volume amps that I ever made.
So I am looking for your opinions because I think I can reduce it more clearly than I am now.

I would like to classify the noise type through the sample.

[martin manning]

The steady background noise sounds like 100-120 Hz hum, so likely it's likely coming from rectified DC. I also hear 50-60 Hz hum when the volume is turned up, which could be from heater voltage leaking into the signal path. The hiss that goes up with the volume is typical tube noise.

Update: Here's an on-line spectrum analyzer I found and the results using your sample: https://academo.org/demos/spectrum-analyzer/

The graph shows the frequencies present with time, with the point where the volume is increased at about 1/4 of the frame from the left.

Looks like a handy tool for ampaholics!

[psychepool]

The steady background noise sounds like 100-120 Hz hum, so likely it's likely coming from rectified DC.

Thank you for your analysis!
How can I reduce the 100-120Hz hum?
What are the common causes and solutions for 100-120 Hz hum?

The increasing 50-60 Hz hum when turning the volume knob is not my concern.
Because it feels to me normal level too.

[martin manning]

The first thing I would try is isolating the reservoir capacitor from the other filters. You may have already tried that, but what I would do is connect a separate capacitor to the rectifier output, and ground it directly at the power transformer's center tap. That will keep the high-current 100 or 120 Hz pulses contained to the reservoir charging circuit.

Also, keep the wire connecting the rectifier to the reservoir cap away from sensitive grid leads.

[psychepool]

BE98130B-D465-4311-831A-0F67D911717B.jpeg

ADA60CF4-9775-4CE6-8C5F-AA60B65A46CF.jpeg

Here is the current ground wiring diagram.

Where can I make improvements in the current situation?
Can I separate and bolt down at other PT bolt the heater center and bias circuit ground from main ground?
Or can I replace the 22uF x 2 parallel filter cap with a larger value or other circuit?

The rectifier diode is UF4007, and my country uses electricity of 220V 60Hz.

[martin manning]

You could try this:

[psychepool]

You could try this:

Do you mean to connect the ground of the preamp power capacitor directly to the starground near the input, not to end of the preamp?

[martin manning]

You could try this:

Do you mean to connect the ground of the preamp power capacitor directly to the starground near the input, not to end of the preamp?

Correct.

[Stevem]

You should first try to narrow down the issue to a part of the circuit by yanking out preamp tubes starting with V1.

[psychepool]

Move preamp filter cap ground to input starground -> no change
Move output tubes cathode ground to main starground -> no change

Remove V1(Preamp stage) -> noise reduces about 1/3 ~ half
Remove V2(Reverb send 12at7) -> no change
Remove V3(Reverb return/recovery) -> noise reduces about half ~ 2/3
Remove V4(Phase Inverter) -> no noise

There's no change with turn the TMB tonestack.
But, there are a lot of changes when rotate the presence and ppimv knob.

[martin manning]

How are the output tube grid wires routed? Are the leads from the PPIMV shielded? If so, where and how are the shields grounded? Typically one end is grounded to the bias supply. Have you tried bypassing the PPIMV?

[psychepool]

Originally, I did not use shield cable for ppimv. But noise did not decrease with shielded cables.

lug1 of the ppimv pot are connected to the bias supply. The shield of the shield cable is connected to the other ground point, not to the lug1.

[Stevem]

Is the 1/4" jack of the reverb return and send grounded at it chassis mounting point and the the output rca jack on the pan getting its ground somewhere else?

[psychepool]

Is the 1/4" jack of the reverb return and send grounded at it chassis mounting point and the the output rca jack on the pan getting its ground somewhere else?

Reverb send/return jack is isolate type RCA jack, not 1/4" jack.

The ground of reverb RCA jacks are connected to preamp ground.(to cathode ground of reverb send/return stage)
The ground of output RCA jack on the pan is connected to reverb return RCA jack ground with reverb cable.

[Stevem]

if you unhook the output cable at the tank and short out the rca end does the hum level drop?