Noise Issue on Note Decay

[R.G.]

You lost me a bit haha. I'm not an engineer.

That just means that you have not wasted the greater part of your adult chasing >>exactly, and in detail<< how electrons mate to make new baby electrons.
EDIT: Fixed left out words "have" and "how". All this time and I still can't accurately type what I'm thinking.

The big things to remember are that (1) all wires are really resistors, and make voltages as V = I*R; and (2) only one place can be designated as 0.000000V at any one time.

Here's the game plan:
1. Move PT Heater CT to filter cap neg.
2. Move PT HV CT to filter cap neg.
3. Single ground wire from filter cap neg. to chassis ground
4. Power tube cathodes will stay where they are (grounded to their individual mounting lugs like in 99.99% of all other amps).
5. Preamp star ground will remain where it is by V1/input/gain control

I think #2 will be really noticeable. #3 may clean up a lot of noise as opposed to hum. Keep in your hip pocket #1, and moving power tube cathodes and preamp star ground. You may need them, or may not. As you said, many amps are done that way.

There is still another chassis star ground point over by the NFB/bias circuit. The filter caps are currently connected to that point, along with the NFB low-pass, and bias circuit. I will need to lift the filter caps from this point so they can ground to number 1/2/3 above.

Hopefully this reduces the noise floor some. It's not perfect, but I can't really rewire this entire amp without completely removing all the boards and desoldering everything.

It's a good plan. Peel just the necessary layers of the onion.

Oh... also, the SMPS for the DC heaters has its own chassis ground over by the power/standby switches. What about that?

I went and looked it up. It's a quality module, with attention paid to keeping the EMI off its input and output. It seems to only isolate the relay coil voltage from the DC voltage that the DC heaters float on. Presumably this is so the relay can be ground referenced to the chassis while the regulated 12V for the heaters that float above ground can also be fed to the input of the converter module. Interesting design choice. I'm not sure what to think, other than it seems complicated.

Same thing for the OT. It's grounded to chassis at the output jacks.

That is a Good Choice. It keeps the speaker return currents flowing back into the OT, not across the chassis to make noise.

Keep in mind that the original issue, noise (hiss?) happens during note decay. That's a big red flag that there may be lurking oscillation/stability issues going on that only happen at certain signal levels. This is a common occurrence in solid state amps, rarer in tube stuff. If I were doing this, I'd get the main grounding issue worked on, along the lines you mention, and then see what the noise on decay does.

[FourT6and2]

Keep in mind that the original issue, noise (hiss?) happens during note decay. That's a big red flag that there may be lurking oscillation/stability issues going on that only happen at certain signal levels. This is a common occurrence in solid state amps, rarer in tube stuff. If I were doing this, I'd get the main grounding issue worked on, along the lines you mention, and then see what the noise on decay does.

Oh... the original issue in this thread was solved and I thought I mentioned that. But might have been overlooked.
That specific noise was coming from a Fortin Zuul+ noise gate. Ironic that a product meant to reduce noise was causing it.

It has a "key" input and output, meant to detect your guitar signal to trigger the noise gate, while keeping the gate in the FX loop of the amp. But the key input is causing issues. I'm trying to work it out with Fortin. But no progress yet.

Besides THAT specific issue, this amp is just noisy in general. Hiss isn't the problem. It's lots of buzzing, humming (60Hz), radio stations with master volumes at zero, and beeping/morse code (no idea what's causing this).
And also plugging in the footswitch for channel changing causes more noise. The amp is quieter without the footswitch connected.

[martin manning]

Here's the game plan...

Interested to hear how much you lower the noise floor with the changes described above.

[R.G.]

+1 Martin!

It's an interesting exercise in how wiring can affect both hum and noise.

I have some other suspicions. The component placement and lead bending is pretty, and so is the careful wire routing. I suspect that the wiring was done with a bed-of-nails wire harness board. The parallel runs and right angle bends are too precise to have been hand placed a wire at a time. The lurking problem there is that unless thought was put into which wires run parallel to which other wires, the capacitive and magnetic crosstalk could be excessive. It's a situation that promotes that. Maybe not, but it's possible.

Writing up Guitar Amp Wiring Notes has left me all twitchy about which wire is where.

[FourT6and2]

UPDATE

OK, in case anybody missed it... the weird noise on note decay was caused by noise gate (ironic). The specific design of the noise gate just doesn't work well. That said, the amp does have some noise that I'm having a hard time tracking down.

I think it's 60Hz hum.

I also tracked down another one of these same exact amps. And it also exhibits the same exact hum. So it's not isolated to my amp. It's a design flaw.

Here was the previous game plan to trouble shoot:

1. Move PT Heater CT to filter cap neg.
2. Move PT HV CT to filter cap neg.
3. Single ground wire from filter cap neg. to chassis ground
4. Power tube cathodes will stay where they are (grounded to their individual mounting lugs like in 99.99% of all other amps).
5. Preamp star ground will remain where it is by V1/input/gain control

I tried this. I lifted all the grounds and tied them together with alligator clips to one point. I tried connecting this temporary star ground to various locations suggested and nothing fixed the hum.

I tried new tubes. No change. I tried new tubes again. No change.

The hum does not get louder/softer with the amp's volume/gain or preamp controls. It is a constant hum.

The ONLY thing that affects the volume of the hum is the bias trimmer. Lower bias = lower hum. Higher bias = louder hum. The preamp is relatively noise free. Maxing out the amp's volume control only adds a bit of normal hiss. No hum.

[Stevem]

I have had this same issue in a few amps and this situation might be the cause in yours .

In particular the Univox U45B model comes to mind as having this issue and its very very prominent in that model amp

Due to how close in the U45B the power transformer is to the output transformer. ( a little over a inch away ) you instantly get 60 hz hum out of the speaker as soon as you turn the amp on even though the tubes are still cold.

This amp has no standby switch so if yours does to test out this potential issue turn both of your switches on at the same time.

If you instantly hear 60 hz hum from the speaker then the amps OT is either too close to the PT, or the OT needs to be turned as in the way Marshall mounts them next to each other in some models, or you have both the issues taking place of 1) being too close and 2) not turned 90 degrees to each other.

Note the OT and PT position in this photo.

I hope this might help in tracking down the root cause of the issue.

[R.G.]

The ONLY thing that affects the volume of the hum is the bias trimmer. Lower bias = lower hum. Higher bias = louder hum. The preamp is relatively noise free. Maxing out the amp's volume control only adds a bit of normal hiss. No hum.

Mmmmmm. Sounds suspiciously like ripple on the bias voltage. Is it possible for you to parallel the bias voltage filter cap with a new(er), and known-good cap? If that changes the hum at all, replace the bias filter cap.

Does this thing have a dedicated bias winding, or get its bias from a cap to one side of the HV winding?

[martin manning]

If the hum is 60Hz then this is a likely possibility. The bias supply is half-wave rectified.

[FourT6and2]

I have had this same issue in a few amps and this situation might be the cause in yours .

In particular the Univox U45B model comes to mind as having this issue and its very very prominent in that model amp

Due to how close in the U45B the power transformer is to the output transformer. ( a little over a inch away ) you instantly get 60 hz hum out of the speaker as soon as you turn the amp on even though the tubes are still cold.

This amp has no standby switch so if yours does to test out this potential issue turn both of your switches on at the same time.

If you instantly hear 60 hz hum from the speaker then the amps OT is either too close to the PT, or the OT needs to be turned as in the way Marshall mounts them next to each other in some models, or you have both the issues taking place of 1) being too close and 2) not turned 90 degrees to each other.

Note the OT and PT position in this photo.

I hope this might help in tracking down the root cause of the issue.

This amp is no different than any other Hiwatt DR504 in that regard. So I doubt that's the problem. But sure, I'll try it.

[FourT6and2]

The ONLY thing that affects the volume of the hum is the bias trimmer. Lower bias = lower hum. Higher bias = louder hum. The preamp is relatively noise free. Maxing out the amp's volume control only adds a bit of normal hiss. No hum.

Mmmmmm. Sounds suspiciously like ripple on the bias voltage. Is it possible for you to parallel the bias voltage filter cap with a new(er), and known-good cap? If that changes the hum at all, replace the bias filter cap.

Does this thing have a dedicated bias winding, or get its bias from a cap to one side of the HV winding?

Both amps were built in 2023. And both amps have the same hum. So I doubt it's TWO bad caps in two separate amps. BUT... yes, I do have a filter cap decade box that I can clip in. Good suggestion, thank you.

Here is a layout for a typical DR504. This amp is fairly similar in terms of the filtering and bias supply.

https://ceriatone.com/wp-content/upload ... ec2021.pdf

And here is a schematic for my actual amp:

Power and Bias.jpg

[FourT6and2]

So here's where I'm at.

1. Appears to be 120Hz hum?
2. Hum is not affected by any of the amp's controls (volume, gain, EQ, etc.).
3. Adjusting bias trimmer does affect volume of hum.
4. Hum disappears when pulling PI tube (V3).
5. Hum disappears when pulling cathode follower tube (V4).
6. I tried paralleling another 100uF cap across bias supply. Did nothing.

All tubes test good and power tubes are matched and bias up perfectly.

PI plate resistors measure 81.9K and 90.6K
Preamp heaters measure 12V DC.

Screenshot 2025-08-16 at 4.25.16 PM.png

[FourT6and2]

Something else I noticed, regarding the design of most 50W Hiwatts, is where the OT CT is referenced. In a typical 50W Marshall, OT CT connects to one side of the choke, along with a 50uF+50uF filter cap (100uF total), to the standby switch, and then to rectified DC right at the output of the rectifier diodes. So you get rectified AND smoothed DC at the OT CT.

In a 50W Hiwatt, there is no filter cap and there is no choke, just a 100R resistor. Could this lead to some ripple?

[martin manning]

So here's where I'm at.

1. Appears to be 120Hz hum?
2. Hum is not affected by any of the amp's controls (volume, gain, EQ, etc.).
3. Adjusting bias trimmer does affect volume of hum.
4. Hum disappears when pulling PI tube (V3).
5. Hum disappears when pulling cathode follower tube (V4).
6. I tried paralleling another 100uF cap across bias supply. Did nothing.

Isn’t the PI V4 and the CF V3? Anyway, if the hum stops when you break the signal path by pulling the CF, and none of the controls affect it, then I would suspect the hum is getting into the signal path after the last control and at or before the CF. Have you tried swapping the CF tube? Failed h-k insulation would be more likely to introduce 60 Hz, but it's worth a try.

[R.G.]

1. Appears to be 120Hz hum?

Yep, that's 120. Thanks for the spectrogram - it shows some interesting stuff. The big first spike is 120, all right; something is also happening at 180, 240, 300, etc. That means that 120 +/- 60 is being created to get those pairs 60 hz apart. I haven't nailed what this is yet, but it's a clue.

2. Hum is not affected by any of the amp's controls (volume, gain, EQ, etc.).
3. Adjusting bias trimmer does affect volume of hum.
4. Hum disappears when pulling PI tube (V3).
5. Hum disappears when pulling cathode follower tube (V4).
6. I tried paralleling another 100uF cap across bias supply. Did nothing.

Hmm. I believe that you already made sure that the PT CT went only to the first filter cap negative. Otherwise that would be the next place to look. In your schematic, I see that the PT CT is where the HV fuse lives. That means that there is a wire from the PT to the fuse, then a wire to the first filter cap negative. That could implicate the wire routing on this wire path, and you might have dirty fuse contacts. Maybe.

Rectifier and transformer snubbing pop to mind next. the 5408 is a standard rectifier, so it's prone to generating RF spikes when the diodes turn off, There's a section on rectifier noise in Guitar Amp Wiring Notes on this. You could try putting 0.01/1kV ceramic caps across each 5408, and also a 0.01 ceramic in series with a 1K resistor directly across the two HV leads from the PT. These dodges help prevent RF ringing from the sudden cutoff of the main diodes. Fast, soft cutoff diodes like FREDs are another way to avoid slam-off diode noise.

[FourT6and2]

Isn’t the PI V4 and the CF V3?

No, not in Hiwatts. The phase inverter is the 3rd tube and V4 is a cathode follower. It's not used for signal amplification. I'm pretty sure it sets the DC bias voltage for the PI and helps provide a low impedance signal to the power tubes.

Anyway, if the hum stops when you break the signal path by pulling the CF, and none of the controls affect it, then I would suspect the hum is getting into the signal path after the last control and at or before the CF. Have you tried swapping the CF tube? Failed h-k insulation would be more likely to introduce 60 Hz, but it's worth a try.

Yes, I have tried swapping the tubes. No change. And make note that the CF is physically AFTER the phase inverter, not before it. The hum goes away if I pull EITHER the PI or the CF tube. Here's a layout of a stock DR504. My amp mimics the relevant circuits, except the Super-Hi has 4 cascaded gain stages and a TMB stack.

https://ceriatone.com/wp-content/upload ... ec2021.pdf