amp buzz

[V2]

Hi All,

A new project amp that I built has 120Hz buzz, and so far I haven't found its source. The buzz occurs with no guitar plugged in, no noisy lights nearby, and in a couple of different areas of my home. The buzz is pretty low in volume and is unaffected by front-panel controls. When I turn off the amp, it seems that the buzz cuts out pretty quickly (but difficult to tell if there is some residual buzz fading out). I have no standby.

The amp in question has a 2204-ish preamp with 3 gain stages (4 with CF), full TMB tone stack and conventional MV.

The buzz goes away when I take out the PI tube and when I ground the signal before the coupling cap to the PI; grounding the signal on the CF cathode does not reduce the buzz.

The PT is a toroid with no CT. Full bridge rectifier with short wires to first filter cap.

I have replaced the first three filter caps (up to PI), dropping resistors, screen resistors.

Any thoughts?

[V2]

Here's a photo of the amp in question; some things have changed since the photo was taken, but I wanted to get something for you to see right away.

[jamme61]

sounds like its in the power amp. check the filter caps.

[sunnydaze]

There is a green wire under the input jack that looks like it supposed to be a ground connection. From the photo, it doesn't appear to be connected to anything.

The pre-amp filters look to be grounded at the same place as the reservoir/power filters. In my experience, that can cause noise. Separately grounding the pre-amp filters from the power filter grounds makes for a quiter amp.

mike

[pdf64]

What controls affect the buzz (likely all of them)?

How about if the signal is grounded after each stage in turn?

Have regular diodes been used for rectification, rather than UF, or FREDs?

If so, does a snubber diode across the input and output of the FWB help, eg 0.1uF ceramic HV?

Buzz is usually 60Hz heaters, rather than 120Hz B+; what is the heater ground ref arrangement?

Tried swapping in a known good 12AX7 to each slot in turn?

[Stevem]

You need to pin down the frequency of the noise.
Does your test meter have a frequency test position ?

Take a sheet of aluminum foil and a clip lead and cover the PT and ground it, does that help the niose issue?
Any grid wire that are not shielded and or lay next to the heater wring can be suspect also.
Are you running DC on to power the filaments , if not then your heater wiring is wrong?!
If you are running DC then with the number of gain stages that you have I have never found a need to power anything more than the first preamp tube with DC and the DC needs to be filtered with no less than 15,000uf.

[Jana]

If it is power supply buzz, it should stop immediately when you flip the power off--no fade out.

It looks and sounds to me like you have the same problem I have described many times. Read my posts in this thread:
https://tubeamparchive.com/viewtopic.ph ... sc&start=0

[V2]

There is a green wire under the input jack that looks like it supposed to be a ground connection. From the photo, it doesn't appear to be connected to anything.

The pre-amp filters look to be grounded at the same place as the reservoir/power filters. In my experience, that can cause noise. Separately grounding the pre-amp filters from the power filter grounds makes for a quiter amp.

mike

Good eye, Mike. I really need to upload a current photo, because this one was taken when I was moving components (TMB up in air) and wires and changing grounds. That green wire was an unused chassis connection. So far I have grounded to chassis at the reservoir cap, at the input, and at the PI. Currently it is connected to chassis at the PI (as in KOC's galactic ground scheme).

and the green wire to chassis that pops out from under the power supply board is for the PT screen. That has also been connected to the reservoir cap and currently is connected to AC ground.

[V2]

What controls affect the buzz (likely all of them)?

None of the front-panel controls affect the buzz. There is some extra hiss of course, but I can't hear extra buzz.

How about if the signal is grounded after each stage in turn?

grounding the signal after stage one or two has no effect. Grounding at the PI input or on the output of my MV (before the shielded wire to the PI) kills the buzz.

Have regular diodes been used for rectification, rather than UF, or FREDs?

If so, does a snubber diode across the input and output of the FWB help, eg 0.1uF ceramic HV?

UFs in the HT rectifier; regular diodes in the negative bias rectifier.

Buzz is usually 60Hz heaters, rather than 120Hz B+; what is the heater ground ref arrangement?

There is no 60Hz hum, btw, which makes me think the heaters are fine.

I'm using a SumR PT from London Power, which has a 12V CT heater supply. I'm running each octal on each side of the supply, and I've got the 12ax7s wired for 12V. I've attached the CT to an elevated source (70V); that used to be connected to a virtual CT using two 100R resistors across the 12V.

Tried swapping in a known good 12AX7 to each slot in turn?

Yep.

[V2]

You need to pin down the frequency of the noise.
Does your test meter have a frequency test position ?

Take a sheet of aluminum foil and a clip lead and cover the PT and ground it, does that help the niose issue?
Any grid wire that are not shielded and or lay next to the heater wring can be suspect also.
Are you running DC on to power the filaments , if not then your heater wiring is wrong?!
If you are running DC then with the number of gain stages that you have I have never found a need to power anything more than the first preamp tube with DC and the DC needs to be filtered with no less than 15,000uf.

- Buzz: my meter has no freq-test. Sound sample of buzz attached.

- I don't have shielded grid wires expect from input to V1a. But chop-sticking those unshielded wires does not change the buzz.

- heaters are DC-elevated (please see earlier post for details).

[Meat&Beer]

Sometimes my solder iron station makes things have weird hum noises when it's on.

Otherwise it could be a filter cap/missed connection (as mentioned) a ground not connected, cold solder, proximity issue, a 1K instead of a 100K type of oopsie...

Bust out the chopstick yet?

Probably not your problem, but the main filter caps look physically small to me. What kinda amp is this?

[V2]

Sometimes my solder iron station makes things have weird hum noises when it's on.

Otherwise it could be a filter cap/missed connection (as mentioned) a ground not connected, cold solder, proximity issue, a 1K instead of a 100K type of oopsie...

Bust out the chopstick yet?

Probably not your problem, but the main filter caps look physically small to me. What kinda amp is this?

- My soldering iron buzzes like a mofo, and so I always unplug it during testing.
- my chopsticks have a semi-permenant spot next to this chassis.
- I use high-quality modern electrolytics. The reservoir cap is a Rubicon 100uF 450 V with 10,000 @ 105 deg. lifespan. In the future, I will compare these directly with the usual specialized caps used in guitar amps.

[V2]

If it is power supply buzz, it should stop immediately when you flip the power off--no fade out.

It looks and sounds to me like you have the same problem I have described many times. Read my posts in this thread:
https://tubeamparchive.com/viewtopic.ph ... sc&start=0

somehow I previously missed your post here and in the previous thread.

My output jacks are isolated from chassis. OT common goes to output jacks; grounded at PI.

[R.G.]

OK, here's a step sideways. Does it do this in a different house/building?

Sometimes buildings are "infected" with noise sources. A high impedance ground to the wall socket can cause a lot of problems, not all of which are obvious. And this is only one example.

I remember vividly an audiophile's story of having incurable audio and mechanical hum from his toroidal-transformer power amp. The curious thing is, it only happened after dark.

Turns out he had one of those pellet-rectifier dimmers on his garage's outdoor light bulbs to make them last longer. The diodes happened to all be "pointing" the same direction and so they loaded one half cycle more than the other, and this was enough DC offset on the power line to make his toroid walk magnetically to one side of the BH curve. Removing the diode "dimmers" was a complete cure, although he spent more in replacement light bulbs.

You might try it in a different building. Stranger things have happened.

[V2]

OK, here's a step sideways. Does it do this in a different house/building?

Sometimes buildings are "infected" with noise sources. A high impedance ground to the wall socket can cause a lot of problems, not all of which are obvious. And this is only one example.

I remember vividly an audiophile's story of having incurable audio and mechanical hum from his toroidal-transformer power amp. The curious thing is, it only happened after dark.

Turns out he had one of those pellet-rectifier dimmers on his garage's outdoor light bulbs to make them last longer. The diodes happened to all be "pointing" the same direction and so they loaded one half cycle more than the other, and this was enough DC offset on the power line to make his toroid walk magnetically to one side of the BH curve. Removing the diode "dimmers" was a complete cure, although he spent more in replacement light bulbs.

You might try it in a different building. Stranger things have happened.

Thanks for the advice, R.G. I am planning to take the amp out of the house on the weekend; there is some reason to believe the noise is tied to my house - but I won't know for sure since until I try it elsewhere (or perhaps get a line conditioner). I wanted to jump the gun in case my symptoms point to something easy that I have overlooked.