Nasty Amp Hum

[Slowy]

The only change was in swapping V1 this morning and that's when the hum started. I never got to hear the replacement.
I took out a 12au7 and replaced it with a 12ax7.

The hum is there regardless and it gets to a volume that makes me very concerned for the speaker so mercifully, it's a brief experience.

So what happens when you put the old tube back in? Could well be a crappy tube.

No change. First thing I did.

[NippleWrestler]

The only change was in swapping V1 this morning and that's when the hum started. I never got to hear the replacement.
I took out a 12au7 and replaced it with a 12ax7.

The hum is there regardless and it gets to a volume that makes me very concerned for the speaker so mercifully, it's a brief experience.

So what happens when you put the old tube back in? Could well be a crappy tube.

No change. First thing I did.

Bums. Could well be crud in the tube socket. Suggest cleaning that out with contact cleaner and pipe cleaner if you have the stuff. On older tubes I've also rubbed the pins in brass wool to remove any corrosion as well.

[Marshmallow]

In Japan, most amp hums are caused by poor house grounding and when the amp is near an internet router. I usually tell them to move the location of the amp and it usually works. Sometimes the unshielded wire coming from volume or input are too close to the components. Could also be the power tube slipping so uneven bias if your amp is a Push pull

[murky]

Assume you’ve tried banging your fist on the top of the amp?

[Bg]

Assume you’ve tried banging your fist on the top of the amp?

Works every time for me

[Litterick]

Uncle Doug says, "listen to the frequency of the hum and it will help you diagnose the problem."



Preamp valves rarely cause hum. The problem is more likely to be something inside the box or the power valves.

[MikeC]

Uncle Doug says, "listen to the frequency of the hum and it will help you diagnose the problem."



Preamp valves rarely cause hum. The problem is more likely to be something inside the box or the power valves.

If the hum is 50Hz it's typically from the heaters. If 100Hz its typically noisy DC (B+) - i.e. bad filter cap(s). A scope or multi-meter that can display frequency is usually necessary. Slowy's hum started suddenly after changing a pre amp tube. Very often a change or a fault that suddenly develops is due to the last thing that occurred before the change or fault occurred. Even if it seems unrelated.

[Slowy]

Uncle Doug says, "listen to the frequency of the hum and it will help you diagnose the problem."



Preamp valves rarely cause hum. The problem is more likely to be something inside the box or the power valves.

If the hum is 50Hz it's typically from the heaters. If 100Hz its typically noisy DC (B+) - i.e. bad filter cap(s). A scope or multi-meter that can display frequency is usually necessary. Slowy's hum started suddenly after changing a pre amp tube. Very often a change or a fault that suddenly develops is due to the last thing that occurred before the change or fault occurred. Even if it seems unrelated.

My hum's not like that. It's a deep, loud, speaker cone flapping dead short sort of hum. I'm not going to turn it on again to play around with because this isn't a hum you can tolerate and I don't think the speaker would for long either.

[MikeC]

Uncle Doug says, "listen to the frequency of the hum and it will help you diagnose the problem."



Preamp valves rarely cause hum. The problem is more likely to be something inside the box or the power valves.

If the hum is 50Hz it's typically from the heaters. If 100Hz its typically noisy DC (B+) - i.e. bad filter cap(s). A scope or multi-meter that can display frequency is usually necessary. Slowy's hum started suddenly after changing a pre amp tube. Very often a change or a fault that suddenly develops is due to the last thing that occurred before the change or fault occurred. Even if it seems unrelated.

My hum's not like that. It's a deep, loud, speaker cone flapping dead short sort of hum. I'm not going to turn it on again to play around with because this isn't a hum you can tolerate and I don't think the speaker would for long either.

The way you describe it sounds like "motor-boating", a very low frequency oscillation. I reckon you should get it to a tech.

[RectifiedAmps]

Divide and conquer! Pull out all of the preamp tubes - just leave the power tubes in and power it up to see if it hums. If it does, the issue is in the power amp section. If the hum disappears it's the preamp, start adding one preamp tube at a time, starting from the socket closest to the power tubes. Note the one that makes the hum come back.

[Slowy]

If the hum is 50Hz it's typically from the heaters. If 100Hz its typically noisy DC (B+) - i.e. bad filter cap(s). A scope or multi-meter that can display frequency is usually necessary. Slowy's hum started suddenly after changing a pre amp tube. Very often a change or a fault that suddenly develops is due to the last thing that occurred before the change or fault occurred. Even if it seems unrelated.

My hum's not like that. It's a deep, loud, speaker cone flapping dead short sort of hum. I'm not going to turn it on again to play around with because this isn't a hum you can tolerate and I don't think the speaker would for long either.

The way you describe it sounds like "motor-boating", a very low frequency oscillation. I reckon you should get it to a tech.

It's definitely a Tech job.

[Slowy]

Divide and conquer! Pull out all of the preamp tubes - just leave the power tubes in and power it up to see if it hums. If it does, the issue is in the power amp section. If the hum disappears it's the preamp, start adding one preamp tube at a time, starting from the socket closest to the power tubes. Note the one that makes the hum come back.

This I can do. Instinct tell me it's power amp but I'll do this and report back. Not tonight though.

[Slowy]

Divide and conquer! Pull out all of the preamp tubes - just leave the power tubes in and power it up to see if it hums. If it does, the issue is in the power amp section. If the hum disappears it's the preamp, start adding one preamp tube at a time, starting from the socket closest to the power tubes. Note the one that makes the hum come back.

20230819_161556.jpg (221.77 KiB) Viewed 3465 times

D'ya reckon this might have something to do with it? V2.


Interestingly for those of you who know and love this amp, it had been running a 12AY7 in V1. The 12AX7s I tried didn't impress so I've gone for my go-to 5751.

[Bg]

Should have gone to specsavers

[sizzlingbadger]

You replace the tube in V1 but manage to bend the pin in V2, now I'm confused.