Help needed with sudden 120Hz hum in Mission Amps 5e3

[rdh006]

Hi,

I have a Mission Amps 5e3 which I built several years ago, which has been a great amp, and provided good, reliable service.
Although I built the amp, I am no amp tech. I am in the Caribbean, no amp techs here, to my knowledge.

Turning the amp on today, I have a loud 120 Hz hum, which was not there before.

Here's what I know and what I have done:

The hum is there with no instrument plugged in and goes away with the volume control turned down.

• I pulled v1 -- hum still there.
• Switched v1 for known-working 12ax7 -- hum still there.
• Replaced original v1, pulled v2 -- hum still there.
• Switched v2 for known-working 12ax7 -- hum still there.
• Replaced original v2, switched 1st power tube for a new replacement -- hum still there.
• Replaced original 1st power tube , switched 2nd power tube for a new replacement -- hum still there.
• Replaced original 2nd power tube
• The rectifier tube is a NOS RCA 5Y3. I discovered that the glass tube is loose in the plastic base and thought I might have found the problem. Switched for another rectifier tube which is working in another amp -- hum still there.

At this point, I would think this is pointing to a filter cap -- would the experts here agree? I can see no visual evidence, no blisters, bubbles or discharge.

What would be the best procedure from here? Do I just replace all the filter caps?

What other likely/possible causes might there be?

Appreciate any help.

[52 Bill]

The hum is there with no instrument plugged in and goes away with the volume control turned down.

The fact that the hum goes away when the volume control is turned down is the big clue here.

Have you inspected the connections in the first stages of the amp?

[eschertron]

Do you have a power-supply-capable cap laying around? Jumper it onto each PSU cap in turn, and see how the ripple hum is affected.

[rdh006]

Thanks for the help, 52 Bill.

The fact that the hum goes away when the volume control is turned down is the big clue here.

I assume you are saying that the fact that the volume control affects the hum means that the hum is occurring before the volume control -- is that correct? If that is the case, I don't understand where 120 Hz hum could come from in that early part of the circuit. But, like I say, I am far from being an expert.

Have you inspected the connections in the first stages of the amp?

I have visually inspected these connections, and prodded them with a chopstick, but I can't find anything loose or broken.

Given that the amp performed normally at a gig on Saturday, and exhibited this problem next time out two days later, it would seem likely that either component failure or a broken connection is the cause. Is that logical?

[rdh006]

Thanks for your reply, eschertron. I can try that

[rdh006]

The amp has 3 x 22uF filter caps. I have jumpered a spare 22uF across each in turn -- the hum remains in all cases.

So filter cap failure is apparently not the cause.

[eschertron]

What's odd is that you report the volume control affects hum, but pulling the first tube does not. I'm not familiar with Bruce's 5E3 but I assume the volume control is early in the circuit, before the second gain stage. Unless the tubes designated V1 and V2 are the power tubes? Which tubes did you pull?

Can you post a schematic or layout?

edit: Is this the amp with 4 inputs, so V1 and V2 are all before the volume control? Then there'd be a 12A_7 tube as V3 post the volume before the PI?
edit: Just checked. The Deluxe only has the two 12A tubes. You pulled both and the hum remained?

[rdh006]

I didn't previously pull both simultaneously. When I pull them both, the hum disappears. With either V1 or V2 in on its own, the hum returns.

Thanks for the help.

PS here is a link to the layout, posted here:
http://music-electronics-forum.com/a...5e3_layout.gif

[mozwell]

It may be worth checking if the heater supply has lost its ground reference, If you have a "humdinger" pot maybe the wiper is bad, or if you have 2x 100 ohm resistors, maybe those have gone, or maybe the ground connection is bad at this point.
Double check all ground connections, especially around the input socket(s)

[rdh006]

@mozwell

I have the 2 x 100 ohm resistors. Is there a way to check if they have gone other than just replacing them?

I'll double-check the grounds, as you suggest

Thanks for the help.

[rdh006]

I'm pretty sure this is 120Hz hum -- I know the open A string is 110Hz, and this is just above that, between Bb and B. As far as I know, in my limited knowledge, 120Hz hum is occurring after rectification. Is that correct? Isn't hum from the heater filaments going to be 60Hz?

[Jazz P Bass]

@mozwell

I have the 2 x 100 ohm resistors. Is there a way to check if they have gone other than just replacing them?.

Measure them with an ohm meter?

[Jazz P Bass]

[QUOTE=rdh006;441501]The amp has 3 x 22uF filter caps. I have jumpered a spare 22uF across each in turn

Not necessarily true.

It could be both capacitors.

Just sayin'

What I look for is a reduction in the ripple (Vac) that is on that supply.
Measure the ripple. Attach a new capacitor in parallel. Measure that ripple.
If the ripple on the preamp stage is high and a parallel capacitor lowers it, then that cap is suspect.
The fact that you have a parallel cap in place is not a cure all.
It is a troubleshooting tool.

[Enzo]

And if you measure the two 100 ohm resistors, they will measure 50 ohms, because the low winding resistance puts them in parallel.

[52 Bill]

Thanks for the layout. Does the hum go away when both volumes are turned down or when only one is turned down? Please explain better.

If either of the 1 meg input grid resistors doesn't connect to ground there will be a lot of hum. Make sure that all of the input jacks are tight and the grounds are clean.