Tweedle Dee HUM... grrr

[MHProd]

So, for no apparent reason, my TD started to hum like crazy. Not a buzz, it’s a low pitched, muffled sounding hum.

It’s only present with the volume turned up, on either channels, with or without anything plugged in.
I built that amp a few years ago, never had that issue.

With V1 removed, the hum is barely audible but it’s there. With V2 removed, it’s gone completely. I changed V2 but hum is still there.

Where should I start?

Thanks!

[pompeiisneaks]

Depends on if it is a ground hum or a filtered rectified hum.

It sounds like it's around V2 since that tube stopped the hum. Check for poor ground connections, or check the filter node that's linked to that specific node and see if it's having a bad ground or the elytic has started going south etc. You can also try to chopstick the wires/components around V2 to see if that solves it.

~Phil

[MHProd]

Thanks!

How would I check for rectified hum?

Continuity for ground is good everywhere.

Chopsticking doesn’t seem to indicate anything.

Also, how could I test if it’s the elytics? Just unsolder and measure them?

Hum issues are super common but in over a decade of building amps, it’s a first. My builds are always close to dead silent. I’ve been lucky I guess.

[pompeiisneaks]

The elytic for V2 is the third from the power section side, of the 33uF caps (there's a 4th over on the side closer to V1, but that's for V1.)

IT could be having issues getting ground, or it could have gone south.

To tell you'd need to determine if the hum is 50/60hz or 100/120hz (depends on where you are in the globe.) 50/60 is mains hum and 100/120 is rectified hum from filtering issues.

It could be ground loop hum too if something in the ground chain anywhere is suddenly poor, like the connections to the ground for the V2 triode. Look over the wires that connect to ground for corrosion or the like as well...

~Phil

[MHProd]

Ok, so I’ve determined it’s a 60hz hum (played on youtube, exact same pitch).

If it was a ground loop, wouldn’t it have been present before? The scheme I used is pretty similar to the one in the layout of Charlie Wilson in the files section.

I checked the continuity in the power section, everything is fine but I might reflow the joint to be sure. What else could I check?

[pompeiisneaks]

There are a few ground connections around V2. The one from the cathode that links the pots and it's ground to the same location as the V1 filter caps ground (the 33uF cap) there's also the ground off of the cathode of the PI side of that one, going through the balance pot etc. Make sure both have good grounds. I would guess some photos of the amp if it differs significantly from the layout in the tweedle dee deluxe thread would help.

Also check that none of the higher voltage PT/OT leads somehow got close to the signal path, and ensure your heater lines 100 ohm virtual center tap didn't somehow get lifted or the like.

~Phil

[MHProd]

Thanks Phil.

I want to confirm something: if the grounding scheme was an issue, I would have had the hum from the start, right? So assuming it’s not a design issue,since the hum is new, I guess it would be a mechanical one and the main culprit in that case would be a bad ground connection?

Checked lead dress, making sure the grid and plate wires are well away from each other in the preamp.
The layout is identical to the original build with the exception of the artificial heaters ct which is between the pt and the board. Also, power chord ground and pt ht ground are tied together near the rectifier. I’ll try to send the mains filters and and heaters ct to that ground to see if it changes anything.

[pompeiisneaks]

Thanks Phil.

I want to confirm something: if the grounding scheme was an issue, I would have had the hum from the start, right? So assuming it’s not a design issue,since the hum is new, I guess it would be a mechanical one and the main culprit in that case would be a bad ground connection?

Checked lead dress, making sure the grid and plate wires are well away from each other in the preamp.
The layout is identical to the original build with the exception of the artificial heaters ct which is between the pt and the board. Also, power chord ground and pt ht ground are tied together near the rectifier. I’ll try to send the mains filters and and heaters ct to that ground to see if it changes anything.

In theory, yes, in practice, over the years, something somewhere may have vibrated or had corrosion build up, wires can and do move a little when the amps are moved etc. If the amp has sat in the exact same location for the entire time w/o any movement, maybe.... The grounding scheme could have 'changed' due to older paths that used to work fine, being less reliable from some corrosion, or bad solder joints etc, therefore causing the grounding to seek new paths for the electron flow.

I wouldn't adjust the grounds that were working, just ensure they didn't somehow get 'worse' measure resistance to ground from the board itself and the other ground points to see if somehow something now has higher resistance to ground instead of near 0 ohms. ( or just use continuity mode ) A common one is input jacks that get more and more 'loose' over time or build up corrosion. That should be ruled out on yours due to the fact that the hum didnt' dissapear with V1 removed, but it did with V2, so that means the source of the hum is around the V2 part of the circuit.

~Phil

[MHProd]

Measured resistance to ground everywhere and everything is between 0.1 and 0.2 ohms. As I mentioned before, continuity is good everywhere. Checked the inputs, all is good. Most of the hum goes away with V1 but not all. It does with V2 though.

I’ll reflow the joints and report back.

[MHProd]

Success!!!

I noticed I had grounded the bleeding resistor from the first cap near the power tubes. I removed it and soldered another one across the cap this time. Most grounds looked good but redid them just in case. I also moved the mains ground to the heaters ct.

I’m not 100% sure what the issue was but the amp is dead silent now.

Thanks again Phil!

[pompeiisneaks]

Great to hear!

~Phil

[norburybrook]

shouldn't the mains ground always be a dedicated one . short/secure and near the input with nothing else connected?




M

[pdf64]

shouldn't the mains ground always be a dedicated one . short/secure and near the input with nothing else connected?

I think that’s all good apart from ‘short’; the ground should be a little longer than the neutral and live, such that if the power cable / inlet got yanked out somehow, live / neutral will be pulled off their terminals first, ground will be last. ie the equipment will stop working before it loses its safety grounding.
Another point to note is that the ground wire needs to be physically secured to the eyelet / lug, eg wrap it around the lug/through the eyelet; we mustn’t rely on the solder to retain it to the terminal. Because if it’s passing fault current, the solder can melt, and we can’t risk the ground wire (if under tension etc) flirting off the terminal. Hence manufacturers often use crimped connections.

[norburybrook]

shouldn't the mains ground always be a dedicated one . short/secure and near the input with nothing else connected?

I think that’s all good apart from ‘short’; the ground should be a little longer than the neutral and live, such that if the power cable / inlet got yanked out somehow, live / neutral will be pulled off their terminals first, ground will be last. ie the equipment will stop working before it loses its safety grounding.
Another point to note is that the ground wire needs to be physically secured to the eyelet / lug, eg wrap it around the lug/through the eyelet; we mustn’t rely on the solder to retain it to the terminal. Because if it’s passing fault current, the solder can melt, and we can’t risk the ground wire (if under tension etc) flirting off the terminal. Hence manufacturers often use crimped connections.

absolutely, yes, I didn't mean short in that sense for the reason you mentioned but close to where the mains IEC socket is

If there's one thing I make sure I do correctly it's that one connection, belt and braces

As someone who's come into this without any 'training' I've always taken safety very seriously and paid close attention. now I'm a little wiser I can be a bit less cautious (Frightened) as I now know the areas I need to be careful of.

Erwin said recently after 6 months in storage an amp or was it a dumbleator still had 110vDC stored in a cap!!!


M