Solved hum problem. Can anyone tell me why?

[fabiomayo]

I had a hum problem on my 50w HRM build. Layout is very different
from the real thing. It used to hum like crazy, similar to how rut described
his amp. I tried elevating the heaters to around 20v, tried shielding the
preamp heater wires, different grounds, messed up with the relay supply,
tried a bunch of things... Nothing...

Yesterday, I isolated the culprit. The very first preamp stage. I was
suspicious of the input shielded wire from jack to grid resistor at the tube socket.
Exchanged that to a long shielded cable that ran outside of the amp.
Noise almost gone. Trimmed it and put it back with the original length,
noise was back... I tried wraping a bit of aluminum paper around the shielded wire and grounding it, noise better. I tried rerouting the wire
underneath the preamp board, still noise. I put an aluminum back plate
where the amp touchs the wood: NOISE GONE (or almost, but that's for
another day).

Two questions:

1) Why this fix killed the noise? An external source seems logical, but what?

2) Why the residual noise diminishes even further when I touch the aluminum chassis?
I'm using a properly grounded three-pong cord...


Thanks!

[DonMoose]

You have a 3-prong cord, but the ground/earth lead may not be adequately bonded to the chassis - a 'star'-style lockwasher helps dig in.

Hope this helps!

[fabiomayo]

Thanks, DM.
I tried puting a jumper (with alligator clips) between the chassis and
the IEC socket ground lug. I was thinking the same thing as you. It did
not have the same effect as putting my hand on the chassis.

My grounds are admitedly very poor But they seem to be holding
up. Could the quality of the aluminum matter? I used a blank I got from
a local naval industry. It was in like new condition, but laying around on
their warehouse.

[Fischerman]

I have an amp that does what I think you're describing. If I 'shield' that area of the amp the hum goes away. This amp just has a bug where some wires are microphonic around a certain socket and it hums. If I put my finger near the components the hum increases...until I touch the chassis with my other hand then the hum decreases (yes I was very careful!). Shielding that area in my amp is just a band-aid and it's still not totally quiet. I got tired of wrestling it and just shield it (it's not that bad esp with the shield).

[fabiomayo]

I have an amp that does what I think you're describing. If I 'shield' that area of the amp the hum goes away. This amp just has a bug where some wires are microphonic around a certain socket and it hums. If I put my finger near the components the hum increases...until I touch the chassis with my other hand then the hum decreases (yes I was very careful!). Shielding that area in my amp is just a band-aid and it's still not totally quiet. I got tired of wrestling it and just shield it (it's not that bad esp with the shield).

That describes my problem exactly! I can live with what is left of hum
even though it makes me want to build another ODS. I guess this is
a good thing

[gahult]

Here's one for ya.
A friend of mine was a repair tech. at a local TV repair shop. Also into Ham Radio. When he ran into trouble with noise in his receiver, at a couple different locations he would take his baseball bat , I thought he had lost it, and go out and beat on the telephone poles nearby. As it turned out they were the power poles with transformers on them. He would have his mobile rig with him turned on. When he hit on a pole and the noise changed he found the source of interference. A bad transformer, which he would then report to the power company. They would come out and replace it and the noise would be gone. My guess (and his) is that nearby lightening strikes and age would cause internal shorts, not enough to blow that transformer, but some would make a lot of noise that was carried in one the AC line.
Grounding would not cure this.
He would do this on customer service calls also. Had to be a hoot to watch him go about his business, so to speak.

Gary

[Fischerman]

That describes my problem exactly! I can live with what is left of hum
even though it makes me want to build another ODS. I guess this is
a good thing

Yea, after I replaced every component at that stage...and every component in the stage before it...and every component in the stage after it...including the actual wires and socket...I decided to punt.

[tubedogsmith]

Where are you grounding that shielded wire at?

[fabiomayo]

Yea, after I replaced every component at that stage...and every component in the stage before it...and every component in the stage after it...including the actual wires and socket...I decided to punt.

Well... At least I didn't go that far...

He would do this on customer service calls also. Had to be a hoot to watch him go about his business, so to speak.

That's crazy!!!! Imagine that... The customer's face...

Where are you grounding that shielded wire at?

At a terminal bridge right next to the tube sockect. The other side of the
shield is taped. I did however tried disconecting the shield and using a
jumper to ground it at different locations in the chassis... No good...

[tubedogsmith]

You may have burned through the hot lead on the grid wire. Try replacing the shielded wire with a new one and grounding it at the jack.

[tubedogsmith]

You may have burned through the hot lead on the grid wire. Try replacing the shielded wire with a new one and grounding it at the jack.

[Bob-I]

I think part of your issue is the closeness of the board to the tube sockets. I'd try moving the board as close to the relay board as possible, maybe evening moving the relay boards to get more space for the main board.

Also, instead of running the heaters above the board, run them against the chassis next to the tube sockets. I really like the Soldano method of running parallel heater wires. I use 3/64" brass rods running alongside the tube sockets.

[fabiomayo]

I think part of your issue is the closeness of the board to the tube sockets. I'd try moving the board as close to the relay board as possible, maybe evening moving the relay boards to get more space for the main board.

Also, instead of running the heaters above the board, run them against the chassis next to the tube sockets. I really like the Soldano method of running parallel heater wires. I use 3/64" brass rods running alongside the tube sockets.

Thanks, Bob-I. It was a indeed a mistake trying to pull this off in such a tiny space. The heater against the chassis was difficult to achieve due to my fake D-style lead dress, with loops around the tube sockets. I saw the parallel thing you did and it got me curious. I'll try that for sure.

In any case, most of the noise is gone and I'm happier now. I just wish I could understand it.

[Fischerman]

fabio,
Here is a build log of a guy who built a SLO clone and he did it real meticulously...looks real purty. I just built a SLO and mine doesn't look as good but it's def a cool way to wire the tubes. Keep in mind that the tube sockets are 'clocked' such that everything lines up...you even have to use certain sockets (Fender-type if the mounts are like the SLO chassis).

[Bob-I]

fabio,
Here is a build log of a guy who built a SLO clone and he did it real meticulously...looks real purty. I just built a SLO and mine doesn't look as good but it's def a cool way to wire the tubes. Keep in mind that the tube sockets are 'clocked' such that everything lines up...you even have to use certain sockets (Fender-type if the mounts are like the SLO chassis).

I will say, the SLO and the Dumble are completley different animals. I built a SLOClone using a turrett board and used the wire loops shown in this build. I tried that with a Dumble build and the results were terrible. It oscillated like crazy and sounded like a bad Fuzz Face. I reworked the lead dress as per a Dumble and the amp sounds great. Don't ask me to explain.