ODS 124 PCB Build

[professormudd]

OK, Prof, see how you have a single buss bar that runs the full length of the pots? I've learned the that preamp stages and tone stack should be grounded at the input jack, while Presence control and Master volume should be grounded with the PI or reservoir cap. I suggest you cut that ground buss in half and try moving the Presence/MV ground elsewhere.

The presence is not grounded. It was not grounded in any layout I have seen. I can try moving the master volume ground and report back.

[professormudd]

I've heard about something called a "hum balance pot" to inject a cancelling noise to kill hum. Is that an option here?

"Inject a cancelling noise" - no, that's not its function. The hum balance pot takes the place of the two 100R "artificial center tap" resistors for the heater winding, and allows you to "balance" those two resistances for least noise.

Here's a quick test: Turn amp on and let it warm up. Listen for the hum. Now, while listening, flip the power switch to OFF. Does the hum stop immediately, or does it fade out slowly? If it fades out slowly, heaters are not the problem.

I had previously reported that it faded. I was mistaken. It's a very low volume hum so my ears decieved me.

The hum stops suddenly when powered off abruptly.

[pompeiisneaks]

OK, Prof, see how you have a single buss bar that runs the full length of the pots? I've learned the that preamp stages and tone stack should be grounded at the input jack, while Presence control and Master volume should be grounded with the PI or reservoir cap. I suggest you cut that ground buss in half and try moving the Presence/MV ground elsewhere.

Oh that's really good to know, and it makes sense, I don't know if I've had an amp with hum issues that I didn't resolve elsewhere but I always use a common buss bar like this. I'll have to start cutting away those on the tail end and give them a ground near the PI etc.

I definitely ground my PI separately from the other preamp on purpose.

Thanks for that tidbit!

~Phil

[professormudd]

Relocating the MV ground did not change anything.

Since this hum exists even when no preamp tubes are installed and it is 120hz, then it stands to reason it is being introduced after the rectifier somewhere in the power supply, correct? Does it provide any more diagnostic information to include that when I initially flip the standby switch it always starts with a loud thud/hum which then seems to clamp down to a steady hum?

I made a clip: https://www.youtube.com/watch?v=aAV9fd1 ... e=youtu.be

[brewdude]

I wonder if the OT is coupling to the PT?

[pompeiisneaks]

That would be 60Hz not 120, and that does sound like 120 to me in the clip, but it's really quiet, and hard to hear. I might be wrong.

The fact that it's present even with all preamp tubes out, does imply the PI, the power tubes or power filtering.

I'd double and triple check ground connections on the filter caps, ensure the tubes have a good path to ground, and that the OT leads aren't accidentally somehow super close if not mixed up with the grid inputs of the power tubes (output from PI).

Other than that, if having all 12A*7's pulled and you still have the noise does narrow it down pretty clearly.

~Phil

[sluckey]

Don't forget that mismatched power tubes will often cause 120Hz hum.

[professormudd]

Don't forget that mismatched power tubes will often cause 120Hz hum.

The first set I bought for this was a "matched" quad that had one tube that was pretty far off, so I got a new set of JJs. I swear I checked when I got em and they seemed okay but re checking the bias on each one I have 37-38mV on all but one, that's 34. I kicked the bias up to 40mV and that one stayed down around 36. I moved the tube to each and every socket, and it followed the tube. So it seems I have one tube out of whack.

Since I recall these being matched when I got them, I think I might have actually damaged this tube before I redid the filament wiring. There's a particular "incident" I caused when chopsticking that caused some arcs to show momentarily on that tube. A bare piece of filament wire contacted another pin. However I thought I got off lucky as the amp still functions the same, the hum is the same, and the tone is the same as before the incident.

So I either was mistaken when I thought all 4 of these matched when I got them, or I damaged this tube and caused it to be out of whack. If it's the latter (as I suspect) then the new quad probably won't fix the hum, but I've ordered a new set nonetheless. This time from a store that's in my area so maybe it won't take forever for them to arrive.

[xtian]

34 mA vs 38 mA is not unbalanced. That's reasonably well matched.

[professormudd]

34 mA vs 38 mA is not unbalanced. That's reasonably well matched.

Thanks.

I was under the impression that it was on the edge of acceptable tolerance. I think previously Phil had mentioned that beyond about 4mA difference is out of whack. Anyway, I know all these tubes will find their ways into sockets eventually.

[pompeiisneaks]

34 mA vs 38 mA is not unbalanced. That's reasonably well matched.

Thanks.

I was under the impression that it was on the edge of acceptable tolerance. I think previously Phil had mentioned that beyond about 4mA difference is out of whack. Anyway, I know all these tubes will find their ways into sockets eventually.

If I said so, I misspoke, I meant usually 4-5 mA is considered in the "matched" range, and I think I mentioned I wasn't sure but 10-15 starts getting into the 'unbalanced' range, but was hoping someone else could confirm. That is where I'm not so confident

~Phil

[xtian]

There's a young man from my town, hardest working musician, no money for gear. He played this beat-to-hell Carvin X100 with a quartet of mismatched power tubes--four different brands! No hum to speak of, and of course he sounded great.

[professormudd]

Well as you can guess, tubes did not resolve my hum. I also took the opportunity to remove the mounting screws/nuts from my choke to see if that was inducing hum, but there was no change regardless of the position.

So I have at least ruled a few potential causes, so there is a positive spin to put on this maddening hunt for the source of this hum.

[xtian]

I believe you reported NO hum with the PI tube pulled, correct? So you can rule out induced hum from transformers, mismatched or bad power tubes tubes.

As a follow up, what happens if you ground the input to the PI tube? If that kills the hum, then the PI is also OK, and the hum is entering from further upstream.

[gtomax]

When in doubt: http://www.geofex.com/ampdbug/ampdebug.htm