HUUUUUMMY new build

[dehughes]

Hey all,

So, I just finished building this amp, and am kinda perplexed at how I cannot get rid of VERY strong AC hum coming out of it. I've made some minor changes to the schematic, but it's mostly what you see here:



I've tried lots of things.....and still can't get rid of the hum. The PT is a Brown Deluxe clone from Mercury Magnetics, so I know it's a solid PT from a reputable builder, AND I know what moderate PT hum sounds like, but what I'm experiencing here is waaaay more than what should be present.

What I do know: The PT hums audibly when on standby, but no more than other amps I've built/repaired/owned which were perfectly quiet when operating. When taken off standby, it hums a TON, but removing the PI kills most of the hum. Removing the first 12AX7 tube doesn't so much....neither does just removing the 6SN7. Removing all the preamp tubes kills as much hum as removing the PI, but even with only the power tubes and recto in place it still hums a bit more than it should....but still less than with all the tubes in there. The guitar signal gets through just fine...but...even with both channels' volume knobs turned all the way down, I can still hear a bit of my guitar signal coming through (bad pot? loop?).

Perhaps I wired the heaters incorrectly? Grounding issues? I'm at a loss, and am open to suggestions.

Here are a few more pics:









I've moved a few things around, but this is mostly what it looks like. Any and all help is appreciated. Thanks!

[txstrat]

Did you use separate ground leads for the main filter caps and the preamp section? That's what caused hum in my 6g3 clone in the first place.
Although it looks absolutely perfect it might be a cause for hum to place all those wires paralleled. Signal wires ought to cross/get near each other in some sort of an angle (90 degrees at best).

[Euthymia]

Clean-looking build! Very tidy.

Do you get any change in hum when you switch that "extra filtering cap in and out?

Is this a proven design that someone you know has already successfully built?

Design-wise, it looks a little light on the PS filtering caps. I'd jumper in a 47/450 across that screen node cap or upstream one node and see what happens.

If that doesn't change anything, where is your heater winding center tap connected?

Do you have an oscilloscope?

[tubeswell]

And did you try subbing in other tubes?

[theunrulychef]

Man, that's one purdy lookin' build you got there. Very nice.

You say it gets better when you pull the PI. I can't tell from the pics, but do you have it grounded with the pre-amp buss, going back to the input side? If so, maybe some hum is getting introduced to earlier stages and then getting bigger as it goes through the amp. You could try grounding it with the power section.

You have a tone cap grounded on the pot back. Is that pot back chassis grounded? I've had some nicer pots that you had to make sure the backs were actually grounded to the mounting panel - didn't have the little tabs.

Heater wiring, they look like they could be running pretty close to the board. Have you tried pulling them back a little, with a little more airspace between them & the board?

[loudthud]

Wow terminal strips! I wish I could find a good price on those. One problem is the way all those 16uF caps have the common ground wire. The grounds for the PI and the preamp filter caps need to go to the ground wire behind the pots.

[Enzo]

I hate to say bend the wires, but try lifting those heater supply wires away from the signal carrying stuff for a listen.

[bnwitt]

What Loudthud said and what Enzo said.

You have two simultaneous channels coming off of your single input jack and converging at the 220K summing resistors before the phase inverter. The 6SN7 branch and the 12AX7 branch. So, I have the same question as Enzo. Is this someone's proven design or a one off experiment? Is there a reason you chose not to have a tone control on the 6SN7 section?

What happens to the hum if you disconnect the output of the 6SN7 or the 12AX7 section one at a time at their respective 220K resistor? Why do you have an 8uf ps filter cap on the 12AX7 and a 16uf ps filter cap on the 6SN7? There are some really long runs on some of your preamp section wires that may be a part of the problem. While your wires are all nice and straight and sharply angled I have to say that every time I have tried to wire an amp like that it was noisey. I have found that the shortest possible wire runs and keeping grid wires away from plate wires is the best way to go and many times that requires flying wires rather than neatly angled runs.
Below are two links to a Princeton Reverb build I did some time back. The first was straight angled wiring and was noisey as all get out. The second was flying wires away from each other and was dead quiet.

http://i298.photobucket.com/albums/m...tt/HPRguts.jpg

http://i298.photobucket.com/albums/m...t/P9270430.jpg

[loudthud]

The grid wires where the source is a volume control or a pair of 220K resistors or the input to the PI will be sensitive to noise pickup. Keep wires as short as possible and away from the heater wires. Consider shielding them.

[dehughes]

Thanks for your help, guys. Muuuuuch appreciated. Here's my attempt at replying to all this great help:

1) Txstrat: Separate ground leads? You mean not grounding all the electrolytics at the same point? If that's what you meant, then no, I have the two preamp electrolytics grounded at the end of the preamp buss bar, and the PI and CT and power tube plate electrolytics at the other end, connected to the tranny ground. I'm considering putting the PI electrolytic on the preamp buss bar and seeing if that helps the issue...

2) Euthymia: Switching in the extra filter cap changes nothing in terms of hum. It is my own design (r.e., I pieced together parts of existing designs...), so no, it's not been tested. As for the filtering, I used filtering in conjunction with the circuits I'm "borrowing": 6G3 Deluxe, Komet Constellation, Top Hat Club Deluxe, 5E3 Deluxe. The heater CT is connected to the tranny bolt along with the PT secondary CT, the electrolytics for the power supply, and the mains ground. I don't have an oscilloscope...unfortunately.

3) Tubeswell: I tried different rectos and power tubes, but I've not tried different preamp tubes. The reason I suspect that its not the preamp tubes, is that even with only one channel's worth of preamp tubes, the noise still persists, which leads me to believe it is a design issue and not a tube one. I will, however, swap in different 12AX7s just for due diligence's sake. Don't have any spare 6SN7s....

4) theunrulychef: I do have the PI grounded on the preamp buss bar, but the electrolytic is on the poweramp buss bar. Perhaps that's the issue? I'd think moving the electrolytic to the preamp bar is a better solution than moving the PI ground to the power amp buss bar... I did move the tone cap off the pot and it is now on the preamp buss bar...just did that after the pics. Pulling the heater wires away from the terminals makes no difference...tried that already.... They aren't as close as they appear in the pics...and I've had other amps similarly wired which didn't suffer from this hum problem, so I doubt that's it....though I'll go move them around again just to be sure.

5) loudthud: Good idea. You're the 3rd or 4th person who suggested putting the PI filter cap on the preamp buss bar along with the PI itself and the other preamp tubes. I'll have to do that...

6) Enzo: I moved the preamp wires around to no avail, and I've tried pulling the heater wires away from the terminal strips as well. I'll continue to try this, though, if the other suggestions don't fix the problem.

7) bnwitt: I chose not to put a tone control on the 6SN7 channel partly out of respect for the Komet design, and partly out of a curiosity to see how much "less is more" I could handle in a guitar amp. It sounds really good...save for the rampant hum, so I think that channel will turn out nicely once I get the hum fixed. I've not tried taking the 220k resistors off to check and see what happens to the hum. I do know that removing one channel's tube doesn't fix the hum. What would separating the 220k resistor show? 8uf and 16uf caps are the stock values for the 12AX7 (6G3) and 6SN7 (Constellation) circuits, so I stuck with that. As for the wire bends, I've built an AC30-ish creation that was even more complex that this one (more pots, wires...ech...) in the same fashion, and it didn't hum like this, so I suspect it's a grounding issue and not a wire-bend thing. Plus, I've moved wires around and it didn't change anything....and the hum is a AC hum (like heaters...or sketchy ground) not the typical parasitic noise from gainy circuits with close wires. You could be right, though, but my instincts lead to checking out some of these other options first.

So, THANKS. I'll get to working through your wonderful list of suggestions and will post back. You guys are wonderful....

[bnwitt]

does the hum stop with the pulling of any one tube? Does the amp hum with all preamp tubes pulled? Does the amp hum with all tubes but the rectifier tube pulled?

[dehughes]

does the hum stop with the pulling of any one tube? Does the amp hum with all preamp tubes pulled? Does the amp hum with all tubes but the rectifier tube pulled?

The hum stops when the PI is pulled, but no less then when all three preamp tubes are pulled.

It hums with all preamp tubes pulled, but far less than otherwise.

Haven't tried that last one....good suggestion! I'll do that...

[dehughes]

So, what about some issue pertaining to the fact that the 6G3 12AX7 side has the cap on the B+ side of the 100k plate resistor, and is then mixed into the 6SN7 channel before the PI? Might that be a problem somehow?

[txstrat]

That cap is in between the 100k and the 15k resistor due to gain factor. That way the next stage gets less signal strenght. Shouldn't be a cause for your hum problem.

[dehughes]

That cap is in between the 100k and the 15k resistor due to gain factor. That way the next stage gets less signal strenght. Shouldn't be a cause for your hum problem.

Right on. Thanks. Wasn't sure if mixing the channels after that stage (instead of before, like the 6G3) would cause trouble for the 6SN7. Good to know.