What to do about hum?

[rutledj]

Thanks for all the suggestions. I've tried everything (except isolating jacks) and nothing cures the problem. I would agree that bad grounding can cause this problem but since the same scheme has worked beautifully in the dumble amp you would think it would work well here also.

I'll keep plugging away at it...

Rut

[mdroberts1243]

Thanks for all the suggestions. I've tried everything (except isolating jacks) and nothing cures the problem. I would agree that bad grounding can cause this problem but since the same scheme has worked beautifully in the dumble amp you would think it would work well here also.

I'll keep plugging away at it...

Rut

My Bluesmaster build is absolutely silent, hum wise, using the distributed grounding scheme and centre tap ground on the heaters.

So, based on your earlier posts let me share what I perceived as the differences in our builds... maybe give you some other ideas.

You talked about taking the cathode grounds to the pot bus-bar and grounding everything at the input jack... I take the bus-bar to the input jack and then run a ground to the chassis from there. At that ground stud I also ground the 1st-tube cathode ground using a separate wire (I don't take the cathode grounds to the bus-bar).

The second-tube cathode grounds and the PI ground is run to a separate ground stud where I also ground the preamp filter capacitors from the PS board.

I run the OT/speaker ground to the power tube cathode ground (or at least very close). The speaker output jacks are also grounded to the chassis (switchcraft).

The primary PS filter (for the power tube plates and screens) is grounded at the same stud I use for the PT secondary centre tap... and I create a twisted pair from this ground and the rectifier board output to run the rectifier board output and ground to the PS board.

Just some ideas to consider... good-luck with the debug!

[mdroberts1243]

Double post due to server error message.

[talbany]

Unusual a Hammond getting that hot just laying out on the bench.. Ive used quite a few Hammond PT's and have never had one get that hot ideling I believe it's telling you something..

[rutledj]

Let me ask this question - If I can acquire a 6v battery and power the filaments with this and the hum goes away does that rule out the gnd problem possibilities?

[drz400]

If you use a 12vdc power supply do they just hook across pins 4 & 5 of the 12xx7 tubes and pin 9 is NA?

Correct and if you want you can run the Power tubes in series and run them DC as well. No need to twist heater wires either.
Here is a better solution and there should be much trouble fitting this one
102-1336-ND from Digikey $29 for one piece, it runs from 100V to 240V , has a trim pot to dial in the voltage to taste and is 12V / 5.4A
Dimensions 4"x2"x1.25" fits on the side of most chassis.
I'm telling you guys I have used this many times and it is a cure all. You can do it without but eve if it is perfectly grounded it will never be as quite as it can be without DC heaters. Also sometimes a tube sounds great but has some heater to grid hum, with DC heaters that never is a problem.

[drz400]

Let me ask this question - If I can acquire a 6v battery and power the filaments with this and the hum goes away does that rule out the gnd problem possibilities?

It would be easier just to slap this Switching supply on there to test, you can just use it to run the preamp tubes if you like. If you do a battery it might have to be pretty serious one to supply the current and might drain quick at 1 amp.

[rutledj]

So what is the correct wiring scheme for using a 12v supply? I mean on the 12ax tubes do they connect in parallel across 4-5 all the way down the line?

[rutledj]

Well, I broke out the lawn mower battery and wired up the heaters to all tubes. Guess what? Hum is still there. aaaargh!

I substituted the switchcraft jacks for the isolated type. No help.

I guess I'll go back and recheck the grounding... again.

Rut

[drz400]

Well, I broke out the lawn mower battery and wired up the heaters to all tubes. Guess what? Hum is still there. aaaargh!

I substituted the switchcraft jacks for the isolated type. No help.

I guess I'll go back and recheck the grounding... again.

Rut

I dont know about using a battery, did you check the voltage?
If you are using DC heaters and still have a hum there is something terribly wrong in the grounding. Personally I like using a buss, ground to chassis at one place, I never get any hum. I dont care for the Marshall/Fender method, they leave too much trust to the chassis.

The other question yes, for 12AX7 you use 4 and 5 and 9 is not used, To use the supply on the power tubes you run the heaters in series.

['67_Plexi]

Thanks for all the suggestions. I've tried everything (except isolating jacks) and nothing cures the problem. I would agree that bad grounding can cause this problem but since the same scheme has worked beautifully in the dumble amp you would think it would work well here also.

I'll keep plugging away at it...

Rut

Not necessarily. It's a black art.
Yes I did get the 60/120 thing muddled up.

Edited......no I didn't get it muddled up....lol...it's me thats muddled.

['67_Plexi]

Put my scope on the output of v1b cap. Got a 60hz signal there so I guess it is definitely heater hum.

Heater hum shows up at double the mains frequency, ie 120Hz, not 60Hz.

Hi Alan,
I think you got the 60/120Hz flipped around... 120Hz hum is always related to the rectified DC supply (B+ ripple is double the mains frequency due to full-wave rectification).

Hi Rut,
The 60Hz could be electromagnetic induction from heater wires or other AC wires (primary/secondary) due to poorly twisted pairs, etc. or ground loops (poor grounding technique)... not just the heaters themselves.
Alan's ideas about grounding and ground loops through the chassis are bang on and the most likely culprit IMHO. The Hoffman article is a good guide to distributed grounding.
I wouldn't think you'd have to go to regulated DC heaters.
Here's another article with some thoughts on hum reduction: http://www.lynx.bc.ca/~jc/hum.html

No Bro, I didn't get it the wrong way around. Any AC supply that has a center tap at a ground potential will be alternating at twice it's original frequency. This could be heaters, or it could be a full wave rectified HT supply. A bit late to the game, but I got there all the same....

[fabiomayo]

Since this thread is back on top... Rut, have you solved your hum problems?

[Bob-I]

I;'ve looked at many of your pics and I see a few things I'd try next.

1) Many of your leads are a bit sloppy in their dress. Precision routing isn't a requirement, but neatness counts. Straighten the leads, run them close to the chassis.

2) I see 2 wires leading off of the cathodes. It appears that one is connected to the rear of the chassis to a ground lug, the other to the front to the ground bus. If what I'm seeing is correct, this is a ground loop by definition. ONE ground point only.

3) While on the subject of cathodes, it looks like you have the trem ckt cathode lead running to the preamp cathodes. I'd break that connection and run the trem to it's own ground lug. There is current flowing there, crossing other wires and possibly creating a ground loop.

4) I hate to criticize someone's soldering work, I have my share of challenges there too. Some of your connections are questionable. Make sure that the wire is stripped as little as possible, meaning the insulation should be as close to the metal as possible. The wire must contact the metal, not just the solder. Some of the leads look like they're inserted in the center of the terminal and soldered in place. I notice this especally on the bare wires the connect 2 terminals on the tube sockets. Bend the leads so they contact the socket and snip off any excess. Same goes for the resistors on the power tubes. Shorten those leads, bend them so they contact as much terminal as possible, then reflow the joints, some look cold.


I've fixed some nasty hums by moving one ground wire. How'd I find, no clue, I just moved them one at a time until the hum left.

On one amp, I'd followed Hoffman's excellent ground plan exactly. It hummed like you wouldn't believe. I moved the cathodes from the ground bus to the chassis, hum gone!!

Another one I moved one power supply lead and silence.

Good luck, take and post pics of the changes and results.

[rutledj]

Thanks all. I had forgotten this thread. Thought I had updated it with my findings. It was a really stupid mistake

The ground for the 2nd B+ cap was not connected to anything! Corrected that and no hum. Amp sounds great, reverb works great as well as the tremolo ckt.

The only thing it doesn't do that I wished it did was distort some! It seems like whenever I implement a Fender build, using the exact same values throughout the preamp, it turns out super clean even at cranked volume. I thought most of these preamp ckts (deluxe, vibroverb, etc) usually started distorting around 3-4 on the volume.

Bob-I: I'll take your advice and go back and make better connections at all points.

Thanks for all your suggestions.

Rut