BIG HUM in Point-to-Point 5F2A build

James Knox

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I’ve been learning about building Hammond Box amps P2P rather than on a circuit board. Experimenting with Lead Dress, Grounding Schemes, and Component Layout.

On this particular build, I’m utilizing the grounding concept from the Valve Wis of grouping “stars” along Single buss bar. Wasn’t working so great but I decided to stay with the “stars” grouped by Filter Cap. The Wis advises keeping the power supply (Full Wave SS Rectifier in my case) on its own “loop” (the first Filter Cap). I then grounded the power tube Components on the second Filter Cap and the Preamp Tube components on the 3rd Cap.

Here is the pic of the Wis hybrid star/bus idea...

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Here is a pic of what I did...

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The red circle is the Power Supply, 1st Filter Cap. The yellow circle is the 6V6, 2nd Filter Cap grounded to the chassis via a grounding lug, and the green is the 12AY7 preamp, 3rd Filter Cap, grounded at the input jack.

Im getting a very bad (60 cycle) and very loud hum. Here are the steps I have taken to mitigate it. No difference turning Vol/tone/NFB pots.

Continuity check all grounds, reflow all ground solder joints, Taken voltages, triple checked schematic following signal path, tried different tubes, jumpered in new filter caps, tried a 6v lamp battery for heater filaments (bust, not enough amps), chopsticked the heck out of the circuit, rewired the heater string from lamp to 6V6.

Voltages are B1= 333vdc, B2=216vdc, B3=179vdc. On the 6V6, Plate is 313, Screen is 211, Grid is -2.08, and Cathode is .005.

After a couple of days researching and staring at it, I’m stumped, lol. Here are some closeups for some fresh eyes...

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FFC97F6A-FE6D-49F0-ABD9-0571E58C96BD.jpeg
 
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schmee

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I didn't really look at what's there very close but I had severe noise on my first 5E3 until:
Keep all the power supply/filter/power amp grounds at or near the PT.
Keep the input grounds and preamp grounds at the input jacks away form the power supply.
 

Lowerleftcoast

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Can you tell if the hum is 60Hz or 120Hz?

60Hz will be the 6.3V heater supply or ground problem. 120Hz will be power filtering troubles.

The heater wiring is mighty close to that blue signal wire.;)
 

James Knox

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Can you tell if the hum is 60Hz or 120Hz?

60Hz will be the 6.3V heater supply or ground problem. 120Hz will be power filtering troubles.

The heater wiring is mighty close to that blue signal wire.;)

Definitely 60HZ

I chopsticked the blue signal wire an inch away from the twisted heater wire... no change.
 

James Knox

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If you first used Valvewizard grounding why did you change it and connect 2nd filter cap to chassis?

My first attempt as a single bus grounded at the input jack with 2 stars. I had the loud hum, So I thought to cut the long single bus bar and ground the 6V6 components near it with its own chassis ground. Didn’t help, still hummn’...
 

Lowerleftcoast

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60Hz
The input and output jacks are grounded properly good solder joints etc.

The other grounds look OK.

Faraday Cage is not complete yet. Maybe a baking sheet temporarily on the bottom opening to check it out?

Lead dress... this one is not your best work JK. Two things that glare at me with this one: -Signal wire(s) not utilizing the chassis for a partial shield.
-Heater wires not using the chassis as partial shield.

You are using the chassis for the power ground path. (The current is flowing from the input jack chassis mount to the chassis ground lug on the other side of the 6V6.) This part of the chassis is no longer a good (shield) place to run that blue signal wire. You don't want the signal wire hanging in the air like it is. Best to put the power ground path back to single point and use the chassis to help shield that blue wire, imo.

The heater wires can be located straight below the sockets so they can touch the bottom of the Faraday cage (the now open side of your chassis). That will get them well away from the other wires and provide some shielding.
 

James Knox

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Faraday Cage is not complete yet. Maybe a baking sheet temporarily on the bottom opening to check it out?

I have the bottom panel of the Hammond Box. I put it on and it made no difference.

Lead dress... this one is not your best work JK. Two things that glare at me with this one: -Signal wire(s) not utilizing the chassis for a partial shield.
-Heater wires not using the chassis as partial shield.

haha, yeah kinda embarrassed to post pics. I have soldered and unsoldered and moved and rerouted a bunch. Looks haphazard cuz I keep changing stuff.

You are using the chassis for the power ground path. (The current is flowing from the input jack chassis mount to the chassis ground lug on the other side of the 6V6.) This part of the chassis is no longer a good (shield) place to run that blue signal wire. You don't want the signal wire hanging in the air like it is. Best to put the power ground path back to single point and use the chassis to help shield that blue wire, imo.

I originally had run one long bus from the tag strip by the 3rd Filter Cap to the input jack and terminated by the Tone Control with all the 6V6 grounds creating a *star there. Because of the hum, I cut the bus and added the chassis ground lug near the 6v6. I really wanted to have a single bus system. It appeared to not be working, so I tried something else (separating the preamp grounds from the power grounds).

The blue signal wire is hanging in the air onl because I read the heater wires. It was first snugged up against the chassis. I’ll put it back there.

The heater wires can be located straight below the sockets so they can touch the bottom of the Faraday cage (the now open side of your chassis). That will get them well away from the other wires and provide some shielding.

My first version had the heater wires running along the floor of the chassis. In an attempt to stop the hum, I rerouted and flew the heater wires from the lamp to the 6v6. I have hesitated flying the heater wires from the 6v6 to the 12AY because of all the resistors airborne to the ground bus. I thought it might cause oscillation interference. Do you think I should fly them instead?

Trying to figure out Best Practices for P2P is proving a steep learning curve for me. I’ve built about 5 of these and all were fine, sounded great, no noise, and now all of a sudden I have 2 with this same malady.
 

D'tar

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Voltages are B1= 333vdc, B2=216vdc, B3=179vdc. On the 6V6, Plate is 313, Screen is 211, Grid is -2.08, and Cathode is .005.

See any issues here? May want to recheck these for accuracy. Do you have guitar signal through the circuit to the speaker. try another 6v6 yet?

Edit..

Amp off etc.. you know the deal... ohm 6v6 p8 to ground.....? may want to remove the elevated center tap until things are running proper.
 
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murrayatuptown

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I am battling a (hum & whine & ticking) problem amp also (commercial, PCB-based).

I used a phone app (Spectroid, but others would work) in front of an amp speaker and saw more things than I knew what to do with (for the moment). Interestingly, 60 Hz was hard to find, but 120, 180, 240, 300, 360 were easily seen. (B+ is rectified by FWB and bias by HWR).

In my case, changing fixed humdinger resistors to a wirewound pot with grounded center tap (to experimental places) made no difference in hum level. (artificial ground with no C.T. on filament winding). Elevating the humdinger pot C.T. with about +24VDC divided off a B+ capacitor made little audible difference. It improved slightly way off center but not really an improvement. So I concluded it wasn't a filament hum problem. The amp had multiple chassis connections that seem like textbook ground loop opportunities so that's my focus now...isolating the existing connections to chassis then trying different arrangements. I can see star ground efforts made on the PCB so I assume they achieved something and I should focus my effort on what is possible external to the PCB.

I haven't even plugged an instrument in because there is plenty of challenge to work on with the shorting input jack closed (and verified).

I may be hallucinating but want to experiment with the continuity between the transformers and the chassis since reorienting isn't practical. Isolating the input & output jacks and PCB from the chassis and experimenting with clip leads produced differences (some better), but not enough (not giving up yet).
 

James Knox

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See any issues here? May want to recheck these for accuracy. Do you have guitar signal through the circuit to the speaker. try another 6v6 yet?

Edit..

Amp off etc.. you know the deal... ohm 6v6 p8 to ground.....? may want to remove the elevated center tap until things are running proper.

I will recheck these. Why would I only have basically no voltage on the Cathode?

I have subbed in a couple different 6v6’s. I do get faint and very distorted guitar audio through the speaker IF the volume is turned all the way up.
 

andrewRneumann

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My first attempt as a single bus grounded at the input jack with 2 stars. I had the loud hum, So I thought to cut the long single bus bar and ground the 6V6 components near it with its own chassis ground. Didn’t help, still hummn’...

Thanks for posting the photo of the first attempt. I think it didn’t work because you grounded the power tube at the far end of the bus—all that current is rushing through the ground bus past the input jack and making noise. You want the power tube grounded close to the power supply—not farthest away from the power supply.

Edit: Removed reference to possible ground loop.

Good luck James, we’re all counting on you. :D
 

dougsta

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After a couple of days researching and staring at it, I’m stumped, lol. Here are some closeups for some fresh eyes...

I don't see any in the photos, did you add a couple of 100Ω resistors to gnd from each heater wire for an artificial CT?

You could easily add them at the pilot light.

Congrats on the P2P build, they never win a beauty contest but when done right have the lowest noise floor and you get to know your amp layout real close.

Doug.
 




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