Rocket Hum

[mickephoto]

I just finished a Rocket made from a Ceriatone kit; Heyboer Transformers from RJ and a Hammond choke. "Upgraded" the filter caps to 3x40 uf Sprague and 3x22uf F&T.

The amp sounds great and voltages are all good, BUT it has a bad 60hz hum (pretty sure it'z 60) even with nothing plugged in and volume at 0.

Some notes to help diagnose...
• If I pull the PI ALL hum is gone so I assume the problem must be in this area. I've switched all tubes and that's not the problem.
• When amp is in Standby you can still here a VERY faint hum from the speaker.
• I checked and triple checked all connections/grounds.
• Retested all components in PI area and they measure fine.
• I tried alternate grounding points for main caps and pre's but it makes no difference in hum.
• Filament wiring is tight.
• If I hold my hand over PI area there is a BIG increase in hum.

Is it possible that a cap (non filter) or resistor can measure fine, but still cause hum? I'm also surprised by the big increase in hum when I move my hand over the PI/center of the amp.

Spent all day trying to find a cure. Any advice most appreciated!

[guitarsnguns04]

In the first pic it looks like the mounting hardware you used for the choke is very close to the board and the connections under. I wonder if it is picking something up there ? Might try flipping that so that the bolt head is on the inside. ..would give you more clearance there? Just an observation. Best of luck

[greekie]

Are the speaker outputs / output transformer common grounded? That is one you can easily forget when using isolated jacks.

[VacuumVoodoo]

Are the speaker outputs / output transformer common grounded? That is one you can easily forget when using isolated jacks.

This could be one cause but than the hum would be to0 strong to be affected by the "magic hand" thing. But one never knows. Needs checking.
Otherwise:

The amp sounds great and voltages are all good, BUT it has a bad 60hz hum (pretty sure it'z 60) even with nothing plugged in and volume at 0.

60Hz is either from heater wiring or a ground loop. Almost always.

• If I pull the PI ALL hum is gone so I assume the problem must be in this area. I've switched all tubes and that's not the problem.
It's rather somewhere in front of PI
• When amp is in Standby you can still here a VERY faint hum from the speaker.
This is usually caused by unfortunate relative positions of power and output transformer
• I checked and triple checked all connections/grounds.
I usually become blind to my own errors after a 2nd look
• Retested all components in PI area and they measure fine.
The source of hum is most probably not in this area
• I tried alternate grounding points for main caps and pre's but it makes no difference in hum.
Heater wiring or ground loop, but I'm repeating myself
• Filament wiring is tight.
It is not very tight & especially the black wire is very close to grid pin #7 on each preamp socket. Try pulling it away from where it sits now
• If I hold my hand over PI area there is a BIG increase in hum.
You are an antenna radiating hum that you pick up from the air, coupling caps and/or gridstoppers pick it up. That's why you should have a bottom cover too.
Is it possible that a cap (non filter) or resistor can measure fine, but still cause hum? I'm also surprised by the big increase in hum when I move my hand over the PI/center of the amp.
Yes, see above

A hum free heater wiring can look something like this:

[img364]http://louderandmore.com/chwilowe/znimk ... eefils.jpg[/img]

[badtweed]

Where do you have the output tubes' cathode bias resistor connected to ground? Do you have it connected to the same ground connection as the output speaker ground?

I see a green wire going from the input jack ground but I cannot tell where you have it terminated.

[xtian]

+1 on covering your chassis with the metal bottom plate, turning off other nearby radio sources (soldering iron, desk lamp, etc). Does this help?

The runs from the input jack to v1 pin 7, and also from the Volume wiper to v1 pin 2 will be sensitive to radio; replace with shielded wire, and ground only one end of the shield, to the input jack, and Volume pot ground, respectively. I didn't bother with the input wire, though, because in the Rocket it's so short and direct. But the Volume one definitely benefits.

[mickephoto]

Thanks for the help all! My first time here. VacuumVoodoo (another Swede?) - great list of advice and some very tight heater wiring indeed.

Back to work now to see if I can figure this out. I'll trace everything back from the PI again. Like VV said...it's easy to go blind on your own assembly.

Speaker jack is terminated in the same place as the cathode bias resistor.

I did try to move wiring around with a chopstick and didn't notice any difference. I'll try rewiring pin 7 away from heaters. Doubt this is it though.

[badtweed]

Thanks for the help all! My first time here. VacuumVoodoo (another Swede?) - great list of advice and some very tight heater wiring indeed.

Back to work now to see if I can figure this out. I'll trace everything back from the PI again. Like VV said...it's easy to go blind on your own assembly.

Speaker jack is terminated in the same place as the cathode bias resistor.

I did try to move wiring around with a chopstick and didn't notice any difference. I'll try rewiring pin 7 away from heaters. Doubt this is it though.

Try relocating the cathode resistor ground to a different location from where you have the output jack ground located.

Also check to see if your bus bar connection to the back of the pots is getting a path to ground via the pots themselves. If so then you have a ground loop that you can easily fix.

I would also make sure that your screws and nuts that are connecting your leads to the chassis are nice and snug. Sometimes applying a bit more torque on those connections can provide a noticeable reduction in hum.

[Cliff Schecht]

I had this problem with my Rocket and another amp at the same time. Turned out that both amps had loose hardware somewhere that was the source of a bad ground connection. It sounds like you are having a similar issue, either a ground is not connected securely or some hardware isn't tightened down. Go over every screw and bolt and make sure they are cranked down!

[mickephoto]

I tried the following and nothing made any difference to reduce the strong hum:
• Moved grid wires away from heaters.
• Switched to shielded cable from volume pot wiper to V1.
• Rotated OT slightly.
• Reflowed all grounds.
• Checked all connections again.

If I remove V1 there is virtually no difference. It only adds some very slight noise/hum from input.

Removing V2 does lower the hum significantly.

Removing V3 or the ground from PI (shown in photo) makes the amp quiet as a mouse.

[badtweed]

I tried the following and nothing made any difference to reduce the strong hum:
• Moved grid wires away from heaters.
• Switched to shielded cable from volume pot wiper to V1.
• Rotated OT slightly.
• Reflowed all grounds.
• Checked all connections again.

If I remove V1 there is virtually no difference. It only adds some very slight noise/hum from input.

Removing V2 does lower the hum significantly.

Removing V3 or the ground from PI (shown in photo) makes the amp quiet as a mouse.

You have a grounding issue and none of those steps you took have anything to do with resolving that.

I suggested a few simple tips regarding your current grounding scheme that you could easily implement. You might want to relocate the ground for the power caps away from the current location and closer to the power tranny mountng screws instead.

Good luck.

[mickephoto]

I suggested a few simple tips regarding your current grounding scheme that you could easily implement...

I did relocate the cathode resistor ground. I also moved the power cap grounds away from pre filters/pots. None made any difference.

Also check to see if your bus bar connection to the back of the pots is getting a path to ground via the pots themselves. If so then you have a ground loop that you can easily fix.

The pots do have a path to ground/chassis by themselves. Gerald Weber advices against this type of grounding, but that's how it is in the layout. Are you suggesting isolation washers?

After all the things I've tried perhaps the best bet now is to remove the busbar from the pots and see if it helps. Leaving only the ground wire from the bus bar to chassis.

I would also make sure that your screws and nuts that are connecting your leads to the chassis are nice and snug...

I retightened everything and it didn't help.

[cbass]

Its hard to tell in the pic.It doesn't look like the CT's are bolted to the chassis.

Are you sure your centertaps(Brown/white orange/yellow) have a good connection to the chassis?

[mickephoto]

Its hard to tell in the pic.It doesn't look like the CT's are bolted to the chassis.

Are you sure your centertaps(Brown/white orange/yellow) have a good connection to the chassis?

Yes, but just to make sure I retightened and re-soldered to the lug again. Also grounded pre and main filter caps to chassis bolt instead of bus bar connection. Again, no difference in hum. Earlier I tried separating main caps while keeping pre filters on the bus bar.

This has been really frustrating to figure out and I've spent more time on it than I want to admit... Is it possible that it could have something to do with the PT, or bad (new) filter caps. I'm not sure what else to try, except...
Removing the bus bar from pots. That's next and final before handing in the towel I think. Maybe I should've started with that:)

[mickephoto]

Regarding the heater wiring..I think it looks clean. If this was the cause for hum one would think it would make a difference when I move the wires with chopsticks, but it doesn't. This must be a grounding thing, or PT...bad caps. Not sure about the last two

Photo also shows the half power switch in case someone is wondering.