2061x clone LOUD hum

[JoshBernstein]

Hi, i recently built a Marshall 2061x with a local electrical engineer that has been building really nice amps for years. It sounds really good, but it hums really, really loud. The weird thing is it occasionally stops, and there is no him whatsoever, and it will stay like that for a few days, and then the hum will come back. Neither me nor my engineer buddy can figure out what it is. Its coming from somewhere after the first preamp tube and before the second preamp tube, as i can pull out v2 and the hum will stop, but if i pull out v1 it does not. I've tried several tube changes, but it hasnt made a difference and im just completely stumped. My buddy said it's 60 cycle hum, but im not 100% sure. Its a ceriatone kit, and ive attatched the layout from the site. Thanks for the help in advance!

[M Fowler]

Need gut shots to help determine.

Where are you located?

Mark

[Stevem]

First I would resistance check all the ground points back to the main star grounding point and confirm zero resistance.

I would reflow all the grounds that are part of the circuit of V1 A and. B, and that includes it power supply node filter, if that does not cure the issue I would replace that 47 uf filter.

Also I would confirm that your center ground stand to the chassis on the heaters 100 ohm resistors is solid and reading zero ohms and that either 100 ohm resistor is not cracked as many times with out a close inspection you can not see a fracture and its-continuity will be intermitant and AC 60 hz heater hum will effect the lowest signal level of the amp the most, and that's V1!

If worst comes to worst you can power V1s heater with a 6 volt lantern battery and check if the hum leaves the building!

[JoshBernstein]

I checked all the grounds and they are reading zero ohms... I reflowed them anyway, and they still read zero ohms, but the hum remains. Something else i noticed was that the hum is effected by the left tone pot, but none of the other pots.

[JoshBernstein]

Also which filter would i replace? Sorry, im new to this; it was my first attempt.

[drew]

Need gut shots

+1

[Stevem]

Ok first lets pin down how much noise / hum level we are talking about so we can narrow down on where to look.

Set your voltmeter for low ac volts and hook across the amps speaker output, how many MV do your read, also if you or your buddy have a voltmeter with a frequency test setting you can check the output and pin down if it's 120 hz hum, or 60 hz hum!

In regards to that tone pot and the hum, you may have a Leaky .022 cap so on V1 I would swap those .022 caps that come off of pin 1 and 6, once you do that does the hum issue then effect the right tone pot?

[M Fowler]

I'm just trying to establish if this is the same Josh Bernstein that lives in my area

[JoshBernstein]

Well im getting about 75 mV accross the speaker outputs.

[JoshBernstein]

And i dont have a frequency tester, but using a tone generator, it seems more like 120 cycle hum

[JoshBernstein]

I used a crude frequency analyzer application on my phone that also says its 120 hz

[Stevem]

Yes, 75mv will be a loud hum out thru the speaker so go thru the other check outs I posted about before !

[JoshBernstein]

Well for the 100 ohm resistors, i cant see any visible fratcures on them, but i ordered replacements anyway. I also ordered new caps, so im waiting on those. I report back in a couple days.

[Structo]

Usually if it is 60Hz hum, it has to do with the heater wiring.

If it is 120Hz, it is probably the power supply and or grounds to it.

[JoshBernstein]

Well it's been a while since ive posted, but ive found that grounding the turret with the three 220k's and the .0047 removes the hum, but i still cant seem to find the source. Also, Grounding out the pots has no effect.