Hum in mojotone 18w build

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Wnively321
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Re: Hum in mojotone 18w build

Postby Wnively321 » Sun 12/16/18 5:18 pm

I've tried the chassis in another part of the shop with other cabs.
It will be Wednesday before I have a chance to look at the amp again. Scratching my head for sure. Reworked heaters again, replaced all tube sockets. My shop is outlets are grounded.

Thanks again for all of the help
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JMPGuitars
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Re: Hum in mojotone 18w build

Postby JMPGuitars » Sun 12/16/18 7:41 pm

Post new photos of the build when you can. Calm down on reworking everything. ;)
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Wnively321
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Re: Hum in mojotone 18w build

Postby Wnively321 » Sat 12/22/18 12:47 pm

Merry Christmas to all
Sorry for the delay, my trip was longer than expected.
Here is the latest update , changed the grounding scheme major improvement! No noise in channel 1 or channel 2 .
I changed the power scheme to the one in the drawing , but that didn’t change any thing.

I still have a audible hum that is there even with the preamp tubes pulled and you can feel it if you place your hand on the speaker cone.
If I intall the preamp tubes the hum leaves if channel I volume is turned up to 4
I have supplied my latest pics, am I wrong in thinking if the preamp tubes are pulled that the noise must be in the PT or OT ?

Thank you
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JMPGuitars
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Re: Hum in mojotone 18w build

Postby JMPGuitars » Sat 12/22/18 5:52 pm

Can you get some heat shrink tubing on that braided cable? Try and cover the whole thing if possible.
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Wnively321
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Re: Hum in mojotone 18w build

Postby Wnively321 » Sat 12/22/18 9:01 pm

Yes, I got the whole cable covered and this is as quite as the amp has been as of yet . Cut the hum by 1/2 Looks like the cable must have been grounding to chassis

Thank you so much
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Wnively321
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Re: Hum in mojotone 18w build

Postby Wnively321 » Sat 12/22/18 11:00 pm

Putting the heat shrink on the braided cable helped so much, along with the grounding they have been the 2 biggest improvement and changed a lot of the issues I was having.
Thank you so much
Tomorrow I’m going to change the power side of the amp to the diagram you provided.
The amp is actually usable
Thanks again
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Re: Hum in mojotone 18w build

Postby JMPGuitars » Sat 12/22/18 11:32 pm

Glad to hear it! That's why I mentioned that wire before, and why I think it's ridiculous that it's part of the kit. Besides potentially grounding out other things, running a shielding wire against ground is wonky, and then there's other potential (hint) problems too.

Hopefully the other change will get you where you want to be. Keep us posted!

Thanks,
Josh
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Wnively321
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Re: Hum in mojotone 18w build

Postby Wnively321 » Sun 12/23/18 6:02 pm

Today I have good news, changed the power scheme and to my surprise a hole new hum issue , crazy right
. After hours of trying to figure out what I did wrong
Would you believe that the only power tube socket I hadn’t replaced was bad.
Replaced the socket ,hum gone , could I say that 1 more time hum gone !!!
The amp sounds like it should. Amazing

I would have never figured this out with out your help.
All of the advice you gave me was 100% correct


Thank you so much

One thing for sure it has been an education
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geoff 1965
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Re: Hum in mojotone 18w build

Postby geoff 1965 » Sun 12/23/18 7:14 pm

great news! glad you have found the gremlin and got rid of the hum,one thing i've noticed zooming in on your pics is there are a few tiny bits of loose solder on the board and in the chassis corners. i would get the vacuum in or air blower and get them out so they don't cause any problems.
enjoy playing the amp and happy christmas & new year.
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