New Build JTM45 Hum - Need Suggestions! See pics

[kburks1230]

I just picked up a brand new JTM45 clone in a trade. Small parts are from Mojotone. The previous owner had a friend build it for him. It really hasn't even been broken in. The guy who built it apparently moved away and never resolved the hum/noise issue that was present upon first power up. The amp sounds AWSOME other than this extra annoying noise in the circuit. It's not a ton of hum but its definitely more than is supposed to be there.

There is hum present when the guitar is plugged in. It is barely noticeable at volumes below 3 but gets louder in proportion to the volume increase. It is most noticable in the treble side and is exaggerated when I crank the presence up. It sounds like what you hear on stage when there is dirty power or dimmers in the same circuit you have your amp plugged into. The sound goes away when I put the amp into standby. I tried swapping the preamp tubes but that didnt change anything. Nothing affects the sound when I move wires around in the amp. The power tubes are KT88 I think.

What should I go after first? Does anyone have recommendations based on the pics or what I have described? Any help would be Greatly appreciated!

See Images:
http://www.flickr.com/photos/58607046@N02/8163880324
http://www.flickr.com/photos/58607046@N02/8163881134
http://www.flickr.com/photos/58607046@N02/8163848755
http://www.flickr.com/photos/58607046@N02/8163882710
http://www.flickr.com/photos/58607046@N02/8163883622

[ToneMerc]

Maybe the pics don't do it justice, but I would go through that build and clean up the wiring and some craftsmanship issues. I would try to document the layout as some of the grounding doesn't appear to be typical.

Here's a link to a grounding scheme that works well if you decide to redo it.

https://tubeamparchive.com/viewtopic.php?t=17017

TM

[Structo]

At first glance I would say most of the problem is lead dress.

Or how the wires are arranged.

The wires from the tubes to the board are too long and will act like antennas.
Shorten them and lay them down to the chassis floor as the pictures below show.

It helps to identify what the frequency of the hum is.
If it is 60Hz then it is most likely heater current being picked up by the preamp.
If it is 120Hz then it is most likely hum pidked up after the rectifiers around the big caps.

Here is a grounding scheme designed by Larry from the Metro forum.

It looks like your amp may be similar.

But notice how the wires are arranged.

If a wire from the tube to the board must cross a heater wire, do it at a right angle for least amount of hum.

Keep signal wires away from AC wires.

The power end of the amp around the power transformer could use some neatening up.

The photo of the 50w amp is a good example of proper lead dress in a Marshall.


Hope this helps.

[Phil_S]

About the only tidy thing in the whole build is the heater wiring. It's almost uncanny how tidy the heater wires are and the rest are done so poorly. It is likely the hum is an easy fix with some cleanup of the wiring.

[kburks1230]

You guys rock. Thanks for the good input.

I guess I'll start around the input and preamp section... Laying those signal wires back against the chassis and shortening them to mimic the example pictures of that cleanly wired amp. If anyone has the inclination to draw notes over my pics pointing out the problem areas that would help me A LOT. This is the only second amp I've ever worked on. I am trying to keep up.

I noticed this guy ran a lot of wiring on the top side of the board instead of laying them under and against the chassis. Does having the signal wires against the chassis generally help in protect them from noise?

[pdf64]

Do the heaters have a good ground reference, ie check resistance between each side of the heaters and chassis/ground?
The green/yellow wire may be it, but it's as well to confirm that.
The B+caps all seem to share the same ground reference; that's a big no-no.
There will be significant ripple current (along the chassis) between the chassis grounding point of the B+ center tap and the grounding point of the reservoir cap.
Best to ground them at the same point.
However, making a build neat and tidy can cause problems; there are certain wires where coupling between them would be bad. A tidy wiring harness can force these into close proximity.
Good lead dress doesn't necessarily look super tidy, though it doesn't have to look a mess.
Pete

[M Fowler]

I've used the Larry grounding as well but in my last JCM800 build I ran the preamp grounds to the can cap grounds and my amp has no noise or hum.

https://tubeamparchive.com/viewtopic.ph ... highlight=

Mark

[Reeltarded]

Nice enough except for the insane hairdo on the wiring. Awesome easy fix.

[kburks1230]

Too bad they don't sell wire hair gel

[Reeltarded]

The normal stuff works on wire too! Ask my hair!

[pdf64]

I noticed this guy ran a lot of wiring on the top side of the board instead of laying them under and against the chassis. Does having the signal wires against the chassis generally help in protect them from noise?

If the amp cab has some sort of screening over the open face of the chassis, then it shouldn't make much difference whether wiring runs above or below the board, all else being equal.
Pete

[kburks1230]

Will I need to relocate the signal leads to the opposite side of the turret board or can I get away with just shortening them as long as they are positioned away from heater wires? I do have copper shielding in the cab.

[Structo]

Marshalls verses Fenders.

They had a different idea about wiring amps, across the pond.

I like using shielded cable for inputs.
Only ground the source end of the shield and only use the middle conductor for the destination.

Marshall purists don't like that stuff because they think it hurts the tone.


By the way, below is the first Marshall amp.

[kburks1230]

I spent some time shortening wires. I used shielded coax from the input jacks to the board. The hum is still there. How does this look?

[kburks1230]

Do the heaters have a good ground reference, ie check resistance between each side of the heaters and chassis/ground?
The green/yellow wire may be it, but it's as well to confirm that.
The B+caps all seem to share the same ground reference; that's a big no-no.
There will be significant ripple current (along the chassis) between the chassis grounding point of the B+ center tap and the grounding point of the reservoir cap.
Best to ground them at the same point.
However, making a build neat and tidy can cause problems; there are certain wires where coupling between them would be bad. A tidy wiring harness can force these into close proximity.
Good lead dress doesn't necessarily look super tidy, though it doesn't have to look a mess.
Pete

PDF64,

What are you referring to with the B+ caps. Which are those? Best to ground what at one point? Thanks!