Marshall 2061x build hum

[JoshBernstein]

Hi all,

I posted about this in the marshall forum, but I thought Id post it here too

I recently built a Marshall 2061x using he ceriatone chassis and layout. 

The amp sounds really awesome, but its got one thing that bugs me. Upon power up, there is a deep hum (sounds like 60hz to me, but I dont have a scope. As the tubes warm up, the hum gets louder, reaches a peak volume, and then subsides to a lower volume hum where it stays constant. 

It occurs even with no preamp tube or phase inverter, even with the post phase inverter master volume turned all the way down to ground the input of the power amp tubes. I tried putting in a new cap can but no dice. Here are some pics, and i will post a link to a video in a second. The video is taken with only the power tubes in and the grids of the power tubes grounded. I generally don't use the standby, but if i let them amp warm up on standby and then flip the standby switch, there is no swell, the hum just starts at its final idle level. Power tubes are a known good matched pair, I have tried a couple other sets as well. Anybody have any ideas?

https://youtu.be/Nwh5gfoO3SA

[statorvane]

Hello. I built one of these too about a year ago., A blast to play. Not too much to this amplifier so it shouldn't take too much time to identify the problem(s).

You seem to have eliminated the preamp and PI, as well as the power tubes, so there's not too much left to check.

Try chop-sticking around with just the power tubes and listen for any microphonics. If you hear something, it's a dead give away there is a loose solder joint(s) somewhere. I spent two weeks chasing down what eventually turned out to be a loose solder joint on a JCM 800 clone I built for my son. Now this is the second thing I check, after tube swapping, if I have problems.

From your photos there are a lot of things coming together between the power tubes. I'd focus first on the heater wiring. I suggest you rewire the power tube heater wiring, twisting the wires tighter, they are pretty loose there.

I see you used the Marshall layout where the control grid resistors are located on the tagboard. I mounted mine, and the screen grid resistors as well, directly to the tube socket pins, vertically. Also your OT wiring is laying right on top of V4. I'd try and move them away; more between the power tubes.

Just some ideas looking at your layout. There could also be a component failure, but I have never run across on before (I've been lucky).

Best of luck with that little monster!

[didit]

Hello --

You've been "creative" in your grounding, and in sloppy ways predicting hum. Look there.

Best .. Ian

[JoshBernstein]

Thank you both for your suggestions! Chopsticking hasnt revealed anything so far, nothing microphonic. I moved the ot secondaries up the center of the output tubes to move them a way a bit. I redid all my grounds, running them all back to a singular star ground at one of the pt bolts, but no change. Ill reflow all the joints in yhe power amp and see if anything happens. I have also clipped in some known good filter caps across the all of the filters with no change. After that I will try moving resistors onto the sockets and redoing the heaters. Does anybody else see anything suspect?

[sonicmojo]

Is there anything connected to pin3 on V2?

[xtian]

Josh, pull out the power tubes also. Do you still hear the hum? If yes, then it's the PT radiating into the OT.

[JoshBernstein]

Is there anything connected to pin3 on V2?

Yes, it is jumpered to pin 8 on both v1 and v2, i accidentally ran the jumper over the heater on v1 and under the heater wire on v2, ill have ti fix that for consistency

[JoshBernstein]

Josh, pull out the power tubes also. Do you still hear the hum? If yes, then it's the PT radiating into the OT.

I just pulled the power tubes and no hum that I can hear, even with my ear up against the speaker

[Malcolm Irving]

Could be unbalance between the power tubes. Check DC voltages on screens of the power tubes - the voltages should be similar. Also check DC volts on their plates. Same with the cathodes and grids. If that all checks out, it could be the two power tubes you have are badly matched.

Push-pull amps rely on balance between the power tubes to cancel the hum that comes from B+. When you power up, one of the tubes may start amplifying quicker than the other, causing a temporary imbalance.

[The Ballzz]

@JoshBernstein
First, please allow me to apologize in advance for what may sound cruel, as that is not the intention!

Long is the list here, of best practices not followed and/or ignored! I estimate that you may not find any one "smoking gun," but instead that you have a stack up of issues. This list includes, but is not limited to heater string, lead dress, grounding scheme, etc.

Also, while many utilize a PT bolt for a ground connection, most will tell us to avoid that like the plague. I realize that at face value, it may seem like a good idea, but think about what a power transformer actually does and the pulsing magnetic fields surrounding and going through it. Do ya think maybe some form of hum may get "INDUCED" there? One of the most critical aspects of building a great amp surrounds implementing the best ways to isolate our DC (and by proxy, our gloriously finger<pickup<guitar induced AC) from that nasty, constantly pulsing AC mains supply, without compromising the sound! Looks easy/simple, but really, not so much!

Now, if all songs got played in the key of a slightly flat "B" (slightly sharp "G" in the UK, etc.) that hum could be utilized as accompaniment, but otherwise, it just ruins the whole experience!

Best of luck on your search!

Just sayin'
Gene

[JoshBernstein]

Could be unbalance between the power tubes. Check DC voltages on screens of the power tubes - the voltages should be similar. Also check DC volts on their plates. Same with the cathodes and grids. If that all checks out, it could be the two power tubes you have are badly matched.

Push-pull amps rely on balance between the power tubes to cancel the hum that comes from B+. When you power up, one of the tubes may start amplifying quicker than the other, causing a temporary imbalance.

I did check voltages and such the other day while troubleshooting, and everything was extremely close beetween power tubes!

@JoshBernstein
First, please allow me to apologize in advance for what may sound cruel, as that is not the intention!

Long is the list here, of best practices not followed and/or ignored! I estimate that you may not find any one "smoking gun," but instead that you have a stack up of issues. This list includes, but is not limited to heater string, lead dress, grounding scheme, etc.

Also, while many utilize a PT bolt for a ground connection, most will tell us to avoid that like the plague. I realize that at face value, it may seem like a good idea, but think about what a power transformer actually does and the pulsing magnetic fields surrounding and going through it. Do ya think maybe some form of hum may get "INDUCED" there? One of the most critical aspects of building a great amp surrounds implementing the best ways to isolate our DC (and by proxy, our gloriously finger<pickup<guitar induced AC) from that nasty, constantly pulsing AC mains supply, without compromising the sound! Looks easy/simple, but really, not so much!

Now, if all songs got played in the key of a slightly flat "B" (slightly sharp "G" in the UK, etc.) that hum could be utilized as accompaniment, but otherwise, it just ruins the whole experience!

Best of luck on your search!

Just sayin'
Gene

I dont find that cruel at all! While this is not my first build, I am still learning and practicing. This is actually a rebuild of my first amp ever, which can be seen in this thread here:

https://ampgarage.com/forum/viewtopic.p ... 60#p392860

I appreciate all criticisms as I do want to continue to improve my builds.

Some of the less than perfect wiring is due to the fact that i cut my transformer leads extremely short my first go around and I didnt want to extend the leads on this run. I have also tidied a few things up, and am doing my best to improve my lead dress.

In this case, it seems to be just one smoking gun, I had accidentally tied both the screen grids and plates of the power tubes to the same power supply node. After fixing that, the amp has a very low noise floor. There is only some mild hiss with both the volume and ppimv cranked! Thank you for the help everyone!

[The Ballzz]

@JoshBernstein
Interesting that you mentioned "I had accidentally tied both the screen grids and plates of the power tubes to the same power supply node." When looking at the Ceriatone diagram, as well as your pics, I can't see where pin # 3 of the power tubes is connected to anything, except for ground through the 150R/47uf network, unless something is connected under the board? In other words, I see no power there!

Maybe I Had Too Much Fun In The 60s/70s?
Gene

[sluckey]

pin 3 is the cathode

[The Ballzz]

See? Too much fun in my youth!

My Bad,
Gene

[JoshBernstein]

See? Too much fun in my youth!

My Bad,
Gene

Not a problem, I got a smile out of it!

I will continue to slowly tidy things up in the amp, though based on the noise level I dont think it is necessary to twist the heaters any tighter. They may be a little loose but apparently they have enough twists to keep them from causing trouble.

Would you recommend i move the star ground elsewhere? My original thought was to have the preamp section grounded at the first preamp tube, and the power supply things near the power transformer. Currently everything is all tied together at one of the pt bolts.

I think the preamp leads are generally okay, the messy part is the power supply which I will go back and fix. I can't do too much about the ot leads as I had cut them very short on my first try.