120hz hum in new build

[bepone]

ok with shorted input on the amp, how much is humm 120hz at the speaker in mVpp?
also the same question i grids on out tubes?
MV put to full, all volumen also

[beasleybodyshop]

I'll take those measurements after the kids go to bed.

I was looking through some layout of actual Matchless amps and it's puzzling. All the filter caps are literally stacked together, with their grounds all bussed together. How in the hell is there not a noise issue doing it that way??

I ended up moving a few caps to the main board, localizing them to their respective circuit grounds which then all go to the buss. Still a hum, and now there's a loud buzz from channel 2 (top boost channel) that's significantly loud when turned up. Man I just can't win haha.

I added a bridge rectifier and large filter cap to run the preamp tubes on DC heaters. No change.

[beasleybodyshop]

Ok....so part of this mystery is solved. I inadvertently wired the standby switch across the mains and screen nodes. I use a 220K across my standby switches to help mitigate the thump but in this case, it was inadvertently bypassing the filter choke

With the standby clipped out of circuit, the hum from the master volume has significantly declined.

Now, however I have another issue. The hum was masking another noise, a loud buzz that happens when you turn up the channel volume for the Top Boost channel. Grounding the output coupling coupling cap for the first input stage makes the hum go away. I've tried swapping in different preamp tubes, no joy.

Looking at the first tube stage for this channel, I have all the grounds tied together for that stage and then grounded to the buss bar.

I clipped another 22uf cap in parallel with the filter node for that input stage, no change. Wiggling the preamp tubes doesn't yield any change (looking for potentially bad tube sockets)

Plate voltage for the input stage is 158v, that seems about right. Man, what is wrong with me here? Lol.

[bepone]

so choke was missing, ok
next step, probably is grounding, in good grounding there is no noise at all..lets see how did you ground every stage and every gain/volume potentiometer

[bepone]

buss ground cannot be supplied from main electrolitic capacitor gnd, buss must be on the next low level ground .. where is the layout for this amp how it is done? to me looks like little bit out of order, random positioning?

bank of electrolytic caps on the right side is not connected or? what is the purpose?

[rangdipkin]

looks like the right hand side bank of caps have individual ground wires (white) running to the ground buss. Maybe connect those grounds locally at the caps with a single connection to the pot ground buss from the lowest voltage supply node? Also consider moving the can cap ground connection to pot ground buss over to this bank as well if those are your first couple filter nodes.

[beasleybodyshop]

Ok, I double checked where all the grounds are:

Power Tube Cathodes - Grounded to B+1/B+2 Cap can negative tab.

B+1/B+2 node - HV Center Tap connected to cap can negative tab. This point is then grounded to main buss.

B+3/PI - Grounded to bottom of PI tail resistor. This point is then grounded to main buss.

B+4/Cathode Follower - This node, and the CF rK are both grounded to the main buss at the same spot.

B+5/TB channel - Grounded to same spot on main buss as the first gain stage rK and its corresponding volume pot ground. Channel's Tone Stack (ground connection off Bass pot) is terminated at this point as well.

B+6/EF86 channel - Grounded to same spot on main buss as its rK and its corresponding volume pot ground.

PPIMV - Dual gang pot grounded directly to buss.

In jacks (insulated from chassis) - Ground connections from both channel jacks, AND main buss connect together at one spot on chassis near input jacks.

Output Jacks - Grounded to B+1/B+2 cap can negative.

Also for grins i did swap in a brand new set of EL84s, no change. Added pics of current situation in chassis. Disregard the flying lead from the PI out to PPIMV, that was just a temporary experiment

[beasleybodyshop]

buss ground cannot be supplied from main electrolitic capacitor gnd, buss must be on the next low level ground .. where is the layout for this amp how it is done? to me looks like little bit out of order, random positioning?

bank of electrolytic caps on the right side is not connected or? what is the purpose?

No real layout. I was working off the schematic linked in the first post.

Originally i had all the filter caps on the right side, but after noise issues presented themselves I tried moving a few stages locally to where they were in circuit. Didn't seem to change the issue.

[bepone]

locally is better but you need to follow currents in the ground, you cannot just connect preamp gnd node to the main filter rectifier node (biggest currents there). need to follow the distribution stream.

also how many chassis gnd points you have (should be only one, close to input jack)?

[bepone]

what is the orientation of the output and mains transformer? (main transformer can be major cause of humm)

[beasleybodyshop]

locally is better but you need to follow currents in the ground, you cannot just connect preamp gnd node to the main filter rectifier node (biggest currents there). need to follow the distribution stream.

also how many chassis gnd points you have (should be only one, close to input jack)?

Where would you suggest grounding B+1/B+2 nodes then? I've tried grounding that cap can node off the main buss onto another spot in the chassis and it didn't make and change.

At this point, I technically have 3 chassis ground points, all at the same location: Channel 1 input jack grounds, Channel 2 input jack grounds, and the main buss all ground to chassis at the same point.

[beasleybodyshop]

what is the orientation of the output and mains transformer? (main transformer can be major cause of humm)

Power and output transformers are close to each other, with laminations oriented 90 degrees from each other. Chassis is a 90s era Vox AC30 chassis. In it's original form it did not have mains transformer noise issues.

[beasleybodyshop]

Looking at layouts of Matchless amps, and the Trainwreck Rocket it just seems like the filters cap ground are all bussed together and connected to a ground point near the rectifier tube? How do they get away with that? lol.

[beasleybodyshop]

Ok, so i moved all the filter caps back to their original location, and ran a buss wire that just connects all the filter cap grounds together like I see in matchless gut shots. This connection then goes back to the rectifier socket solder tab that connects to chassis.

Still a buzz when i turn up either channel volume.

I'm beginning to think that this must not be strictly speaking a grounding issue. I've tried 5-6 different ground schemes now with no real change to the noise excepting for the standby switch error.

In looking at the trainwreck rocket layout, it shows HV center tap and Heater CT connected together away from everything else. I'll give that a try next.

[bepone]

heaters ground to the input jack not main filter rectifier gnd , and lets see