1965 Bassman Noise 🤷‍♂️

CodySorgenfrey

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Hey everyone! I'm looking for some experienced members to help me sniff out what is going on with this amp, any help would be greatly appreciated, however I have tried a lot of different things already so please read the long story!

The long story not so short.
I got this amp on Ebay for a pretty good deal ($600). I knew from the pictures that it had been around the block, most of the amp was not original, but nonetheless, I've wanted one for a while and this is what I could afford. I am comfortable working on electronics, I've built a few guitar pedals from scratch and laid out some PCB boards, done some smart home things with ESP32 chips, etc. so I figured I can pour some money into fixing it up if I need to.

When the amp arrived, it had a broken 6L6GC from shipping and mostly original filter caps. So new pair of 6L6GCs from TAD and F&T filter caps ordered. Once those got here, I opened it up to balance and bias it as well as replace those electrolytics. With everything looking good I fired it up and it works and sounds good tone wise, but there was a lot of noise.

I have been on a mission for the last month literally 2-4 hours a day trying to fix this hum. I don't believe this level of noise is normal or acceptable, but I'll let you all be the judge of that. Here's what I know and what I've tried so far:
  • What I know
    • Noise is not affected by any controls.
    • Noise is 100% gone if V4 is removed.
    • Noise is ~75% gone if V3 is removed.
    • Removing V1 or V2 changes the noise but doesn't reduce it.
    • No extra noise chop sticking any components.
  • What I've tried
    • Bias was set using transformer shunt method.
      • Inner 6L6 68% 20.3 Watts = .0453A * 448.0V
      • Outer 6L6 67% 20.0 Watts = .0446A * 448.6V
    • Humdinger pot on heater wires.
    • Dropped heater voltage with resistors from 6.9VAC to 6.3VAC.
    • Re-ran heater wires to be correctly in phase from tube to tube.
    • Reflowed all solder joints.
    • Reworked preamp grounding to use a bus and only connect at the input jacks.
    • Reworked power amp grounding to use transformer lug instead of soldered to chassis.
    • Replaced all tubes with brand new tubes.
    • Complete disassembly and reassembly of the amp, testing and replacing out of spec components.
      • This went so far that there is almost NO original components in this amp apart from some blue ajax molded caps and rectifier diodes.
    • Ran under-board wires on top.


I've included a 100% accurate schematic of the amp as it is today. Here's a key to the schematic's color-coded info:
  • Green OK indicates parts that came with the amp that tested good and were left. Everything without this Green OK label has been replaced with brand new quality parts.
  • Orange measurements are what Fender claims at a given point.
  • Purple measurements are what the amp is currently running.
Scope images:
Amp Images:
Let me know if you have any questions or if you may know what's going on here. I have no idea what to do now. Thanks!
 

AntonyB

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Which version and year is this bassman?

This is a lot of info to go through.

Is this "noise" hum? You have not described it nor how loud it is... I am going to assume it's a hum that is due to DC leaking into your grids.

My first few thoughts:
- removing V4 cuts noise is great, that means it's a preamp noise, good job
- removing EITHER V1 or V2 "changes noise"... but you didn't say if you removed BOTH?
Because removing both would confirm it's V1 or V2 leaking DC into the circuit.

My hunch is going toward a leaking coupling capacitor (ie DC is making through to the next grid) because your V3 pin 7 is messy. Signal should be clean by then.

Also, I remember Skip Simmons mentioning that he had a hum once that came from DC showing up on the tone pots... check that too (DC voltage leaking from the tone caps). Check your T/B pots leg 3 to ground and see if you have DC there.

I'll spend a bit more time on this later... thanks for the detailed info.

Also, looking at the pics, did you replace all the carbon comp resistors with metal film?
Because that could be another issue. If those metal film don't have the right watt rating (and they all look like 1/4W) you could be having a bad plate resistor.

Finally, with this amount of mod, your noise could definitely be coming from MULTIPLE sources added together...
 
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CodySorgenfrey

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Which version and year is this bassman?

This is a lot of info to go through.

Is this "noise" hum? You have not described it nor how loud it is... I am going to assume it's a hum that is due to DC leaking into your grids.

My first few thoughts:
- removing V4 cuts noise is great, that means it's a preamp noise, good job
- removing EITHER V1 or V2 "changes noise"... but you didn't say if you removed BOTH?
Because removing both would confirm it's V1 or V2 leaking DC into the circuit.

My hunch is going toward a leaking coupling capacitor (ie DC is making through to the next grid) because your V3 pin 7 is messy. Signal should be clean by then.

Also, I remember Skip Simmons mentioning that he had a hum once that came from DC showing up on the tone pots... check that too (DC voltage leaking from the tone caps). Check your T/B pots leg 3 to ground and see if you have DC there.

I'll spend a bit more time on this later... thanks for the detailed info.

Also, looking at the pics, did you replace all the carbon comp resistors with metal film?
Because that could be another issue. If those metal film don't have the right watt rating (and they all look like 1/4W) you could be having a bad plate resistor.

Finally, with this amount of mod, your noise could definitely be coming from MULTIPLE sources added together...
Hey Antony thanks for taking the time to go over this and help!

It’s a 1965 Bassman, AB165 circuit (with a few mods like bias level pot, switchable NFB loop and humdinger heater pot). The schematic I linked is accurate to what’s in the chassis.

The noise is a hum. It sounds to me like 60Hz (hence all the heater circuit tinkering) but the scope output is a complete mess at the speaker Jack so it’s a lot of frequencies. It is fairly loud. I’d say speaking level volume. Loud enough that I can only listen to it hum for like 10 minutes before I’m annoyed.

I’m on the same page about the DC. I’ll take careful measurements again and check about V1 & V2 removed when I get a chance. It would make sense since those are the only old parts left in the amp.

Yes I did replace all of the carbon comps with metal film. They’re half watt like the schematic specifies. I didn’t want to but they were all well out of tolerance and I want this amp to work well for a long time.
 

Jasonpatrick

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I had a 1967 ab165 bassman that would hum until I reinstalled the 220k? Flyover resistors coming off the power tubes. That amp was noisy regardless because of the added stage to the normal channel, which is what makes it roar (thanks cbs) :)
 

CodySorgenfrey

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I had a 1967 ab165 bassman that would hum until I reinstalled the 220k? Flyover resistors coming off the power tubes. That amp was noisy regardless because of the added stage to the normal channel, which is what makes it roar (thanks cbs) :)
Thanks for the suggestion! All the resistors are new and tested so that’s unfortunately not our noise maker.
 

Lowerleftcoast

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There is a ground loop. The ground bus attaches to the ground terminal at each input jack and the jacks are grounded to the chassis.

Overall Circuit.jpg

Unrelated to above: Check that the switches on the jacks make and break contact as they should.
 

CodySorgenfrey

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There is a ground loop. The ground bus attaches to the ground terminal at each input jack and the jacks are grounded to the chassis.

View attachment 1052755

Unrelated to above: Check that the switches on the jacks make and break contact as they should.
Yeah the switches do make good contact. I wish this was the fix! But I only attached the bus to the ground tab of the bass channel jack. It’s attached to the switch tab only on the normal channel jack.

Although the input circuit is quiet complicated at the jacks so if I missed something and it is indeed a ground loop please let me know.
 
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Paul G.

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I would start by pulling preamp tubes, one at a time. A bad tube can hum at 60Hz. Check all ground connections, the solder blobs on the chassis can crack so it looks like they're ok, but the wire is floating in a void. A nice big iron and reflow. Make sure all your pots and jacks are tight and have star washers. That's your ground bus. Have you rewired for 3-prong plug and disconnected the "death cap"? If not, do it now, if yes, check your wiring. Hot (black) to center of fuse, then switch. Neutral direct to transformer primary. Ground to its own lug on the chassis. Check that heater wiring is flying and not running close to any signal wires.

That's all I've got for now.
 

CodySorgenfrey

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Ok so this morning I got some time to monkey with this amp again.

I pulled V1 & V2 together and the noise is still there but it's at ~50% volume.

I lifted one end of each of these caps and measured the DC to ground. Here's what I found:
C03 0.040V / 212V = 0.02% (new silver mica)
C04 0.147V / 212V = 0.07% (old blue molded)
C05 0.166V / 212V = 0.08% (old blue molded)
C06 0.160V / 212V = 0.08% (old blue molded)
C10 0.115V / 210V = 0.05% (new silver mica)
C11 0.230V / 210V = 0.11% (old orange drop)
C12 0.134V / 210V = 0.06% (old blue molded)
C15 0.028V / 252V = 0.01% (new orange drop)
C17 0.216V / 274V = 0.08% (old orange drop)

I've also tested each of these for DC current to ground. They all test out at 0.0uA.

Since some of these caps are brand new, that begs the question, how much leakage is too much? Which ones should I replace, and which ones can I leave?
 
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schmee

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Lots of info. It sounds like an issue with the input jacks/shunt to ground to me.
-I assume your shielded wires from the jacks to pretubes have the shield wire only grounded on one end?
-The NFB switching scheme can cause a bad hum depending on wire routing and switch location. I had that problem with a Vibrolux once.

-A new heater wire isn't protruding/touching a nearby socket solder tab possibly?

-Because the amp is new to you, Is it possible the original owner has the OT primary wires switched at the power tubes? This will often cause a loud squeal, but I have had them just hum loudly.

-A Bassman is pretty simple, If worse comes to worse I would start removing any mod from stock one at a time and see if it gets cured. I'd remove the NFB switching first.

-I assume you have actually installed it back in the head to test it to be sure it just isn't the lack of the cover shield and your guitar proximity to it?
 

CodySorgenfrey

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Ok I'm continuing to work through various possible solutions here. Again, I really appreciate everyone's help!

Which version and year is this bassman?

This is a lot of info to go through.

Is this "noise" hum? You have not described it nor how loud it is... I am going to assume it's a hum that is due to DC leaking into your grids.

My first few thoughts:
- removing V4 cuts noise is great, that means it's a preamp noise, good job
- removing EITHER V1 or V2 "changes noise"... but you didn't say if you removed BOTH?
Because removing both would confirm it's V1 or V2 leaking DC into the circuit.

My hunch is going toward a leaking coupling capacitor (ie DC is making through to the next grid) because your V3 pin 7 is messy. Signal should be clean by then.

Also, I remember Skip Simmons mentioning that he had a hum once that came from DC showing up on the tone pots... check that too (DC voltage leaking from the tone caps). Check your T/B pots leg 3 to ground and see if you have DC there.

I'll spend a bit more time on this later... thanks for the detailed info.

Also, looking at the pics, did you replace all the carbon comp resistors with metal film?
Because that could be another issue. If those metal film don't have the right watt rating (and they all look like 1/4W) you could be having a bad plate resistor.

Finally, with this amount of mod, your noise could definitely be coming from MULTIPLE sources added together...

I have double checked, and the capacitors are good, no current leaking. Although I do get DC voltage reading to ground. Is this normal? See my previous post for values.


There is a ground loop. The ground bus attaches to the ground terminal at each input jack and the jacks are grounded to the chassis.

View attachment 1052755

Unrelated to above: Check that the switches on the jacks make and break contact as they should.

I tried isolating input jacks on normal channel to eliminate the ground loop. That unfortunately wasn't the cause of the noise. I've also tried it with no jacks at all for the normal channel, input straight to the ground bus just for kicks.


I would start by pulling preamp tubes, one at a time. A bad tube can hum at 60Hz. Check all ground connections, the solder blobs on the chassis can crack so it looks like they're ok, but the wire is floating in a void. A nice big iron and reflow. Make sure all your pots and jacks are tight and have star washers. That's your ground bus. Have you rewired for 3-prong plug and disconnected the "death cap"? If not, do it now, if yes, check your wiring. Hot (black) to center of fuse, then switch. Neutral direct to transformer primary. Ground to its own lug on the chassis. Check that heater wiring is flying and not running close to any signal wires.

That's all I've got for now.

Thanks for the suggestions! I have reworked the grounding scheme already, there isn't anything soldered to the chassis anymore.

That wire that's been abandoned off the PT is the filament center tap; I've used a pot with the wiper connected to ground to balance the two windings and reduce their voltage as this PT isn't original and the filament winding was unbalanced and a tad high.

Pots and jacks have star washers and good connections to ground.

Yes, this is using a 3-prong cord, ground has its own lug of the PT, black to power switch, white to PT primary. Death cap removed. I wired to the switch first, then the fuse to be sure the fuse isn't hot if the amp is plugged in but off.

Heater wires are twisted nicely and up in the air, they only come down at the tube sockets with no loops.


Lots of info. It sounds like an issue with the input jacks/shunt to ground to me.
-I assume your shielded wires from the jacks to pretubes have the shield wire only grounded on one end?
-The NFB switching scheme can cause a bad hum depending on wire routing and switch location. I had that problem with a Vibrolux once.

-A new heater wire isn't protruding/touching a nearby socket solder tab possibly?

-Because the amp is new to you, Is it possible the original owner has the OT primary wires switched at the power tubes? This will often cause a loud squeal, but I have had them just hum loudly.

-A Bassman is pretty simple, If worse comes to worse I would start removing any mod from stock one at a time and see if it gets cured. I'd remove the NFB switching first.

-I assume you have actually installed it back in the head to test it to be sure it just isn't the lack of the cover shield and your guitar proximity to it?

I went ahead and tried your suggestions because I really appreciate any help. It makes sense none of this worked though because it's mostly related to the power amp circuit and that circuitry is running quietly with the preamp tubes removed.

Yes, the wires from jacks/pots to tubes are shielded and only grounded at the control end.

I removed the switchable NFB. It's now wired as per Fender's layout (except above the board instead of under).

I've verified OT primary leads are correct as per Fender's layout.

I've verified no heater or any other leads shorting pins on sockets.

I tried the head in cabinet to verify it's not just RF interference. There is a lot of RF interference when it's out of the head, but I can tell the difference between that noise and this hum.


Does anyone have any more ideas? I know we can figure this out if we keep at it! 🙏
 

schmee

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Ok I'm continuing to work through various possible solutions here. Again, I really appreciate everyone's help!



I have double checked, and the capacitors are good, no current leaking. Although I do get DC voltage reading to ground. Is this normal? See my previous post for values.




I tried isolating input jacks on normal channel to eliminate the ground loop. That unfortunately wasn't the cause of the noise. I've also tried it with no jacks at all for the normal channel, input straight to the ground bus just for kicks.




Thanks for the suggestions! I have reworked the grounding scheme already, there isn't anything soldered to the chassis anymore.

That wire that's been abandoned off the PT is the filament center tap; I've used a pot with the wiper connected to ground to balance the two windings and reduce their voltage as this PT isn't original and the filament winding was unbalanced and a tad high.

Pots and jacks have star washers and good connections to ground.

Yes, this is using a 3-prong cord, ground has its own lug of the PT, black to power switch, white to PT primary. Death cap removed. I wired to the switch first, then the fuse to be sure the fuse isn't hot if the amp is plugged in but off.

Heater wires are twisted nicely and up in the air, they only come down at the tube sockets with no loops.




I went ahead and tried your suggestions because I really appreciate any help. It makes sense none of this worked though because it's mostly related to the power amp circuit and that circuitry is running quietly with the preamp tubes removed.

Yes, the wires from jacks/pots to tubes are shielded and only grounded at the control end.

I removed the switchable NFB. It's now wired as per Fender's layout (except above the board instead of under).

I've verified OT primary leads are correct as per Fender's layout.

I've verified no heater or any other leads shorting pins on sockets.

I tried the head in cabinet to verify it's not just RF interference. There is a lot of RF interference when it's out of the head, but I can tell the difference between that noise and this hum.


Does anyone have any more ideas? I know we can figure this out if we keep at it! 🙏
Boy, that's a stumper. But it seems it's always something easy to overlook.

"I've verified OT primary leads are correct as per Fender's layout."
I dont really think this is it, but just want to point out if by chance it's not a Bassman original OT, it's possible the Brown and Blue colors are a wrong match. ie: dont follow the layout colors. Had that issue a few times.

With that hum in the power section, I wonder if something is amiss in the Filter Cap board, ground or wire routing etc. You are plugging the speaker into the main outlet not the extension speaker outlet by mistake aren't you? I guess that may not hum anyway, just be silent.

I have had power tubes go in clocked wrong on old Fender amps, the loose center hole on the octals may allow the tube to go in wrong.
 

Lowerleftcoast

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I removed the switchable NFB. It's now wired as per Fender's layout (except above the board instead of under).

I've verified OT primary leads are correct as per Fender's layout.
@schmee and I would like the NFB disconnected, not rerouted. We are grasping for straws but one straw will solve the problem.

I am curious how the (filter board) doghouse is wired.
Using the picture below...
I assume there is an under-board wire where the yellow line is.
I assume caps 1&2 are in series and grounded at the transformer side of the amp.
It appears cap 6 is grounded on the input side of the amp.
Are my assumptions correct?
Where are caps 3, 4, and 5 grounded? Doghouse1.jpg
 
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CodySorgenfrey

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Boy, that's a stumper. But it seems it's always something easy to overlook.

"I've verified OT primary leads are correct as per Fender's layout."
I dont really think this is it, but just want to point out if by chance it's not a Bassman original OT, it's possible the Brown and Blue colors are a wrong match. ie: dont follow the layout colors. Had that issue a few times.

With that hum in the power section, I wonder if something is amiss in the Filter Cap board, ground or wire routing etc. You are plugging the speaker into the main outlet not the extension speaker outlet by mistake aren't you? I guess that may not hum anyway, just be silent.

I have had power tubes go in clocked wrong on old Fender amps, the loose center hole on the octals may allow the tube to go in wrong.

I can switch the OT primaries if that 100% eliminates this as an issue. The OT is the original 125A13A listed on the schematic so the colors should match.

Just to clarify, the hum is not in the power section. when V4 (Phase inverter) is removed the amp is silent. So that makes me think this isn't a power or bias filtering issue. This is also what has led me to believe the mods I have made (Power section grounding to PT lugs, heater pot/resistors, bias pot) have been fine.

Perhaps the grounding bus in the preamp section is a mistake? But the hum was still there and just as bad when I had the original brass bus in there.

Thankfully my power tubes only fit in the one way and that matches the labels on the pins.


@schmee and I would like the NFB disconnected, not rerouted. We are grasping for straws but one straw will solve the problem.

I am curious how the (filter board) doghouse is wired.
Using the picture below...
I assume there is an under-board wire where the yellow line is.
I assume caps 1&2 are in series and grounded at the transformer side of the amp.
It appears cap 6 is grounded on the input side of the amp.
Are my assumptions correct?
Where are caps 3, 4, and 5 grounded? View attachment 1053068

I will remove the NFB circuit all together in the morning and report back! You never know.

Here's a photo of the filter board underside, as well as a drawing on the top side. Your assumptions are correct. 1, 2 & 3 are grounded together at the PT. 4, 5 & 6 are grounded together at the preamp bus. I've also tried 3 grounded at the preamp bus but that didn't change anything. I couldn't find much online for how this is supposed to be from the factory so let me know if you know.

I did have cap 2 flipped at one point in my haste and frustration. I only had the amp on with it backwards for 10 seconds before I realized it. Since it still tests at 80uF and isn't shorted I think it's ok still. I could always try replacing it with a new one, but I don't think that's the issue. Plus, it's not the cap connected directly to ground, so I think if anything it just let cap 1 and the 220k get hit with the full force of 450V.
 

schmee

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There is a ground loop. The ground bus attaches to the ground terminal at each input jack and the jacks are grounded to the chassis.

View attachment 1052755

Unrelated to above: Check that the switches on the jacks make and break contact as they should.
Not a fan of that ground buss. Or rigid wire busses in general.

More grasping and observations:
A nice clean photo of the whole board and chassis would help.
Are the wires I marked in pink and yellow on my drawing there still... under the board?
I wonder if the proximity of that yellow wire from the Filters to location B is too close to your ground bus?
Bassman layout.jpg bfbASSMAN.jpg
 

CodySorgenfrey

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Not a fan of that ground buss. Or rigid wire busses in general.

More grasping and observations:
A nice clean photo of the whole board and chassis would help.
Are the wires I marked in pink and yellow on my drawing there still... under the board?
I wonder if the proximity of that yellow wire from the Filters to location B is too close to your ground bus?
View attachment 1053379 View attachment 1053380

I will be taking the whole amp apart, testing the boards for conductivity, and cleaning or replacing them. I'll put it back together according to Fender's stock layout to avoid any ground issues I may have introduced.

I tried to get some solid photos of the whole amp and closeups of key areas. I'll take some more and maybe throw in a video! I appreciate you going through the build via photos, I'm sure that's very frustrating work.

Yes, the 100K is under that cap, and yes, those underboard wires are still there and have continuity. No, those 1.5K resistors aren't missing, I just moved them from directly over the tube socket to going around the side. I'll put them back as per Fender's layout.

When the amp was using the original brass grounding bus the hum was still there and just as loud, so I don't think it would be that B+ wire near the ground bus but maybe I just traded one issue for another. We'll see once I have it cleaned up and rebuilt.
 




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