DIY B15N - Noise Issue

[ndawson]

I'm trying to locate noise coming from my B15N amp I'm putting together. Attached are some photos, schematic I'm using and audio file of the noise. Audio file starts with volume pot all the way down, then to 9 o'clock, then to 12 o'clock, then gradually down. Recording with my iphone, 12 inches away from the speaker cabinet. Loudest volume is around 20 seconds. Bass guitar is plugged in and noise doesn't really change with tone and volume pots either fulling on or off. Tube voltages seem fine. Tubes are from a stash of NOS tubes that a family member left me. The output tubes aren't matched, but don't think this noise is coming from that. I've switched the output tubes with a couple other 6l6's and still the same noise.

Modifications to the schematic on this amp:

Second input jack on first 6sl7 channel is eliminated and it's 2 resistors.
Second 6sl7 channel is eliminated and it's components to the second stage.

Any ideas where the noise could be coming from? Bad tube, bad wiring on my part?

Thanks,
Nate

B15N Top.JPG

B15N Inside.JPG

b15n-1_schematic.pdf

B15N Noise.m4a

[Stevem]

That’s 120 hz hum you have taking place and it’s more likely then not due to your layout.

One wrong thing I think I see is that work cable off of your input jack is grounded at both ends and it should only pick up its ground for the shield at the jack end, not the tube end.

Other common things that make for that type of hum/ noise is grounding of the power supply .

The center tap of the power transformer B plus must go to the filter first and then off to the chassis.

A star grounding layout is pretty fool proof in terms of low noise.

Another thing that you should look at is the it’s imperative that you keep your grid leads short, and especially the ones going from the PI tube over to pin 5 of your output tubes.
In terms of this it’s best to put your PI coupling caps right on pin 5 of those tubes.

This should be looked at for any other grid leads back in the preamp.

In terms of using process of elimination to narrow down where your noise starting at you can do this in your preamp section .

If you place a short across your plate load resistor that will kill the gain from that stage and give a clue as to weather it’s a good circuit section or noisy one.

[ndawson]

Thanks for the reply. The grey cable on the left has it's shield grounded at the star ground point at the input jack. The other end connects to pin 1 of the preamp tube and nothing else. I think that's what your referring to as the work cable.

The PT center tap connects to the first filter cap negative. From there, it daisy chains down to the PI ground section, then to the input jack star ground point. The only point where the ground touches the chassis is at the star ground point of the input jack. Should I be isolating each of the sections grounds to it's own chassis/ground point? I'm pretty sure I've got more of a buss grounding scheme here going from the first supply filter caps, PI section, preamp section, input jack star ground. I could always star ground all of the points back to the input jack I guess.

I'll look at the PI coupling caps and place them at the tube socket.

As far as shorting the plate load resistor...just don't want to damage the tube. Will that be alright for a quick check?

Thanks again.

[Stevem]

Then just don’t power that preamp tube section , lift the plate load resistor on one

Curious here, not that it matter to your issue, but how many watts is your OT rated for?

[R.G.]

[...]
The PT center tap connects to the first filter cap negative. From there, it daisy chains down to the PI ground section, then to the input jack star ground point. The only point where the ground touches the chassis is at the star ground point of the input jack. Should I be isolating each of the sections grounds to it's own chassis/ground point? I'm pretty sure I've got more of a buss grounding scheme here going from the first supply filter caps, PI section, preamp section, input jack star ground. I could always star ground all of the points back to the input jack I guess.

I don't know if you have read Guitar Amp Wiring Notes yet. If you haven't, you might like it. It touches on most of the issues that can make 60Hz and 120Hz hum. It's here:
https://www.dropbox.com/scl/fi/vpijj3al ... j5hmn&dl=0

You're way ahead having already tied the PT CT to the first filter cap negative. I'm guessing you did this right and nothing else touches the wire carrying the CT to the first cap negative.

You don't mention how the output tube cathodes get their current back to the first filter cap negative. Do the output tube cathodes connect to the PI local ground?

Just some random comments: ground is a point, not a wire or a buss, or the chassis. If you define your amp's "One True Ground" as the first filter cap negative, it makes things easier and sidesteps some issues.

For my amps, I would never use the chassis as a conductor, only as an rf shield. You're already doing this right by having the input jacks' sleeve contacts be the only connection to the chassis. Don't pollute your existing good work by connecting random other things to chassis.

Does the amp use a tube rectifier or solid state diodes? We had a similar question not long ago where the 120Hz magically disappeared when the SS diodes rectifying the high voltage were bypassed with 0.1 or 0.01 ceramic caps to eat the RF blasts that ordinary slow SS diodes produce when they slam off in recovery.

[ndawson]

Looks like a great article. I'll read through it. Thanks.

Yes, I've got the PT CT to the first cap negative. Nothing else attaches at that point.

The output tube cathodes connect in between the PI local ground and the second filter cap. Should I be connecting them to the PI local ground? I normally just run some 14/romex copper ground from the input jack to the first filter cap. Then I attach the different sections directly to that copper wire.

I've got a tube rectifier for this amp.

Should I be attaching the OPT ground wire to the output jack ground connection (it's isolated from the chassis) or back to one of the local ground points? It's currently attached to the output tube cathode ground section.

I think I'll read your Wiring Guide as I'm sure it has the answers I'm looking for.

Thanks again.

[ndawson]

Reading your article, the OTG is the first filter cap. Looks like the output tube cathodes will go back to that 1st cap negative. And the OT common will go to the PI local ground.

From the photos I've seen of original Ampeg B15N's, it looks like the grounding system is using a buss wire. Would having the local ground points for each section be true to the original grounding scheme? Or should I create local grounds for each section and individually send each section back to the first filter cap negative?

[ndawson]

I decided to shorten things up inside and moved the tubes around in the fashion of the original layout of a b15n. Attaches are some new photos. I still have the same noise present. I've followed the grounding scheme as the first cap as the OTG. The PT CT is attached here as well as the output tube cathodes.

At the input jack (I'm only using one of them instead of three in the original schematic that I'm basing my build on), could that be where the noise is originating? I'm not using the second 6sl7 channel and I'm also not using the second input jack and it's 120k and 1m resistors. I'm thinking that's the issue...?

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[R.G.]

Should I be attaching the OPT ground wire to the output jack ground connection (it's isolated from the chassis) or back to one of the local ground points? It's currently attached to the output tube cathode ground section.

Current leaves the OT speaker winding, goes through the speaker and has to return to the other end of the speaker winding. If the amp didn't use feedback, grounding or not grounding the speaker jack would not matter. But you have feedback from the 16 ohm end of the speaker winding, so the 0/common end of the speaker winding has to connect to the same ground as the PI somewhere. The best place to connect it would be where none of the speaker current can cause ground-wire-resistance shifts. I prefer the output jack isolated from chassis, then a wire from the sleeve contact back to the >>OT speaker winding common<< which is where the current came from. The OT common can then be grounded to signal ground either at the PI local ground, or at the OTG. It's probably better to tie it to the PI local ground.

Reading your article, the OTG is the first filter cap. Looks like the output tube cathodes will go back to that 1st cap negative. And the OT common will go to the PI local ground.

That's a good way to do it.

From the photos I've seen of original Ampeg B15N's, it looks like the grounding system is using a buss wire. Would having the local ground points for each section be true to the original grounding scheme? Or should I create local grounds for each section and individually send each section back to the first filter cap negative?

Star grounding isn't the only way to get quiet, hum-free grounding. It's just the only way provable to do it without a lot of experimentation. It's more work, too. In a manufactured amp, it's fine to experiment a lot to get a grounding scheme that is quiet enough, if not perfect, and that is also cheaper in terms of labor to mass produce. So yeah the original probably did use a buss setup. The thing about buss grounding is that it's highly dependent on exactly where everything is physically, and exactly where they connect to the buss. So unless you can reproduce the wire locations, you're still up for some tinkering.
I prefer making local star grounds; for instance the preamp tube, tone/volume and recovery tube all gathered at a local star; then that star with its own wire to the One True Ground individually, your second option. This is a predictable quantity. As an aside, it is possible that the original B15 didn't have all that great hum performance to start with. Maybe its designer got it to be good enough, and shipped it. If so, local stars and one true ground will be lower hum.

All of this is sidestepping your original question, though. I'm with Steve - it sounds like 120Hz hum, not 60Hz. It also sounds buzzy, not a clean low frequency signal. If you have the PT CT going to the first filter cap negative, and no connections to the wire between the PT CT and the OTG, that sidesteps the major source of 120Hz hum, presuming only that your B+ ripple is under control. I had a look at the schematic. Did you wire the AC power from the line cord to the power transformer the way that schematic shows? If so, it could well be conducting in AC mains junk. Fluorescent fixtures with bum grounds are notorious for generating this kind of thing. Got fluorescents nearby?

Or did you modify it to a three wire ground?

[ndawson]

I've got a three wire (hot/neutral/ground) power cord connected. I have the green/ground wire bolted to the chassis where in comes in which is another modification to the schematic I made.

No fluorescent fixtures around, just led lighting. I will take the amp to another location in the building to see if it the noise goes away.

Here's how I'm wiring my output jack: I'm using a TRS jack, isolated from the chassis. From the OT, I've got the yellow wire going to the tip, green wire going to the ring and the black wire going to the sleeve. From the sleeve point at the output jack, I ran a second wire to the PI local ground. I'm using a Hammond AO-20936-3 OT that I think came from an old M3 organ. I used this on my other SB-12 build and it worked fine for that but maybe I'm wiring this wrong here.

I do have the output tube cathodes tied to the first cap negative. But the PT CT connects directly to the OTG, first cap negative so that should be ok?

I don't have a scope, but I tried to measure ripple with my meter and didn't notice any AC on the B+.

[ndawson]

I tried a different spot in my house, but still have the buzzing coming through the speaker.

I will say that the bass amp sounds coming from the amp sound great...just the buzzing that will probably be an issue when recording.

[R.G.]

I've got a three wire (hot/neutral/ground) power cord connected. I have the green/ground wire bolted to the chassis where in comes in which is another modification to the schematic I made.

OK, that's good. I was wondering about that capacitor ground connection in the schemo. But three wire should be good.

No fluorescent fixtures around, just led lighting. I will take the amp to another location in the building to see if it the noise goes away.

Oh, well. Another good theory bites the dust.

Here's how I'm wiring my output jack: I'm using a TRS jack, isolated from the chassis. From the OT, I've got the yellow wire going to the tip, green wire going to the ring and the black wire going to the sleeve.

I'm not sure what color goes to which OT secondary tap. Can you relate this to the schemo? It has 0, 8 and 16, presumably common, 8 ohms, and 16 ohms. With three wires going to a TRS jack, it seems like a mono plug would short the ring terminal to the sleeve. I don't know this would have anything to to with your hum, but ...

From the sleeve point at the output jack, I ran a second wire to the PI local ground. I'm using a Hammond AO-20936-3 OT that I think came from an old M3 organ. I used this on my other SB-12 build and it worked fine for that but maybe I'm wiring this wrong here.

If sleeve is connected to the common on the OT secondary, running a wire from the sleeve contact on the jack to the PI local ground would be fine. The big point is that the OT secondary common return current does not run through the chassis; it has its own wire. That's good.

I do have the output tube cathodes tied to the first cap negative. But the PT CT connects directly to the OTG, first cap negative so that should be ok?

Yep, that's OK - good and in keeping with star grounding practice.

I don't have a scope, but I tried to measure ripple with my meter and didn't notice any AC on the B+.

Yeah, I mentioned that mostly for completeness.

Good practice to test different AC sockets. That probably eliminates the external buzz theory.

[ndawson]

I don't have any new PS filter caps but I have some NOS multi-can caps that I could try out and see if that solves the buzzing sound. I wish I had a scope I just don't do this enough to warrant the need for one.

I did try replacing the 5u4 rectifier tube but still have the buzz. Seems like this is probably power supply/b+ related?

[SoulFetish]

It sounds really buzzy to me. Do you use any dimmers for your lighting? It sounds a lot like the noise we get at the house where my band practices. The dimmers in his house radiate miserably.
I know you swapped in other output tubes. But here is tip which can save you a lot of time spent chasing noise demons around your amp.
One thing you can do is power up your amp so you can hear the problem, then pull your preamp tubes in order, 1 at a time, starting with V1. Pause, to see if the noise changes or goes away at some point point in the tube chain. (You can power down or put it in standby while pulling your preamp tubes to avoid any loud pops). If there is no change in noise, the you can eliminate that tube and it’s corresponding amplifying stage as the cause of the problem for the moment. If the noise does go away after pulling one of them, then first thing to do is swap in a known good tube, or even just another tube to see if you have a bad preamp tube. If its not a bad tube then you have a good starting place to start troubleshooting. Look closely at the stage/stages that that particular tube handles.
Keep in mind that a bad tube can imitate all kinds of n noise problems that are normally caused by other fault conditions.

[ndawson]

No dimmers here but yeah that's what it sounds like. I've got an area in my garage studio that has power lines overhead and I can't use certain mics without situating them at certain angles to cancel the noise. It's a pain. On the other end of the house, dead quiet. That's where I took the amp and the noise followed so I know there isn't any outside interference causing and it's in the circuit itself.

The buzz does goes away when pulling first and only preamp tube. Still a very ever so slight hum in the background but that's more than likely ground related. The main noise is gone. I've only got three 6sl7 type tubes in my possession so I've got one in the preamp stage and one as the phase inverter. I can't imagine they are all three bad. I've rotated them all around with the same noise being produced in any and all orders. I only pulled the preamp tube and left the other 6sl7 phase inverter tube in place for the test. Maybe I should order some know working preamp tubes.

Thanks for the tip/s.