Mojotone NC3015 Debugging help

[_ej_]

I've got a Mojotone NC3015 build up and running. The circuit largely matches the schematic/layout given by Mojotone with a couple changes.

• I deleted the Normal channel. This was accomplished by grounding the grid of V1A and removing the inputs and volume pot from the circuit.
• I tweaked the grounding scheme. I disconnected the bus bars from the back of the pots and grounded the can cap and PT centertap to the end of the bus.
• I shielded the wires going from the input the V1 and from V1 to the volume pot. I also moved the grid stopper closer to the tube socket.
• I replaced the amber lamp with a blue LED (https://www.amazon.com/dp/B01F1SDE4E?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1).

I have gone through the amp twice and verified it's wiring matches the layout with the exception of the modifications above (highlighter trick was super helpful). I've powered up the amp and it sounds fantastic however I have almost no headroom. From 0-1 on the volume knob I get clean volume but beyond that I just get overdrive. I've checked the voltages and everything is reading low when compared with the schematic. I've confirmed the values of the voltage divider resistors in the B+ circuit are correct.

V1a/b:
Plate - 105V
Cathode - 1V

V2a:
Plate - 114V
Cathode - 0.8V

V2b:
Plate - 187V
Cathode - 117V

V3a/b:
Plate - 199V
Cathode - 52V

V4/5
Pin 3 - 11.3V
Pin 9 - 312V

B+ Circuit Voltages:
XA: 351V
XB: 312V
XC: 264V
XD: 186V

I've attached the schematic. The voltages look a bit strange to me (2V drop across the 2.2K resistor between XA and XB) but the tech at Mojotone insists the number are correct. Do you guys think there is an issue with my amp causing my low B+ voltages or is the schematic wrong? Also any thoughts on my headroom issues? The amp seems quiet compared to other AC15's I've played and I'd like more clean volume.

Here are gut shots:

[AlNewman]

What are your voltages at pin 7 of the power tubes?  You may want to adjust your cathode resistor.

[_ej_]

What are your voltages at pin 7 of the power tubes?  You may want to adjust your cathode resistor.

338V for both

[AlNewman]

Seems a bit hot for a 12W tube.

I'd try to lower wattage in the power tubes to get within acceptable range  by raising the cathode resistor.  I doubt it will help you with the volume range you're experiencing, but your tubes will last longer, and it will get you closer to the B+ voltages on the schematic.

The distortion could have something to do with the shared cathode on V1.  It may be better to only run heaters on the unused triode.   

It does look like Mojotone's voltages are out to lunch, but maybe my math is wrong.  I'd just approach it according to what your findings are and throw their voltages out the window.

It looks like the dropping resistor in your amp is 7k, not 10k, between XB and XC?  I see purple as the 1st band.

[HotBluePlates]

I've got a Mojotone NC3015 build up and running. The circuit largely matches the schematic/layout given by Mojotone with a couple changes.

• I deleted the Normal channel. This was accomplished by grounding the grid of V1A and removing the inputs and volume pot from the circuit.

... I have almost no headroom. From 0-1 on the volume knob I get clean volume but beyond that I just get overdrive.  ...

Thank you for the large photos, they helped confirm the issue.  You likely need only ground the unused channel-mix resistor (220kΩ) at the circled turret in the photo below.

   - The stock circuit reduces the signal-level of each channel by about half due to the interaction of the 220kΩ channel mix resistors.
   - The output of the Top Boost channel is normally reduced via the voltage divider these mix resistors form; the path of this divider is highlighted in the schematic below.

   - When you deleted the Normal channel Volume but kept everything else, you removed the ability of the channel mix resistors to reduce signal-level.

You should be able to run a single wire form the turret to the ground buss, and reduce the excessive signal level.

[bmccowan]

First - Great example of a troubleshooting post - schematics, good photos, voltages!
Consider these as additional comments to HBP's recommendation - which is likely spot on.
I built one of those - albeit for the ridiculous reason that I like the faceplate I modified the power amp for 6973s, and used iron from a Hammond AO-39 which resulted in almost identical voltages.
I found channel 1 to be too hot - almost no headroom. I change V1 to a 5751, V2 to a 12av7, and V3 to a 5751, and I reduced the bright cap to 20pF. More to my liking, but I am not a fan of high gain amps.

[Platefire]

Yelp, good info from OP to go by and good call by HBP. Be interested to see the report of how the amp is acting with the ground added

[_ej_]

Thank you for the large photos, they helped confirm the issue.  You likely need only ground the unused channel-mix resistor (220kΩ) at the circled turret in the photo below.

   - The stock circuit reduces the signal-level of each channel by about half due to the interaction of the 220kΩ channel mix resistors.
   - The output of the Top Boost channel is normally reduced via the voltage divider these mix resistors form; the path of this divider is highlighted in the schematic below.

   - When you deleted the Normal channel Volume but kept everything else, you removed the ability of the channel mix resistors to reduce signal-level.

You should be able to run a single wire form the turret to the ground buss, and reduce the excessive signal level.

Thanks so much man! I haven't had a chance to really dig into because my kid is napping but adding that ground has eliminated the constant hum and I have nice clean volume up to at least 25% on the volume knob. Once she's awake I'll dig into it a bit more but it seems like my biggest problem is fixed! The noise floor seems right around what I'd expect. Its not dead quite but I'm happy with it. I might try moving some leads around to see if I can reduce it further.

I am still getting a hum that increases with the Top Cut knob. Any idea where I should start to try and get rid of it?

First - Great example of a troubleshooting post - schematics, good photos, voltages!
Consider these as additional comments to HBP's recommendation - which is likely spot on.
I built one of those - albeit for the ridiculous reason that I like the faceplate I modified the power amp for 6973s, and used iron from a Hammond AO-39 which resulted in almost identical voltages.
I found channel 1 to be too hot - almost no headroom. I change V1 to a 5751, V2 to a 12av7, and V3 to a 5751, and I reduced the bright cap to 20pF. More to my liking, but I am not a fan of high gain amps.

Nice to hear from someone else who built one of these. Yeah, I'm fairly convinced that the schematic is wrong. To drop only 2V across that 2.2K resistor would mean the tubes were only drawing like 0.9mA.

[HotBluePlates]

I am still getting a hum that increases with the Top Cut knob. Any idea where I should start to try and get rid of it?

Pull V2 (2nd gain stage & cathode follower) from its socket.

If the hum goes away, you might need to swap this tube to a different socket (see rest of the post).

If the hum remains, try tacking a large-ish cap (25µF to 250µF) across the phase inverter's 1.2kΩ self-bias resistor.
If the hum remains, tack that large cap from phase inverter cathode to ground (temporarily); that's across the 1.2kΩ and the 47kΩ resistors.

_______________________________

Some tubes have heater-to-cathode leakage and exhibit hum unless a large bypass cap is used to bypass all AC from the cathode to ground.

However, sometimes we have circuits where we cannot do that (cathode followers, long-tail inverters, stages with local negative feedback, etc).  When a large cathode bypass cap cannot be used, tubes must be selected to have low heater-to-cathode leakage.  Unfortunately, most tube vendors either do not screen for this issue, or don't want to acknowledge that they know they're selling "hummy tubes, that are okay in a lot of circuits with big cathode bypass caps."

_______________________________

The thought process is the Cut control is only supposed to reduce high frequencies.  Your photo suggest this control is wired correctly, so we wonder if the control is basically reducing the output of the phase inverter.

The next thought is whether the hum is originating at the phase inverter, or elsewhere.  Best plan is to isolate "which stage is humming" to determine the cause & a solution.

Many folks fixate on "grounding" when encountering hum, and forget that sometimes tubes are defective in a way that hums.  That defect is laid bare when there is not a heavy-bypass for AC from cathode to ground.  I once had a prominent boutique builder ask me about this same problem, and was convinced the tubes were not at fault, because swapping in other 12AX7s did not fix the issue.  Turns out, every 12AX7 in a case of 100 had the same heater-to-cathode leakage hum.

After that experience, I look for ways to prove whether the tube is leaking before moving on to other checks.  There are tube testers that can check for this, but they are mostly expensive and/or obscure.

[_ej_]

I'll try that out tonight, thanks!

Yeah, the knob is not a top cut at all. It acts way more like a top boost, it reminds me of the treble knob on an amp with active EQ.

I've been assuming its more akin to presence knob but I need to dig into some reference material to really figure out what its doing.

[HotBluePlates]

Yeah, the knob is not a top cut at all. It acts way more like a top boost, it reminds me of the treble knob on an amp with active EQ.

I've been assuming its more akin to presence knob but I need to dig into some reference material to really figure out what its doing.

1 + (-1) = 0

The "Cut" control is a resistance in-series with a cap.  All that runs from one output of the phase inverter to the other output of the phase inverter.
The Cap means it wants to mostly pass the highest frequencies.
The series (variable) resistor allows adjustment for "highest frequencies" to include "high frequency" and/or "mid frequency."
The phase inverter has two outputs of opposite-polarity.

The Cut control reduces treble right before the output tubes by allowing some of one phase inverter output signal to sneak-over & cancel the other phase inverter signal output.  Depending on the control setting, only super-high stuff gets canceled, or some progressively-lower stuff is allowed to pass through & cancel out.

______________________________

"Hummmmm" is normally 60Hz or 120Hz, and way below the range of stuff affected by the Cut control.  Normally, that would make me think the control was wired incorrectly, but what I believe I see in your photos shows the control wired correctly.

So then I wondered whether this "volume control on the phase inverter" was reflecting hum being injected at the phase inverter.

[_ej_]

Pull V2 (2nd gain stage & cathode follower) from its socket.

If the hum goes away, you might need to swap this tube to a different socket (see rest of the post).

If I pull V2 the amp is basically dead quiet. All three of the 12ax7 tubes in the amp are essentially equally noisy in that position.

Is the only option to find a quieter tube?

[HotBluePlates]

I am still getting a hum that increases with the Top Cut knob. Any idea where I should start to try and get rid of it?

Pull V2 (2nd gain stage & cathode follower) from its socket.

If I pull V2 the amp is basically dead quiet. All three of the 12ax7 tubes in the amp are essentially equally noisy in that position.

Is the only option to find a quieter tube?

Momentarily ground V2 Pin 2.  Most likely the hum will remain.

If the hum does remain use any handy capacitor, and temporarily tack one from the cathode follower grid to ground.  You want a value large enough to bypass hum, so 0.047µF or bigger oughta do.

The goal here would be to figure out whether the hum is being added by leakage at the follower, or if it is present at the gain stage before.  This added cap splits the two stages up, so hum from Pins 1-3 will not contribute to the follower section on Pins 6-8.

[_ej_]

Momentarily ground V2 Pin 2.  Most likely the hum will remain.

Grounding pin 2 eliminates the hum.

[HotBluePlates]

Momentarily ground V2 Pin 2.  Most likely the hum will remain.

Grounding pin 2 eliminates the hum.

You can keep moving the Temporary Ground towards the input jack:
   - Ground at the Volume control wiper.
   - Ground the plate (via a capacitor) of the input stage.
   - Ground the grid of the input stage.

We are looking for the circuit point where hum is killed if we move towards the output section, but remains when moving back towards the input jack.  Eventually you will isolate the stage where the hum is entering the circuit, and can then look around for causes & cures.

[_ej_]

Momentarily ground V2 Pin 2.  Most likely the hum will remain.

Grounding pin 2 eliminates the hum.

You can keep moving the Temporary Ground towards the input jack:
   - Ground at the Volume control wiper.
   - Ground the plate (via a capacitor) of the input stage.
   - Ground the grid of the input stage.

We are looking for the circuit point where hum is killed if we move towards the output section, but remains when moving back towards the input jack.  Eventually you will isolate the stage where the hum is entering the circuit, and can then look around for causes & cures.

Maybe I'm trying to fix something that isn't an issue. When no guitar is plugged in the input is grounded and the hum is gone.

I've tried two different guitars (both with humbuckers) and it's present but only with the volume knob up.

[Esquirefreak]

Does the switch/breaker on the input jacks function properly?

Max

[shooter]

Does the switch/breaker on the input jacks function properly?

When no guitar is plugged in the input is grounded and the hum is gone.

[Esquirefreak]

Sorry, I read it backwards! 

/Max

[_ej_]

I figured the easiest thing to do would be make a quick video of what I'm hearing.

When the amp is turned on with nothing plugged in there is a very quiet pulsing sound.

Once a guitar is plugged in there is a hum, that hum can be affected by the Top Cut knob but at this point I think im just hearing the hum be filtered by that control, I don't think it's the cause.

Honestly it sounds like a bad guitar ground to me but it happens with all my guitars. When comparing with my Fryette Pitbull this amp has much more hum.

Is this potentially just a lead dress issue? It's not so loud that the amp is unusable but it's definitely louder than Id like.

i=pnBVQPu21Z2QBe9D

[Esquirefreak]

Have you tried shielding the chassis with for example a piece of sheet metal?

[_ej_]

Have you tried shielding the chassis with for example a piece of sheet metal?

No but I can throw some tinfoil over it and see if it helps.

[bmccowan]

Video - yup, that's too much hum and a bit of buzz eh?
Did you try what HBP suggested?

You can keep moving the Temporary Ground towards the input jack:
   - Ground at the Volume control wiper.
   - Ground the plate (via a capacitor) of the input stage.
   - Ground the grid of the input stage.

We are looking for the circuit point where hum is killed if we move towards the output section, but remains when moving back towards the input jack.  Eventually you will isolate the stage where the hum is entering the circuit, and can then look around for causes & cures.

That is a great way to find the section causing the issue.

[SEL49]

That fits the symptoms of an instrument cable with a broken shield. Could also be caused by miswired input jacks. Check cable and input jack wiring.

Oh BTW, remove all connections to V1 pins 1, 2, and 3. That includes the jumper between pin 3 and pin 8.

[bmccowan]

Just trying to help you rule out things. I fired mine up and plugged in a P90 guitar...very quiet. Removed the chassis cover - no change. Same result twiddling with the treble cut pot. Since I have an 12av7 in V2 position I changed it to a new 12ax7. Still quiet.
So I think HBP's process and SEL's suggestions are good steps.
I once had a bad pot that caused similar buzz/hum. HBP's process would find that without wasting time replacing potentially good components.

[_ej_]

I'll try working backwards through the amp tonight and I'll also verify the input jack wiring.

I did try several guitars and guitar cables with no change. It is also present on the hi and low inputs.

I removed everything going to V1a.

[SEL49]

This is how I wire Hi/Lo Cliff jacks...

[HotBluePlates]

Maybe I'm trying to fix something that isn't an issue. When no guitar is plugged in the input is grounded and the hum is gone.

That fits the symptoms of an instrument cable with a broken shield. Could also be caused by miswired input jacks. Check cable and input jack wiring.

Oh BTW, remove all connections to V1 pins 1, 2, and 3. That includes the jumper between pin 3 and pin 8.

This is how I wire Hi/Lo Cliff jacks...

I think SEL49 has you on the right course now.

[_ej_]

Maybe I'm trying to fix something that isn't an issue. When no guitar is plugged in the input is grounded and the hum is gone.

That fits the symptoms of an instrument cable with a broken shield. Could also be caused by miswired input jacks. Check cable and input jack wiring.

Oh BTW, remove all connections to V1 pins 1, 2, and 3. That includes the jumper between pin 3 and pin 8.

This is how I wire Hi/Lo Cliff jacks...

I think SEL49 has you on the right course now.

I rewired the jacks and confirmed they were correct. Sadly it did not fix my issue.

I worked backwards through the amp but the hum always goes away when I ground the signal all the way back to the input.

I haven't tried grounding the plate of the input stage, when you say to ground it via a capacitor do you mean ground it after the 500pF cap? Or something else?

I did notice that when I wiggle the grid pin on the input stage tube socket there is static and noise. I didn't hear any static when I similarly disturbed the plate pin, could a faulty socket be the issue?

[shooter]

could a faulty socket be the issue?

Yep, any sloppy connections, bad solder, wire dress is a gremlins best friend.  a sloppy pin connection forms a "resistance greater than zero".  your meter won't measure it most times, but noise will jump all over it.

[HotBluePlates]

... when you say to ground it via a capacitor do you mean ground it after the 500pF cap? Or something else? ...

A capacitor is a short-circuit for AC but an open-circuit for DC.

If you want to kill AC at a tube plate, connect a cap from the plate to ground.  AC is now 0v, but DC is not 0v because we don't want to short-circuit the power supply.

[_ej_]

... when you say to ground it via a capacitor do you mean ground it after the 500pF cap? Or something else? ...

A capacitor is a short-circuit for AC but an open-circuit for DC.

If you want to kill AC at a tube plate, connect a cap from the plate to ground.  AC is now 0v, but DC is not 0v because we don't want to short-circuit the power supply.

Gotcha, any particular value?

[HotBluePlates]

Capacitive Reactance tells you how many Ohms a capacitor is at a given frequency.

   Fill in "Frequency" (maybe with 60 Hz?)

   Fill in "Reactance" (how many Ohms?  5,000,000 or 5,000?)

   Read off "Capacitance" (change the Units as needed)


You will see it takes about 0.05µF to present 50kΩ of Reactance at 60Hz.

The cap needs to be large enough to be "very low reactance" or "a short circuit" compared to the stuff around it in the circuit.

[tubeswell]

Gotcha, any particular value?

0.1uF 600V will be more than sufficient

[_ej_]

Made a little progress today. I replaced the V1 tube socket. This reduced the hum significantly but did not eliminate it. I would say its at a level I would consider acceptable but I'm still going to try and completely eliminate it.

When I ground the input jacks the hum remains but if I ground the grid of V1b it is nearly completely eliminated. So it seems there some kind of issue with the wire going from my input jacks or potentially the way I've wired the grid stoppers. Any suggestions on how to improve that? I might run 2 discrete shielded wires in stead of the 2 conductor I have in there now.

Also, if I ground the V1B plate the noise is completely eliminated so I'm thinking I might also have a noisy tube in that spot.

[_ej_]

I just rewired it to try and reduce the noise.

I've removed the low input and put i  a shielded single conductor wire and a single grid stopper resistor (47k just because it's what I had on hand).

The amp behaves about the same as the previous post.

Honestly the hum isn't horrible, it's significantly improved from where it was originally.

Referencing my previous post, any thoughts on what could be causing this hum? I would really like to get rid of it.

Here's another quick video so you can hear what I'm dealing with. Starting out I have the volume around 9 o'clock. You can hear the hum is very manageable. However it scales with the volume knob. It also isn't eliminated or reduced by turning the guitar volume down.

I am wondering if my V1 tube is just noisy. I might order a low noise tube just to try. Any suggestions? I was thinking a NOS GE 5751

[Willabe]

Very carefully try putting a metal sheet on top off the open chassis. See if that helps any, it should.

 Are those humbuckers in that guitar or single coil P90'S?

Does the humm lessen as you move away from the amp, or turn the PUP's away from facing the amp? 

[Willabe]

i=pnBVQPu21Z2QBe9D

I wouldn't plug into an open amp chassis that's sitting on top of a cab with 1 hand and a guitar hanging on me.

Put the guitar down/in a stand, so you have 2 hands free. Plug in with the amp off, not in stand by either.

You can dump/slide that amp chassis right off that cab. And your in-stinks are going to be to try and grab that chassis. Just like when we drop a kitchen knife and we in-instinctively want to catch it.   

[HotBluePlates]

Here's another quick video so you can hear what I'm dealing with.

Unplug everything from the Input jack.  No guitar, no pedals.  I think there's a 99.99999999% chance all the "hum" goes away.


The only noise I hear in the video comes from "Guitar Pickup" ---> "Pedal Gain."

[_ej_]

Here's another quick video so you can hear what I'm dealing with.

Unplug everything from the Input jack.  No guitar, no pedals.  I think there's a 99.99999999% chance all the "hum" goes away.


The only noise I hear in the video comes from "Guitar Pickup" ---> "Pedal Gain."

Yeah that's the guitar straight in. It's an ES335 with humbuckers.

So the hum remains when the guitar is unplugged. It's disappears completely when the V1b plate is grounded via a cap.

I was thinking it was just normal hum but this guitar straight into my other amp (with similar gain) is much quieter and like I said, the hum remains when I ground the input.

[tubeswell]

When the amp is turned on with nothing plugged in there is a very quiet pulsing sound.

Once a guitar is plugged in there is a hum, …

Honestly it sounds like a bad guitar ground to me but it happens with all my guitars.

Possibly your cable. You tried a different cable?

[shooter]

I deleted the Normal channel.

It's disappears completely when the V1b plate is grounded via a cap.

I mighta missed something, so...
V1b is in the normal channel


 

[Willabe]

Very carefully try putting a metal sheet on top off the open chassis. See if that helps any, it should.

The metal sheet will act as shielding. Use a cookie sheet. 

A member did this here a few weeks ago, killed off most of the last humm.

[Willabe]

In OP, in the pictures;

1. On 1st preamp tube socket, you have a black wire from pin 2 (?) that goes to a ground lug bolted to the chassis? And what's next to it, on pin 3? 

2. Did you ever solder up the white wire by the PT, power cord wire?

[_ej_]

I deleted the Normal channel.

It's disappears completely when the V1b plate is grounded via a cap.

I mighta missed something, so...
V1b is in the normal channel


 

I always assumed the B referred to the triode connected to pins 6/7/8. If that is correct then the schematic doesn't agree with the wiring diagram. Regardless, I removed all connections to the normal channel gain stages.

[shooter]

b is the 2nd half, but all I have here is a schematic


so which plate makes it all go away??

[tubeswell]

When the amp is turned on with nothing plugged in there is a very quiet pulsing sound.

Once a guitar is plugged in there is a hum, …

Honestly it sounds like a bad guitar ground to me but it happens with all my guitars.

Possibly your cable. You tried a different cable?

[_ej_]

When the amp is turned on with nothing plugged in there is a very quiet pulsing sound.

Once a guitar is plugged in there is a hum, …

Honestly it sounds like a bad guitar ground to me but it happens with all my guitars.

Possibly your cable. You tried a different cable?

I thought I had mentioned this at some point but regardless, happens with multiple cables and guitars.

I was using this exact guitar and cable earlier today thought my VHT with no noise.

Also the hum remains when nothing is plugged in. This behavior is new since I replaced the tube socket. Before I had a much louder hum that was eliminated by unplugging the guitar cable. 

[_ej_]

b is the 2nd half, but all I have here is a schematic


so which plate makes it all go away??

If I ground the plate on the first stage of the top boost channel

[Willabe]

In OP, in the pictures;

1. On 1st preamp tube socket, you have a black wire from pin 2 (?) that goes to a ground lug bolted to the chassis? And what's next to it, on pin 3? 

2. Did you ever solder up the white wire by the PT, power cord wire?