Very loud hum on Marshall 100w circuit

[Ampzone]

I am getting a very loud hum when I power up a Marshall 100w Super Lead I recently inherited. I have been fighting with this amp for a while. I have found If I remove the presence wire coming off the 8ohm tap on the output transformer it goes away, its quiet and you can play the amp. Replace the presence wire coming off the 47k resistor and boom! it's back, it's a very loud hum that requires you to just shut the amp off. While the amp powers down you then get a whining almost siren like phasing tone that goes up in down in pitch and then dies off as the power has been cut.

There is about 12-14v of DC on the presence pot's on leg. All voltages on V1-V3 are right in line with standard readings. No DC leakage on any of the coupling caps that I can detect. 10uf Bias caps good, no leaky diode in bias supply. Tube swaps do nothing. With the output tubes in, V1-V3 pre-amp tubes removed and the amp powered up the hum is not present. Replace the phase invertor V3 and it's back to loud hum unless you remove the presence wire.

Any ideas or suggestions?

Thanks for any help, this has got me stumped

[Enzo]

Yes. I'd bet my lunch money your output transformer primary wires are reversed.

Or the leads to the speaker jacks are miswired. Or maybe you are missing the little impedance plug and have a jumper wire in its place... plugged into the wrong holes.

[Ampzone]

Yes. I'd bet my lunch money your output transformer primary wires are reversed.

Or the leads to the speaker jacks are miswired. Or maybe you are missing the little impedance plug and have a jumper wire in its place... plugged into the wrong holes.

Excellent suggestions! I have tried the primary switch, that did not change anything I'm afraid. I will double check the speaker jacks and impedance selector, I think they are good but you never know. I will report back directly.

[Enzo]

Good thing I already had lunch then...

[Ampzone]

Good thing I already had lunch then...

The speaker jack and selector look correctly wired. Hmmmmm

[R.G.]

This may sound a little silly and simple, but have you tried swapping out the phase inverter tube?

[Ampzone]

This may sound a little silly and simple, but have you tried swapping out the phase inverter tube?

Thanks for the input. I have. I tried two different brands in each spot V1-V3 with no change. As I said earlier if remove V1-V3 and just leave the output tubes in and power it up it's quiet; put a preamp tube in V3 and it's back to very very loud nasty hum; as you power the amp off there is a siren like tone that winds around in pitch like a synth and then dies as the voltage bleeds off. Now if you were to put V3 back in and pull the presence wire going to the impedance selector off then it would be quiet. I bet something is staring me in the face but I cannot see it

[Ampzone]

I did test the presence pot which is giving me it's right value and trying the stick test with the amp on while moving the presence wire makes no difference.

[R.G.]

Thanks for the input. I have. I tried two different brands in each spot V1-V3 with no change. As I said earlier if remove V1-V3 and just leave the output tubes in and power it up it's quiet; put a preamp tube in V3 and it's back to very very loud nasty hum; as you power the amp off there is a siren like tone that winds around in pitch like a synth and then dies as the voltage bleeds off. Now if you were to put V3 back in and pull the presence wire going to the impedance selector off then it would be quiet. I bet something is staring me in the face but I cannot see it

hmmm... That's oscillation, all right. I'd normally immediately jump to reversed primary wires, just as Enzo did.

Presence is actually an active filter made by feeding back part of the output through Rs and Cs to the PI. As such, it's prone to the same issues as other active filters, plus the issues of the output transformer. That's why reversing the primary leads causes/cures oscillation in some instances - it's reversing the feedback from negative to positive.

So if it's not the tube, and not the primary wires reversal, maybe it's some of the other ills of active filters. At this point, I'm purely speculating.

When you remove the presence feedback, that's breaking the feedback loop, and why it quits. When you remove power supply, the siren sounds are the oscillation changing speed with the power supply as it goes down. Most often, this indicates positive feedback and a blocking oscillator instead of a negative feedback gain-phase oscillator. That's one argument that says that the feedback is still incorrectly positive. Active filter action requires negative feedback gain-phase action that almost but not quite oscillates.

At this point, I'd give grounds and power supply wires a quick once-over. A missing ground or power wire could be making the PI suck its power through signal wires, and that can sometimes cause something like this. If you've already been through checking component values and wire routing, that's the next most likely culprit.

Is it possible that this was modded before you got it?

[Ampzone]

hmmm... That's oscillation, all right. I'd normally immediately jump to reversed primary wires, just as Enzo did.

Presence is actually an active filter made by feeding back part of the output through Rs and Cs to the PI. As such, it's prone to the same issues as other active filters, plus the issues of the output transformer. That's why reversing the primary leads causes/cures oscillation in some instances - it's reversing the feedback from negative to positive.

So if it's not the tube, and not the primary wires reversal, maybe it's some of the other ills of active filters. At this point, I'm purely speculating.

When you remove the presence feedback, that's breaking the feedback loop, and why it quits. When you remove power supply, the siren sounds are the oscillation changing speed with the power supply as it goes down. Most often, this indicates positive feedback and a blocking oscillator instead of a negative feedback gain-phase oscillator. That's one argument that says that the feedback is still incorrectly positive. Active filter action requires negative feedback gain-phase action that almost but not quite oscillates.

At this point, I'd give grounds and power supply wires a quick once-over. A missing ground or power wire could be making the PI suck its power through signal wires, and that can sometimes cause something like this. If you've already been through checking component values and wire routing, that's the next most likely culprit.

Is it possible that this was modded before you got it?

RG you make some great points and it is very helpful. Regarding the amp it's an old 100w super bass converted to lead that I acquired (not working). Some parts were replaced with newer coupling caps etc with stock values. The voltages look right including heater voltage; the transformers, choke are stock but the amp has been changed over the years in the pre-amp to Super Lead setup.

Missing power wire? There are a few voltage taps 120, 220v, 240v all look unmolested. I will check the grounds too.

[Ampzone]

Another little oddity when you plug a guitar into the amp with the feedback lead removed the amp plays normal (no hum or oscillation) but the Vol control does not completely turn the signal off. On "0" you can still hear a little signal. Other than that the rest of the tone controls function.

[Ampzone]

I have three points heading to ground on the pre-amp board; V1A-V2A cathode caps and the bias caps positive to ground. All three are intact The primary CT on the PT is grounded along with the bias.

[JoeM]

What impedance load are you running the amp in when it hums?

[Ampzone]

What impedance load are you running the amp in when it hums?

Hey Joe, thanks for weighing in. I think I found the culprit. RG had suggested I check the grounds which looked good; but just now I noticed the ground wire on the output jack is loose at the lug on the chassis. Enzo made a good call I think on this earlier. I will be repairing the cold joint and powering it back up; this should be it.

[JoeM]

Great! Hope that solves it.