#124 started

[norburybrook]

makes no difference, pulling either valve has no effect on the hum.

M

[martin manning]

What about when you pull the PI tube?

[larsvictor]

having a cable in the send return stops this somewhat

That´s strange, isn´t it ?

Are the jacks insulated to the chassis ? The PI/NFB ground wire goes to the return jack and to the send jack ? Or only to the return jack ?

Just to clarify: In your amp the send jack is the one near the speaker output, in Martin and Tony´s layout the send jack is the one near the OD switch.

[norburybrook]

What about when you pull the PI tube?

total silence.


I've tried different tubes here and it made no difference.

M

[amplifiednation]

What about when you pull the PI tube?

total silence.


I've tried different tubes here and it made no difference.

M

Do we know about the heater string ground reference?

Marcus do you have 100R resistors on each side of the heaters going to ground? Or did you ground the center tap? Without a ground reference, the hum from the heaters is unbearable.

[norburybrook]

having a cable in the send return stops this somewhat

That´s strange, isn´t it ?

Are the jacks insulated to the chassis ? The PI/NFB ground wire goes to the return jack and to the send jack ? Or only to the return jack ?

Just to clarify: In your amp the send jack is the one near the speaker output, in Martin and Tony´s layout the send jack is the one near the OD switch.

jack's are standard switchcraft no Cliff jacks so uninsulated.

yes my jacks are labelled with the send nearest the speaker output but I've wired the same as the layout, I presume they're the wrong way round then? funny that it seemed to work fine when I took the pre out into another amp.

I should correct that.

M

[norburybrook]

What about when you pull the PI tube?

total silence.


I've tried different tubes here and it made no difference.

M

Do we know about the heater string ground reference?

Marcus do you have 100R resistors on each side of the heaters going to ground? Or did you ground the center tap? Without a ground reference, the hum from the heaters is unbearable.

Taylor,

the 6A heater CT is grounded with the other center taps etc at ground 4 on the schematic.

I did wonder about this myself.

However as the amp is silent without the PI tube I'm presuming it's not the heaters.

M

[martin manning]

What about when you pull the PI tube?

total silence.

Ok great, now what happens if you ground the input side of the PI coupling cap* (PI tube installed)? If it goes quiet then the source of the hum is in the clean/OD switching or send/return jacks. If not it's in the PI itself.

*The end with the green wire going to the return jack in the layout.

[norburybrook]

What about when you pull the PI tube?

total silence.

Ok great, now what happens if you ground the input side of the PI coupling cap (PI tube installed)? If it goes quiet then the source of the hum is in the clean/OD switching or send/return jacks. If not it's in the PI itself.

erm....how do I do that martin? where exactly do you mean on the board, not sure where the coupling cap is for the PI ;D

M

[martin manning]

It's the 0.02uF coming from the return jack tip. Ground the jack side of that. I added clarification above.

[norburybrook]

yes, that kills the hum completely



M

[larsvictor]

Ok, then it must be the jacks since turning down the MV doesn´t affect the hum

In the layout there´s an additional wire connecting the sleeves of the jacks (grounds). With non-insulated jacks there is already a connection via the chassis. Or in other words: this is a ground loop.

[norburybrook]

in the schematic though it's showing a chassis ground connection AND a ground with the cathodes.

the layout is also showing switchcraft jacks not insulated Cliff jacks.

Is this wrong?


M

[larsvictor]

Marcus, do you have a black wire between the grounds of the jacks ? I can´t see this on the pics. If there is one, cut it and see (hear) if that changes something.

Yes, in the layout the V2 cathodes are wired to ground 1 in the schematic (#124 1.7) they are connected to the PI ground, that´s correct.

[martin manning]

Technically there is a small ground loop there with both the chassis and a wire grounding the jacks but it shouldn't be a problem. Try grounding the send Jack's tip to prove it. If that results in silence, work your way back upstream grounding the signal into the MV pot, OD relay, etc.