Is that normal?

[Leevi]

I'm working with an amp where is not a grid resistor (normally 68K) on the first amplification stage.

I changed the plastic input jack to Switchcraft metal jack and wired it like always i.e. the signal tip is connected to ground when the guitar is unplugged. It works fine if the guitar is plugged but if I unplug the guitar I get relative big hum.

If I separate the signal tip from ground the hum disappears.

Then I changed the jack to non-switched jack and that works fine.

Any good clarification?

/Leevi

[HotBluePlates]

What is the amp?

Where are there possible ground loops?

[stratele52]

What is the amp?

Where are there possible ground loops?

Yes, first thing ; what is the amp ???? 

[Leevi]

I don't know the brand, I only have chassis of it, can also be a DIY amp.

It's a push pull amp with 2x6V6 and cathode bias. I have added PPIMV to it but
don't know if the problem was there before that.

The structure of PI is like in Marshall and Vox.
The preamp consists of one 12AX7 where the triodes are parallel connected.

The preamp circuit is very common with Volume and tone.

/Leevi

[stratele52]

I don't know the brand, I only have chassis of it, can also be a DIY amp.

I
/Leevi

Most of the time it is easy to see if it is a DIY , post some pictures.

Ground loop ? connect the switchcraft jack but do not scew it in the chassis . If it is a ground loop it wil work with no hum .

[Merlin]

Why not just put the plastic jack back on...

[stratele52]

Why not just put the plastic jack back on...

+1 ;   easy and will work

-1   ; Plastic is not as tough as Switcraft jack

[Leevi]

One solution is to use a non-switching jack which works fine
but I would like to know the root cause for the problem.
/Leevi

[John]

Did you say it does not have the 68k grid resistor to the first stage? Maybe it's not so much "hum" as just air noise.

[stratele52]

One solution is to use a non-switching jack which works fine
but I would like to know the root cause for the problem.
/Leevi

Non- switching jack = hum too

[Leevi]

Non- switching jack = hum too

Not in this case. The hum disappears which is weird.
This the reason why I started this topic.

Then I changed the jack to non-switched jack and that works fine.

/Leevi

[HotBluePlates]

I don't know the brand, I only have chassis of it, can also be a DIY amp.

The hum disappears which is weird. This the reason why I started this topic.

My crystal ball is cloudy...

Being a DIY amp (or at least an unknown amp), the only one who knows what is in the amp and what is going on is you. Also, being a DIY amp, all bets are off about whether the thing was even assembled correctly in the first place.

[Leevi]

is in the amp and what is going on is you.

Yes and my interest is to find the root cause for the problem.
I'm now pretty sure that it is a DIY amp.
/Leevi

[sluckey]

Can we see a hi rez pic or two?

[Leevi]

Couple of pics:

I have changed the preamp triodes to serial connection but the problem was there before the change.

/Leevi

[Willabe]

The power tube K R should not be on the big filter cap next to it. It will heat up that cap and will shorten it's life.


               Brad   

[Leevi]

The power tube K R should not be on the big filter cap next to it. It will heat up that cap and will shorten it's life.

Good point!
/Leevi

[Leevi]

I found the root cause.

I changed the input jack to a shorting jack.
I didn't connect the shunt to the closest ground point
but a point where the hum was not any more caused.

The clarification for the problem is IMO ground loop.

/Leevi