Hum from JCM2000 TSL 60

[MervLizard]

My TSL 60 head has developed an annoying hum. The hum is not effected by any of the control knobs and sometimes disappears after a few minutes.

Any ideas what could be causing this?

 

[TheKman76]

A little more detail please.

Does it occur without an input connected?
When is stops, is it sudden or does it fade?
Does the effects loop make any difference?
If you plug something straight into the return is it still there?

It sounds like something in the power amp stage and could be as simple as a bad valve. When were they last changed or biased?

 

[MervLizard]

A little more detail please.

Does it occur without an input connected?
When is stops, is it sudden or does it fade?
Does the effects loop make any difference?
If you plug something straight into the return is it still there?

It sounds like something in the power amp stage and could be as simple as a bad valve. When were they last changed or

A little more detail please.

Does it occur without an input connected?
When is stops, is it sudden or does it fade?
Does the effects loop make any difference?
If you plug something straight into the return is it still there?

It sounds like something in the power amp stage and could be as simple as a bad valve. When were they last changed or biased?

it still occurs without any input connected.

It seems to fade out when it stops
The effects loop makes no difference at all.
Plugging into the effects loop return makes no difference.
I purchased the amp about a month ago so no idea how long the valves have been in there or when they were biased.
Would you recommend getting a new set of valves put in? I was looking at getting an amp tech to look it over and service it as it’s been a long time since I’ve had a valve amp, my last one was a JCM800 Master Volume head which I sold in the 90’s.
Thanks for taking the time to answer my question it’s very appreciated.

 

[Jon Snell]

Possibly one of the output valves is taking a while to balance with the other/s.
A bias test will confirm that.

 

[TheKman76]

If you don't know the history of the amp I think a look from an experienced tech is a good move.

I'd be replacing the output valves as a first step simply to know that it has a good matched set in there, but this may not actually change anything. If you're confident changing the valves and setting the bias, do this first.

 

[MervLizard]

Possibly one of the output valves is taking a while to balance with the other/s.
A bias test will confirm that.

Checked the bias and it’s 80.3 which according to the information I’ve found on the internet is about right for this amplifier

 

[MervLizard]

If you don't know the history of the amp I think a look from an experienced tech is a good move.

I'd be replacing the output valves as a first step simply to know that it has a good matched set in there, but this may not actually change anything. If you're confident changing the valves and setting the bias, do this first.

Thanks for the reply I’ll order a new set of valves and take it to a tech to give it a once over.

 

[MervLizard]

I’ve replaced the valves and biased them, hum still there waiting for an amp tech to get back to me to arrange to give it a look over. Thanks for help

 

[Tatzmann]

Which valves got replaced by now?
All of them or just the EL34's?

 

[MervLizard]

Which valves got replaced by now?
All of them or just the EL34's?

Just the EL34’s

 

[Spanngitter]

The TSL Series is notoriously known for having bad 330uF / 325V Caps...be careful if you insist to replace them yourself, these SnapIn Type can easily pull the TTH contact ferrule out of the hole and then you are Eff'd up....

 

[MervLizard]

Thanks for the information. I’m not going to attempt to repair it myself, I’ve taken it to a local amp tech who claim to offer a Marshall factory authorised repair and warranty service. So hopefully it’s in good hands!!