5 year old ST70 build suddenly no sound , all tubes light up

Switched out the rectifier 5A4,

Fuse still good

There are no signs of loose solder connection.

Not sure what to check next. Would greatly appreciate any advice.What would cause both channels to go completely simultaneously with all tubes lighting up?

The volume pot ?

Tommytetrode wrote:Switched out the rectifier 5A4,

Fuse still good

There are no signs of loose solder connection.

Not sure what to check next. Would greatly appreciate any advice.What would cause both channels to go completely simultaneously with all tubes lighting up?

The volume pot ?

Compare voltages with the notations in the manual

Thanks for the reply. I do have a multimeter.
Can this be done with amp unplugged and tubes out?
Proper setting on multimter?

Tommytetrode wrote:Thanks for the reply. I do have a multimeter.
Can this be done with amp unplugged and tubes out?
Proper setting on multimter?

Do read the manual !
Of course you need the amp turned on with proper load on the speaker outputs. Be careful !

Tommy, you need to read some of the 'stickies' on the home page. If you don"t have a manual then you can print one @ home page.
Do be careful; Lethal voltages inside that amp!!

Always test amp with speakers and inputs plugged in. Input does not have to be active.
Clean and tension any socket pins that are accessible. Replace center driver tube.

And let us know your progress. All the members here will be glad to help.
(and listen to Peter) Dale

Can you set the bias on the output tubes? If not, you probably don't have B+.

Thank you for helping out :-)

Some basic questions:
The amp has a removable bottom plate. How do I access points here with multimeter if amp is plugged in with tubes and speaker terminals used.
I can rock the amp back resting with the heavy transformers down, exposing the all the wiring points but then the speaker terminals are obstructed.

One hand should be in back pocket and this makes sense. Do rubber shoes, long sleeves, and rubber gloves really contribute to additional safety?

I will check the stickies but my first scan showed only one post from Bob on troubleshooting.

Also...The black lead should clamp on one of the metal plate chassis screws( for ground) and the red multimeter lead would touch various points to check on voltage?

Tommytetrode wrote:Thank you for helping out :-)

Some basic questions:
The amp has a removable bottom plate. How do I access points here with multimeter if amp is plugged in with tubes and speaker terminals used.
I can rock the amp back resting with the heavy transformers down, exposing the all the wiring points but then the speaker terminals are obstructed.

One hand should be in back pocket and this makes sense. Do rubber shoes, long sleeves, and rubber gloves really contribute to additional safety?

I will check the stickies but my first scan showed only one post from Bob on troubleshooting.

Also...The black lead should clamp on one of the metal plate chassis screws( for ground) and the red multimeter lead would touch various points to check on voltage?

You can have the amp upside down, resting on the transformers and the can cap.
If the tubes are too tall, use a woodpiece under the transformers.

From Bob's troubleshooting list:

2. Problem - All the tubes light up but no sound
  Area to check - Nearly always a bad rectifier tube - just because the filament lights doesn't mean the rectifier is good. Check the bias voltage - A no bias situation = bad rectifier tube

Might wanna double check that all the socket pins are making good contact.