Lost the heater on the left 12AT7 of my ST-120

Hi All,

I've got a weird one with my amp: After losing the left channel I noticed that the heater in the left 12AT7 wasn't happening. After tube rolling everything, including the rectifier and of course symptomatic tube, (which works fine in the middle and right sockets) nothing plugged into that socket gets warmed up - visibly or to touch,
Pulling all of the tubes and running a meter on the socket's heater pins shows 6.3V as expected and re-loading all of the tubes still shows 6.1V on the PCB where the heater cct wires terminate. (I'm running KT120's so expect that's why the 6.1V with tubes in).

A bit of background: When it started failing, a couple of times a jiggle of the back-left KT120 seemed to 'clear' the problem. That tube has since been rolled out (as a single swap) and no change to the fault.

Its so obvious why it's not working, but I can't find the actual cause...

Any suggestions where to go from here?

AB

antipodean wrote:Hi All,

I've got a weird one with my amp: After losing the left channel I noticed that the heater in the left 12AT7 wasn't happening. After tube rolling everything, including the rectifier and of course symptomatic tube, (which works fine in the middle and right sockets) nothing plugged into that socket gets warmed up - visibly or to touch,
Pulling all of the tubes and running a meter on the socket's heater pins shows 6.3V as expected and re-loading all of the tubes still shows 6.1V on the PCB where the heater cct wires terminate. (I'm running KT120's so expect that's why the 6.1V with tubes in).

A bit of background: When it started failing, a couple of times a jiggle of the back-left KT120 seemed to 'clear' the problem. That tube has since been rolled out (as a single swap) and no change to the fault.

Its so obvious why it's not working, but I can't find the actual cause...  

Any suggestions where to go from here?

AB

check for bad solder points, check for cracks in the board.
Easiest i think is to use dvm and ohm yourself following the filament from one transformer connection.

A lockdown and a weekend - perfect time to sort out the amp ... It was Peter, a dry joint as expected.
The wierd thing is that I had 6.3v everywhere I expected it to be when I used a meter. I can only think that the meter's minuscule current didn't upset anything but running a couple of heaters did.

I'm not 100% exactly where, but a poke around with the iron, solder sucker and some fresh solder has indeed sorted things out.