Second Stak amp I have worked on the first was a 230 Watt one. This is 130 Watt 4 EL34 design that had a dead power supply transformer which I replaced with a Sound City 120 Transformer. A nice amplifier well made!

 

Lovely Stak 130 Watt head which had a dead power transformer..

Pic of the wiring for the old trans

New transformer in position. It is a little bigger ansd comes from a sound city 120 which has 6 EL34 tubes so will work well with just 4 😉

 

New poles drilled in the chassis for the wider transformer.

New Transformer in place

 

New transformer wired up and tested..

Original bias circuit was not adjustable and had the tubes running too hot..

Voltage doubler installed and a variable bias pot added.. all good now. Al;sp replaced the coupling caps from the PI as these often go bad..

 

Gut shots…

Very similar to a Fender Bassman Circuit

I needed to add 2 100 Ohm resistors from the Filament supply to ground as the transformer had no centre tap on the filament supply. This helps reduce hum.

 

Master volume goes to 12!

Shot of the Transformer working.. real quiet no hum… 480 Volt B+

2 new caps were needed in the

 

All running well! and sounding loud and proud . A lovely amp!