Initial Power-On Procedure

Hi,

I've just finished building my ST-70 with the VTA board (low gain version).

I build the amp from scratch using an unbuilt kit from the early sixties and the VTA board. I tested the output transformers beforehand but not the power transformer. So, I have no idea on how the components will react; they are new but 50 years old!

I intend to proceed step by step in turning this amp on to avoid any catastrophic failure. These are the steps:

.- pulling out all tubes, connect resistance loads, check the AC voltages at the different secondaries of the power transformer: the B+ one, the filament ones;

.- if the power transformer is working fine, then I will insert the rectifier tube and check the DC voltages and inspect their forms on a scope;

.- only then, if the values seem on range, I would insert one set of output tubes and check and adjust roughly the bias; I will do the same for the other channel;

.- I will next insert the 3 driver tubes and, after appropriate warmup time fine-tune the bias;

.- I will proceed to the AC balance test;

.- I will again check voltages and finally I will inject a 1000 Hz signal and check for clipping on a scope.

Have I forgotten something here or am I doing a useless or much worse a harmful procedure ?

I would appreciate your feedback


Yvon

Connect power resistors as speaker load ( unless that is what you mentioned with
"connect resistance loads," ).

If the electrolyte capacitor is 50 years old i would suggest you start with slowly
charging it with an external source ( increase 50V a time, allow current to diminish, then
increase 50V again. ). Or you may start with a fresh one .. Otherwize it sound to be a good plan.

Dear Peterh:

The resistance loads that I mentioned are indeed power resistors (Dale 8ohms 250 W). The QUAD capacitor is also new one bought with the VTA board.

I appreciate you input.

Best regards,

Yvon