Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on September 19, 2023, 03:41:42 pm
-
Can a variac be used in place of a lightbulb limiter for startup of a new build? If so, what a/c setting would one first use and the procedure?
-
I use a variac and not a LBL in testing builds and repairs. But they do not do the same thing. The LBL will detect if there is a short in the circuit. The variac limits the input voltage which in turn limits the voltages delivered out of the power transformer. Mistakes in the circuit, including. shorts, are much less likely to cause damage at the lower voltages. I usually start at about 45V AC input and check to see that I have voltage in and out of the rectifier and at the heaters. Unless something goes wrong, like smoke, spark, fires, blown fuse, I pretty quickly bring it up to around 70-80v input. That's usually enough to light the pilot and produce some sound (guitar plugged in, speaker connected of course.) I then check for voltage at each plate/grids/cathodes and slowly bring it up to full line voltage, watching and listening for any issues. A final voltage check at the tube pins should make sense, but I am not expecting them to exactly match a given schematic.
Now that may or may not be the accepted best practice for using a variac - its just what has worked for me.
Before I do this, I go through the amp checking each connection. Better to find an error that way than either with a LBL or variac.
-
I use a variac and not a LBL in testing builds.
Do you first do the above without tubes, if all ok, then with tubes..?
-
Well - here is the difference between what I should do and what I usually do.
It would be smart to always test first without tubes. And if a new scratch build, or an amp that has had problems, I usually do so.
If its a repair/rebuild involving new filter caps and other electrolytics, I usually leave the tubes in place. The variac set low is protecting them to some degree.
I wonder if someone here has a standard, step-by-step best practice, for this?