I do... To look at things like output stage stability, dc offset into a resistive load, small signal ac behavior and the like. If the amplifier is not unconditionally stable the ballast lamp will probably save your output devices.
the lamp as a series current limiter has merit... but for really *dead* stuff, like blown output transistors, and shorted stuff, I like the variac and ammeter because you can't get above about 10vac on the primary before the current starts to soar - so you never get to the point where anything more can happen to the amp... of course if you don't have a variac and ammeter, then the bulb is a best bet.
Maybe NYC Transit Authority bulbs are the best - rated at 130vac and glowing that dull, rich olde tyme orange glow - and lasting about 60 years or more??
_-_-bear
PS. the lamp is just to get the thing up and running... without smoking anything. Real testing for performance still has to be at operating conditions...
Maybe NYC Transit Authority bulbs are the best - rated at 130vac and glowing that dull, rich olde tyme orange glow - and lasting about 60 years or more??
_-_-bear
PS. the lamp is just to get the thing up and running... without smoking anything. Real testing for performance still has to be at operating conditions...
Bear, humor aside, there is a REAL reason to use BOTH a Variac and a series lamp. First, the Variac removes any potential 'avalanche' when a circuit requires a minimum voltage, even to turn on. However, it does NOT stop a current pulse from blowing the fuse or other components.
The lamp, alone, brings up the voltage, almost to full potential, then limits the current within a short time, usually keeping the fuse from blowing or parts broken.
The COMBINATION gives you the same approximate 'avalanche' voltage, yet limits the current to a respectable value. THEN, once you get the unit running 'properly' you can even increase the Variac voltage to OVER the normal input voltage, in order to compensate for the normal lamp voltage drop and actually run the preamp or amp under nominal conditions. Then, if something, like a transistor that was partially damaged, but not completely, can be found and it may even short, but the rest of the amp parts will be protected, even if you are away from your bench.
The lamp, alone, brings up the voltage, almost to full potential, then limits the current within a short time, usually keeping the fuse from blowing or parts broken.
The COMBINATION gives you the same approximate 'avalanche' voltage, yet limits the current to a respectable value. THEN, once you get the unit running 'properly' you can even increase the Variac voltage to OVER the normal input voltage, in order to compensate for the normal lamp voltage drop and actually run the preamp or amp under nominal conditions. Then, if something, like a transistor that was partially damaged, but not completely, can be found and it may even short, but the rest of the amp parts will be protected, even if you are away from your bench.
John is spot on, and exactly why I do the same thing. More than a few things have been saved this way, and I have direct experience of things failing on reduced line voltage with just the variac with damage ensuing. Variacs do not limit current, but the bulb does - and instantaneously.
If the fuse blows with the secondary disconnected, you have shorted turn(s) in the toroid. Depending how it is clamped pressuree of clamping may have shorted turns.
Use the trick of putting a 100 Watt light bulb in series with the line input. The magnetizing inrush would never blow a 10 amp fuse.
IF you have dual primary windings to accomodate 120 volts as well as 220, you should check that the connections of one of the primary windings is not backwards... DON'T even trust labels...
Use the trick of putting a 100 Watt light bulb in series with the line input. The magnetizing inrush would never blow a 10 amp fuse.
IF you have dual primary windings to accomodate 120 volts as well as 220, you should check that the connections of one of the primary windings is not backwards... DON'T even trust labels...
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.