Light Bulb Current Limiter

When first powering on a newly built amplifier, use a bulb limiter or Variac. It will keep you from destroying some very expensive parts if there is any problem with your amp. The picture shows a 75-100 watt lamp, but I typically use a 25-60 watt to start with. If there's something wrong with an amp, it will draw too much current and the light bulb will blow. But it's better to blow a light bulb then a $150 transformer.

Variac, or variable autotransformer, is a device to vary the output voltage from a steady AC input voltage (wall outlet voltage). It can maintain a steady voltage during a wide range of load conditions, allowing for very smooth variable control of voltage.

light bulb current limiter schematicThings you’ll need.

  1. Three conductor power cord
  2. Light bulb socket
  3. Switch and outlet module. I wired the switch in series as a kill switch.
  4. Single switch and one wall outlet junction box
  5. 25-100 watt light bulbs (florescent bulbs will not work) 25 WATT INCANDESCENT LIGHT BULB

To build it:

  1. I started with a single switch and one wall outlet junction box. I then used a Greenlee 7/8-Inch Multi-Hole Step Bit and some metal files on the switch side to cut a hole for the light bulb socket. You'll need a cordless drill for the Greenlee bit.
  2. Insert the power cord through one of the knock out holes of the junction box.
  3. Wire the switch and the light bulb socket in series.
  4. Make sure all grounds are wired properly.inside light bulb current limiter

Recheck your wiring. Then put the metal cover for the junction box in place. Install a light bulb into the socket and plug the limiter into the wall outlet. The bulb will not light if there isn’t an amp plugged into the limiter. Remember everything is in series. Plug a known-working amplifier into the current limiter outlet and turn the toggle on. Turn the amp to standby while watching the lamp. The lamp should flash and then glow an amount that reflects the "idling" current of the amplifier. For the 25W lamp, this may be fairly fully on. For higher wattage bulbs, this will be dimmer.

 

Here is the start up procedure I used for my amp.  

Before turning on the power:

Recheck all mechanical and wire connections in the amp. Make sure all screws and nuts are tight. Carefully inspect all solder joints. Recheck all connections and compare to the layout and photos. Spray some compressed air to clean out any dirt or solder. Most amp problems at start-up are due to silly things that are overlooked during assembly.
Set the bias trimmer to the minimum setting and install the 2 amp fuse.

Start with a 25 watt bulb. That will supply the lowest current and will help to form the electrolytic filter caps. If all goes well, then put in a 60 then a 75 watt bulb. Remember whenever you power up a tube amp with tubes in it, be sure to have a speaker connected or it can destroy the output transformer and/or the power tubes.

Have a set of start up tubes (an older set that you wouldn't care if they broke, but still in good working condition). Make sure that your preamp tubes are the type that the heaters will glow, some don't.

Power-up with Bulb Limiter, No tubes installed first.

complete light bulb current limiter With NO tubes installed, plug the amp into your bulb limiter and turn on the main switch. The bulb should have no glow. If the bulb glows, recheck all wiring until you find the error. Switch the amp from standby to ON. Your bulb limiter should flash at first and then return to a dull orange glow. (The flash is from the caps getting their initial charge.)
Using a digital multi-meter (DMM) on the AC setting, check for heater voltage at V1. Checking at V1 will check heater connections for the entire harness. Note: The bulb limiter will affect your voltage readings, so don’t panic if you have less than 6.3VAC. Switch your DMM to DC and check for high voltage DC at the first two filter caps. The first cap should show less voltage than the second. If you show the same voltage on both caps, the power board needs to be rewired. Check for high voltage DC at all plate resistors, including the preamp. Check for negative DC voltage on the bias pot (potentiometer) wiper. If everything checks out, turn the amp off and wait for the caps to bleed down. (Since there are no tubes installed, the caps take a little longer than usual to bleed down.)

Power-up with Bulb Limiter, Tubes Installed

Install tubes and connect a speaker (or speaker load) to the speaker jack.

Connect the amp to your bulb limiter and turn on the main switch. The limiter should glow slightly as the heaters draw current. Check all tubes to make sure the heaters are glowing. Switch the amp from standby to ON. The limiter will flash and should then return to a dull glow. Repeat all voltage checks. Remember that all voltages will be lower due to the bulb limiter. If everything checks out, turn off the amp and disconnect the bulb limiter. You can now plug the amp into a normal AC outlet and do your final checks.

Final Power-up

Note: Before powering up for the final test, adjust all controls as follows:
Vol – 7:00 (off)
Treble, Mid, Bas – 12:00
OD Drive and OD Level – 9:00
Master – 7:00 (off)
Presence – 12:00

Connect a speaker (or speaker load) to the speaker jack. Turn on the amp and wait a few minutes for the tubes to warm up. Switch the amp from Standby to ON.
Using a DMM, check and adjust the bias current to 30-40mA. (The bias taps convert mA to mV so you’ll be reading on the millivolt scale.)