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How to change a light bulb?? (pilot lamp in Harman Kardon A50K/A500)

ViperZ

ViperZ

Well-Known Member
Well, that's embarrassing, but I can't figure out how to get to the pilot lamp (Type # 1847?) from inside the power button of Harman Kardon A50K (A500) integrated tube amplifier?

Does the front panel have to come off? Or do I need to take the button body apart from the back (it has two metal twines holding the dielectric body).

Thank you!
 
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You need to remove the faceplate. All the knobs pull straight off, two phillips screws hold the faceplate on. You will need to remove the screw holding the lamp bracket to the chassis.
IMG_4529.JPG IMG_4530.JPG

Be careful to not lose the violet push button when you remove the faceplate.
 
Good to know if I need to do the same on mine. BTW, my button lost all its color, it's now beige translucent.
 
I looked at my options for the light bulb. 150mA bulb or LED. Buying LED would significantly reduce strain on vacuum tube heaters, which are in parallel with the light bulb, but would probably introduce noise to those tubes? So stick with the bulb? I wonder maybe I can find bulb that doesn't need so much current?
 
The #47 bulb is the correct replacement as it is 6.3 v just like the tube heaters. If anything, it wouldn't be putting a stress on the tubes but rather the power transformer.
 
I ran the amp for about 3-4 hours today, half the time with new light bulb. So far so good. Power transformer was hot after two hours at pretty high volume, but still cool enough for me to touch it. I remember my old Dynaco ST-70 transformer running stupid hot compared to this one (swapped it for upgraded bigger one years ago though).
Just need to receive and install the bottom cover (right channel hums a bit without bottom cover; using borrowed cover from another amp for now).

The sound is just fantastic! Thank you everyone for being super helpful!!
 
Pinball parts companies have a plug in violet LED on the bay. If you search there for
(10 Pack) - 6.3 Volt LED Bulb Flat Top 44/47 Base (BA9S) Pinball - PURPLE
you should find them. The sell in a 10 pack, but then the cost is $4.99 +$1.99 shipping. I know you only need one, but figure each LED is good for maybe 10 years or so, you should be good for 100 years.
They are substantially brighter and violeter (more violet) than a #47 lamp and the violet color helps with the button that typically has faded. The cardboard shield on the #47 bulb fits loosely on the LED, either leave it off or use a dab of silicone. I've not noticed any effect on the sound.
 
The LED won't introduce noise, but personally I find them to be hard on the eyes unless rectified. They flicker slightly otherwise. Can't see it so much when looking right at it, but I notice it out of the corner of my eye. Either way it hurts to look at. A diode, a cap, and possibly a drop resistor to cut the brightness will make it right tho.

or use a standard bulb and stick a purple cap over it.
 
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