• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Voice of Music 1428

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L

Lawrk

Member
Joined 2017
#1
Hi, I'm restoring a Voice of Music 1428 Tube Amp and found the incandescent power indicator is burnt out. The Sams schematic doesn't specify a part. Per the attached schematic it runs to the center tap of the 6 Volt filament transformer witha 10 ohm series resistor. Can anyone suggest a replacement bulb or LED combo for this bulb? Since this indicates power in a high voltage amp I want greatest reliability which may be an LED.


thanks
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thanks in advance
 
#6
Ok, what you have is known as a grain-of-wheat bulb. These are T-¾ size lamps. Many of them are 5v at 60ma. But they also come in 12v and perhaps lower then 5v. Since yours is connected through a resistor to ½ the 6.3v filament voltage, they're probably low voltage. Perhaps get some 5v lamps and operate it from the 6.3v through a suitable resistor. Or just the 3.15 half as shown. Lamps operated at 10% lower then rated voltage last a very long time.
 
#9
HollowState said:
Ok, what you have is known as a grain-of-wheat bulb. These are T-¾ size lamps. Many of them are 5v at 60ma. But they also come in 12v and perhaps lower then 5v. Since yours is connected through a resistor to ½ the 6.3v filament voltage, they're probably low voltage. Perhaps get some 5v lamps and operate it from the 6.3v through a suitable resistor. Or just the 3.15 half as shown. Lamps operated at 10% lower then rated voltage last a very long time.
Click to expand...
Thank you. I should be able to find a 4.5 Volt grain wheat bulb and run it off the 3 Volts as suggested for longevity
 
#12
HollowState said:
Ok, what you have is known as a grain-of-wheat bulb. These are T-¾ size lamps. Many of them are 5v at 60ma. But they also come in 12v and perhaps lower then 5v. Since yours is connected through a resistor to ½ the 6.3v filament voltage, they're probably low voltage. Perhaps get some 5v lamps and operate it from the 6.3v through a suitable resistor. Or just the 3.15 half as shown. Lamps operated at 10% lower then rated voltage last a very long time.
Click to expand...
Do you see any issue using a 5 Volt one off the 3.1 Volt tap? Bulb in question is: 5 Volt, 115 mA Grain of Wheat Bulb
 
#13
I don't see any problem with it as long as your PT handles the extra current drain. And it should be able to. Hopefully the intensity is bright enough. Personally I would use a 60ma version lamp, but you'll just have to try the 115ma bulb and see.
 
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