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Old 1st September 2012, 06:42 AM   #1
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Default DC Pilot Light energy source - cathode resistor?

Hey there - this may seem preposterous, but here goes . . .

I'm wrapping up a GA-40 clone/projector conversion that is a lot of action in a small space. I intend to put a 12vdc pilot light in my amp due to cramped chassis and, well, lets say its in the hole already. I've had some luck putting it after a diode coming off one leg of the 6.3v filament supply. It only draws 50mA and this amp is running a bit high at 6.5vac on the filaments due in part to having two less tubes than originally spec'd, so the load isn't hurting anything. As you may be able to imagine, the light is flickering out of phase with the guitar signal - interesting at first, but a bit annoying after a while.

The amp has a little hum, not bad but could be improved, and I thought to float DC over the filament supply - and then I had the thought - could I just run the pilot light off of the cathode of the output tubes? or would that fry things? what are your thoughts? besides the ones where I get a 6.3vac light or use a bridge rectifier (added buzz).
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Old 1st September 2012, 06:51 AM   #2
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One diode from the filament voltage, one 100 F (or larger) 16 V to GND of filament supply. 12 V DC pilot light in parallel to the 100 F (or a resistor in series with a 1 W power LED in parallel to the 100 F) and you're done. No luck needed, no flickering, just simple thinking for a good solution. When you choose the 12 V DC pilot light please mind that the voltage will be around 9 V DC so it will not glow optimally but it will glow better than it does in the current setup anyway. If it is a LED pilot light (which I assume) you could open it to adjust the series resistor for 9 V DC use to be able to use it with full "power".

I would not even think of messing with kathodes just for a LED.
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Last edited by jean-paul; 1st September 2012 at 07:12 AM.
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Old 3rd September 2012, 10:29 PM   #3
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Thanks for the response, and the edits as those helped clear up what you meant. The diode with a 100uf cap parallel to the light added noise to the amp - the cap raised the tone of the noise, but it was there with the diode as well.

I have since discovered that the Cathodes on the Output Tubes are connected to the center-tap of the filament winding, elevating the filaments with about 14vdc. Perhaps this is what was causing the dimming of the light when the cathode demanded more current? or the opposite?

Any thoughts on diode/cap noise suppression?
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Old 4th September 2012, 01:07 AM   #4
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just use a full wave bridge, a single resistor and LED.

You don't need to filter for the LED. 120Hz pulses you will not notice.

The noise is from the rectifier / capacitor combination

If you get a "super bright" or high efficiency red LED then 10-20 mA will be good.

6.3Vrms -1.2V for diodes and -2V (or whatever the diode drop is) divide by .015A

very easy to try
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Old 4th September 2012, 02:13 AM   #5
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thanks for the idea Doug - I'd tried that already, there was added noise from the bridge rectifier. In addition, the lamp i'm using calls for 10vdc at 50mA. It will light off of the bridge rectifier, but not if I drop much voltage. 2.2k resistor dimmed it out.

Regarding noise, I just checked a possible option - hooking the (-) of the bridge rectifier to the (-) of the lamp, without attaching it to the ground circuit. Would there be any harm leaving it hooked up that way? I can't see where this would cause a problem.

i like your signature, by the way - conducive to growth. thanks again.
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Old 4th September 2012, 03:04 AM   #6
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how about two LED's and a resistor direct to the 6.3VAC

hook the diodes in parallel - one reversed.

They will protect each other from reverse voltage.

no bridge rectifier

each LED's turn-on might be soft enough not to produce noise due to dI/dT

it might be another interesting experiment to see if just running the wiring with a resistor load would introduce noise.
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Old 4th September 2012, 05:57 AM   #7
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so . . . do you think a "closed circuit" on the bridge rectifier is trouble? like i said, it works noiselessly that way so long as the (-) isn't attached to the rest of the amplifier ground circuit. Instead, I only had it on the negative end of the lamp.

Actually, this rat-shack lamp doesn't have a positive or negative marked, just two leads coming out of the back. I chose it due to its minimal protrusion into the amplifier, which is pretty cramped for space. However, it was marked 10v dc, and I verified that it draws 50mA with a meter.

I'm going to try adding a smoothing cap and maybe a series resistor to see if the brightness can be increased.

thanks for your input.
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Old 4th September 2012, 06:56 PM   #8
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perhaps what i should be using instead of a bridge rectifier, is a voltage doubler circuit
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Old 4th September 2012, 07:17 PM   #9
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You could make it a dissertation to a university study: how to connect a LED
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Old 5th September 2012, 02:34 AM   #10
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of course you are correct. perhaps its still essence of newbie wearing off. maybe one day i will graduate to your level of understanding.
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