Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hylaphone on March 02, 2026, 01:55:23 pm
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Hi folks, I've been asked to add an LED to indicate B+ on/off in a tube power amplifier. The B+ is 480V, C/R/C/R/C and each C is two series caps with 220K balancing resistors.Would it be best practice to simply light the LED from the last C node through a honkin resistor?Or - say the last C node is 400V, could I tap from the midpoint (200V) of the series caps so the LED resistor isn't dropping so much voltage? And if so, would I need to adjust the top 220K balancing resistor so currents are equal between the two caps?If this is confusing I'll try and post a schematic....Thanks
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I guess I would ask: What is the purpose of specifically indicating B+ voltage vs presence of voltage in the heater circuit, or a neon bulb off of the primary input?? Ie, we normally use indicators to tell if the amp is powered on or off.
I guess it could be done safely enough, BUT you're going to dissipate a lot of power through the series resistor. @400VDC it would be around 7 W, and at 200VDC it would be about 4W. (There's your honking resistor. . .)
You would be borrowing 10-20 mA from the B+; that is is enough to lower the B+ voltage a bit in the distal legs. That's probably not much of an issue.
Unless there is a specific good reason for this modification, I feel it is ill-advised.
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They make a handy calculator for this. This page has the numbers plugged in for 200VDC.
https://ledcalculator.net/#p=200&v=1.8&c=1&n=1&o=s
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It's okay to say no to a customer once in a while...
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They make a handy calculator for this. This page has the numbers plugged in for 200VDC.
https://ledcalculator.net/#p=200&v=1.8&c=1&n=1&o=s
FYI, you only need a 1mA-2mA to light an LED for use as an indicator. No need [for] 10-20 unless you need to see it half way down the block :icon_biggrin:
[edited]
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You can buy a low current led, which is happy at >1mA. I wouldn't try to put it in series the chain of the mixing resistors on series caps, because that would probably defeat the purpose. I also wouldn't put it at the end of the chain at the preamp, because it could create noise. Possibly between the screens and PI, after the dropping resistor with a connection to ground. Set your resistor at about 2 mA, and you may have to adjust the dropping resistor slightly before the led node to maintain the same voltages.
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Hi folks, I've been asked to add an LED to indicate B+ on/off in a tube power amplifier. The B+ is 480V, C/R/C/R/C and each C is two series caps with 220K balancing resistors.
Put the LED in series with one of the balancing resistors, nothing else required.
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Hi folks, I've been asked to add an LED to indicate B+ on/off in a tube power amplifier. The B+ is 480V, C/R/C/R/C and each C is two series caps with 220K balancing resistors.
Put the LED in series with one of the balancing resistors, nothing else required.
Thats clever, thank you! And likewise to all the other responses.
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Or you can buy a ‘type 47’ 6.3Vac LED and stick it in any old 6v bayonet lamp holder. No extra resistors required.
https://ebay.us/m/SU3S8m (https://ebay.us/m/SU3S8m)