audio-talk

My Transcendent Beast OTL power amp design includes a Fender amp-style incandescent 12V pilot light bulb on the front panel, powered from the heater circuit.

The Beast has two heater circuits (from the two 12V secondaries) which are floating. One circuit supplies the bank of output valves that have their cathodes at -170V, and the other circuit supplies:

1) the opposite bank of valves where the cathodes are at 0V
2) the heaters for the input valves, where the cathodes' potential varies between a few volts and 200V or so (I think). The centre tap on one of the input valve heaters (12BH7 I think), is connected to ground through a capacitor, so in DC terms I think it's still floating.

Obviously wiring up a bulb brings the circuit within potential reach of human fingers. Now if my understanding is correct, it should be perfectly safe to supply it from either circuit as I can't' see that either of them have been actively 'elevated' to any HT potential - but would probably be better to do it from the latter circuit that heats cathodes at 0V.

Or - should I use DC blocking caps just to be really safe?

Thanks in advance. I think I know the answer already but I just want to sanity check it.

A fender bulb (at least the ones I have seen) is a bulb behind a bezel, so to get to the bulb would take some effort, the valve on the top of the unit would be easier to get to.

Thermionic Idler wrote: ↑Sun Dec 17, 2017 11:40 pm Obviously wiring up a bulb brings the circuit within potential reach of human fingers.

Same as for a mains powered neon one, but that's not a problem.


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PS: if you're clever tho you identify which side has the dropper resistor and connect that to mains live, so the rest of it is at neutral potentail.



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Heh yeah - until the day comes when it's plugged into balanced power and then neutral is at -120V potential.

I actually re-examined the bulb assembly this afternoon (to check I had the cutout correct in CAD) and you're right Nick - you'd actually have to unscrew the bezel, remove the bulb and poke a finger quite a ways in, all with the amp on. I can't actually get any of my fingers down the hole far enough. The bezel itself will be mounted against a grounded surface so I'm not going to worry too much about it, though I will use the 0V circuit obviously.

Thanks for the input.