Power lamp in series with PT

[paggerman]

Hi all, got a build under way which I have previously asked advice on - I look forward to sharing the results.

Now then, I am a little perplexed by the impact of having a power indicator bulb in the 230VAC mains supply chain. From the live side of the 3 cord cable, I connect in series to fuse, to switch, to a little 220v bulb to PT primary. Neutral goes straight to PT Primary.

This is where I get a little murky on my theory. The amp has 3 pre-amp valves (using 2x6N2P and 1x6J32P) and two 6V6 for power output. that, to me, indicates a significant amount of power that needs to go through the bulb to power the rest of the gear. I have seen tranformerless amp schematics with all sorts of 'funny' voltages for heaters (50v, 16v, 25v) which when added together conveniently land at or around the mains voltage for the country of origin. And each of those heaters would consume their rated current. But why doesn't all the 'extra' current that runs through the bulb cause it to burn out? Or will it? Or maybe, like a bulb limiter, it will be very dim if the amp operates normally.

Perhaps I am confusing conventions with reality (e.g. current flows +ve to -ve, but electrons flow the other way.

Can anyone help me understand what will happen when I plug in (assuming I have built the amp cotrrectly)?

many thanks
Jeff

[Jazz P Bass]

The lamp should be in parallel with the transformer primary.
Not in series.
Why not use a 6.3V lamp & run it off of the heater (in parallel) like just about every other amp?

[paggerman]

It's always the simple answer, isn't it? Thanks very much for the quick response. Reason for a 220v bulb? Because I have one.

[olddawg]

I've always been leery of using line voltage lamps on front panels. Too much chance for calamity. Much better to use an led and resistor or a neon lamp and resistor off of the mains.

[NorCalTuna]

I fixed an ampeg once for a friend like this. It came in running horribly and making very little power, and the my buddy was like "and the power light works like a output indicator!" It was good for at least two giggles and a snicker or two. Just connect the transformer primary to the switch and rewire/ think about losing that 120v lamp... I wonder if some guru spread this around lol.... because...

It may/will work as is. The lamp will act as a current limiter but it will likely turn on and even play (it may be the most glorious tone since hendrix!) depending on the light bulb. Another way to look at it is that your amp has a built in "light bulb limiter" (google that) but it's likely too small to be reliable at full power.

[Enzo]

I think you may have already solved your problem, b ut back to your earlier question. I don;t know the 6N2P, is that like a 12AX7? I will assume it is for now, it shouldn't matter. You have a pair of 6V6 heaters which draw about 0.45A each, call it an amp total. You have three preamp tubes, so 12AX7s on 6v draw 0.3A each, so three of them draws close to an amp. So 2 amps from 6vAC winding.

If there is 2A drawn from the 6v winding, that does not mean 2A is also drawn from the mains. It is power transformed through the transformer, not amperage. 2A at 6v is 12 watts. 12 watts coming through the transformer means 12 watts drawn from the mains. AT 230v for the mains, 12 watts is only a draw of 0.052 amps. 52 milliamps of current is all that is flowing through your bulb for the heaters. I have no idea what other currents are being demanded, but that is all the "all that extra current" amounts to for the heaters.


Current is the same through a series circuit. So in an old transformerless circuit They took a 50C5 power tube, a 35W4 rectifier tube, and three 12v tubes and ran them in series. All those tubes have heaters with the same 0.15A heater draw. SO the same 0.15A will heat them all if they are in series. They naturally drop their nominal voltage across their heaters due to Ohm's Law. SO all those various heater voltages add up, and as a group they draw only that little 0.15A from the mains.