Bad idea to run indicator lamp this way?

[Gaz]

Check out how this old Hiwatt used to power the "Standby Indicator" lamp. It uses the potential difference between the two HV lines (approx 200V) to power what I assume is a 220-240v rated lamp.



Is there any risks doing this? I'm wanting to use a typical neon lamp like this:

http://www.mouser.com/ProductDetail/...Z%2fxJTStus%3d


Thanks for any advice!

[Enzo]

Why would there be risks? I am not aware of a problem with Hiwatts blowing up from panel indicators.

The neon indicator (a bulb with resistor in one part) needs 200 some volts to light, it doesn't know or care where they come from The thing is rated for the mains voltage, and if they wired it up to run straight off the B+ to ground - either one - it would WAY over voltage it. SO it indicates when high voltage power is applied to the circuits. When it is powered, there is about 200v across it, perfect. They cleverly used a standard off the shelf part in a situation where the voltages were too high for it if used conventionally.

[Gaz]

Thanks, Enzo, I thought it was very clever as well. I also thought to move the standby to before the rectifiers, and I'm thinking I could still use the same idea.

What I'd really like to do is find a way to use a 'progressive' OFF-STANDBY-ON switch like this one (https://www.tubesandmore.com/products/P-H520).

Is there a way I could use a single half of this switch to function as a Standby switch? In the CT in series with the fuse? Or is that a bad idea? I have limited panel space, which is why I'm asking...

[Enzo]

Um... I am about to go home for the night, I'll have to think about it after food and sleep happen.

[Gaz]

Haha, I appreciate it very much. I need to hit the hay too - I was hoping to catch some early bird's brilliance before crashing

[Steve A.]

Without changing the secondary wiring you could use a 3PDT center-off toggle switch and flip it one way for ON and the other way for STANDBY. It's not like a car where you need to shift it in 1st, 2nd and then 3rd...

Steve Ahola

[jmaf]

I would not move the standby switch to the AC side. We get away with switching higher DC voltages than the switches are rated for because of the low current in tube amps high voltage circuit. But if you move the switch to behind the rectifiers, the switch is going to get hit with the capacitor inrush current + 2 times it's peak rated AC voltage.

As for your initial question, I don't like the idea of 650 V feeding into a thin glass lamp that's embedded into the panel. But I've also never seen a Hiwatt kill anyone in 20+ years messing with tube amps.

6,3 V lamps do the job well, no need for 100x the voltage.

[mhuss]

It's a 240v neon lamp, standard mains voltage in the UK (and elsewhere). As Enzo pointed out, you need at least 70v to get a neon lamp going.

By connecting it to the 240 volt winding, you get 240v for the neon indicator no matter what the input mains voltage is (provided the input voltage switch is set properly) - the PT acts as an 'autoformer.' You could do the same thing with a 120v lamp and the 120v winding, but since these were made in the UK, 240v indicators were doubtlessly standard items.

[jmaf]

It's a 240v neon lamp, standard mains voltage in the UK (and elsewhere). As Enzo pointed out, you need at least 70v to get a neon lamp going.

I thought OP was asking about this neon lamp:



One of the legs on that lamp is 650V above chassis potential.

[mhuss]

Ah, you're right, my bad.

FWIW, I've never seen one of these fail in a dangerous way. The neon eventually dies, of course, but I've never seen or heard of any cases where the HV "got out" of the chassis. The way these indicators are constructed, if the exposed face breaks, the rest falls inside, and the HV is normally a good 1-2cm behind the face plate.

[Gaz]

I would not move the standby switch to the AC side. We get away with switching higher DC voltages than the switches are rated for because of the low current in tube amps high voltage circuit. But if you move the switch to behind the rectifiers, the switch is going to get hit with the capacitor inrush current + 2 times it's peak rated AC voltage.

As for your initial question, I don't like the idea of 650 V feeding into a thin glass lamp that's embedded into the panel. But I've also never seen a Hiwatt kill anyone in 20+ years messing with tube amps.

6,3 V lamps do the job well, no need for 100x the voltage.

That's the first time I've heard anyone suggest leaving the the Standby after the rectifier, and I'm very intrigued. Surely you've come across this article by Merlin:

The Valve Wizard

Would the resistors across the switch contacts help the situation you're describing? Are you also opposed to moving HV fuses to the AC side of the rectifier for the same reason?

[Gregg]

There is a very simple solution to the Stby switch dilemma: just switch the secondary AC on and off. That's it.

[Gaz]

There is a very simple solution to the Stby switch dilemma: just switch the secondary AC on and off. That's it.

That's what what jmaf was advising against and my question was concerning.

[jmaf]

That's the first time I've heard anyone suggest leaving the the Standby after the rectifier, and I'm very intrigued. Surely you've come across this article by Merlin:

Merlin discusses leaving the standby after the rectifier on that very article, and explains the tradeoffs of the various possible positions.

Would the resistors across the switch contacts help the situation you're describing? Are you also opposed to moving HV fuses to the AC side of the rectifier for the same reason?

I put HT fuses on the DC side, after the big caps. Marshall puts them on the tube cathodes on the JCM 900s, I like to place HT fuse right before the OT center tap.

I don't use resistors across the standby switch. I've never seen a Carling standby switch fail. Marshall standby switches can pop sometimes after years of heavy use.

[Gaz]

Merlin discusses leaving the standby after the rectifier on that very article, and explains the tradeoffs of the various possible positions.



I put HT fuses on the DC side, after the big caps. Marshall puts them on the tube cathodes on the JCM 900s, I like to place HT fuse right before the OT center tap.

I don't use resistors across the standby switch. I've never seen a Carling standby switch fail. Marshall standby switches can pop sometimes after years of heavy use.

Yes, but I don't see where he mentions anything about the pitfalls of AC side switching, especially in one leg of a bridge rectifier, which was the part of your post that made me curious. I'm aware that right after the rectifier is not the worst place for the switch, but I was under the impression it was always advantageous to put it on the AC side if possible.