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Author Topic: NG Supply indicator lamps  (Read 1007 times)

afterlifestudios

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NG Supply indicator lamps
« on: July 03, 2019, 06:02:00 pm »

Is there any negative (other than visual) impact of using incandescent or led indicator bulb in an NG supply?  (I’ve read that neon can act as voltage regulating in some applications.  But in the NG supply, line voltage just goes across the lamp, so I’m thinking this isn’t one of those applications?)

One of my NG’s bulb socket has been swapped for a miniature beyonet (t-3 1/4 I believe) and I’m having trouble locating 110v neon in that base type.

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uwe ret

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Re: NG Supply indicator lamps
« Reply #1 on: July 03, 2019, 11:35:57 pm »

Any type of 110V ac-powered illumination (Neon, incandescent, LED, fluorescent, plasma...) will work in this application. The sole considerations will aesthetics and physical fit. It may be easiest to find an LED fixture (lamp and socket).
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #2 on: July 04, 2019, 01:20:58 am »

Excellent.  Thank you, Uwe.  There are lots of available incandescent T-3 1/4 (9mm) base bayonet bulbs available near me for less than $1.
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #3 on: July 06, 2019, 01:29:32 pm »

One more question.  What about the heat?  This 110v incandescent bulb gets very hot to the touch...  The lamp holder has a “plastic” (?) screw on cover. I guess I’ll keep a close eye on it today...
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klaus

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Re: NG Supply indicator lamps
« Reply #4 on: July 06, 2019, 02:10:26 pm »

Some people like their pilot lights bight, others like it barely visible. Install a 2W resistor in series with the bulb feed, after temporarily soldering in a trimmer, to determine your preferred illumination level for the bulb.
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Klaus Heyne
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #5 on: July 10, 2019, 05:45:37 pm »

Well.  I just discovered that the indicator lamp sockets in BOTH of my NG PSU’s are run off the 105v DC.  Not the 110vAC like the schematics I’ve seen show.  So perhaps the neon IS part of the DC stabilization.

(Both u47 systems were bought at different times from different people in different places. One is Tele badged, the other is Neumann, so the chances that this was a mod done by the same person is extremely slim. And all the wire used corresponded with the rest of the PSU.)

I do not have a schematic that shows this arrangement.

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klaus

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Re: NG Supply indicator lamps
« Reply #6 on: July 10, 2019, 06:36:19 pm »

I cannot see how the pilot light in NG supplies could act as voltage stabilization: it's positioned on the AC side, pre-input transformer, rather than the DC side with its five heavy-duty filter caps. 

All of my official Neumann schematics for the NG and NG 1/2 show only AC bulbs, all positioned before rectification.
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Klaus Heyne
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #7 on: July 10, 2019, 06:58:32 pm »

I was surprised too.  But that’s what I’m saying: It is positioned after rectification and is being feed DC.  A wire comes from the last 40uf cap and brought back to the indicatotor bulb socket. Then the other side of the indicator socket is brought back via another wire along that same path to ground at the mic connector.  It doesn’t exactly look stock.

Photo attached.

What are the odds that two separate previous owners in 2 separate countries did the same mod using the same method and layout etc?

Should I rip that stuff out and wire it as per the schematic?
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uwe ret

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Re: NG Supply indicator lamps
« Reply #8 on: July 10, 2019, 08:33:15 pm »

The heat shrink tubing and the two square 1kΩ/5W power resistors are clear evidence that the power supply shown in the photo has been serviced and modified.

None of the various implementations of NG-style power supplies ever utilized the neon indicator bulb (or anything else) for voltage regulation/stabilisation. The current to fire the small indicator bulb is far to small to be of any effectiveness for voltage regulation, that task was accomplished via a type 150B2 gas discharge tube in early regulated supplies.

If you cannot source a neon bulb, to avoid potential heat damage from incandescent bulbs I recommend a light emitting diode (LED) to illuminate the indicator. With a suitable current limiting resistor a wide range of desired brightness can be achieved. There even may be available LEDs operating of 110Vac, just like the original Neon bulb.
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #9 on: July 10, 2019, 09:00:14 pm »

Thank you Uwe. I will remove the modified b+ lamp wiring (green twisted wires in photo) and connect the lamp socket to the AC110 as the schematic shows.   I have 110v neon bulbs but they were not firing, which is how I ended up discovering this.  They would only fire if the PSU was not under load (mic not connected), because they were getting a LOT more voltage.

Does anyone have simple layout diagram or photo of how the AC lamp sockets were originally wired?  As I mentioned, BOTH my NG’s have this b+ indicator lamp mod so I don’t have a normal one to copy.

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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #10 on: July 11, 2019, 01:01:32 am »

Is it possible that the mod is for over voltage protection?
Or is that the same thing I already asked above?
Just trying to figure out why two separate tech’s did this mod...
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klaus

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Re: NG Supply indicator lamps
« Reply #11 on: July 11, 2019, 12:12:57 pm »

They probably did this because:

1. Original Neon bulbs of the kind used in Neumann NG supplies are hard to find and now quite expensive.

2. DC bulbs are the norm for powering pilot lights in power supplies, so techs are familiar with it, even without a schematic (before they became widely available on the internet).

As mentioned by several posters: power supply pilot lights just indicate that power is getting to the unit, nothing else.
That does not mean that it's always done intelligently: early N52 supplies used a pilot light feed in series with the heater voltage output: when the lamp's filament broke, the mic no longer received heater voltage.
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Klaus Heyne
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #12 on: July 11, 2019, 01:29:27 pm »

Thanks Klaus.  Both these NG’s have the bulb socket in parallel (not series) with the b+, and someone (two separate people) deliberately made the effort to do so.
I just thought before I put it back to stock (where it truly does just indicate if there’s AC making it past the switch), I’d try to decipher the reason for the mod. It doubt it was bulb scarcity. The socket was not changed on one NG, and the other was changed to bayonet base and came with 4 spare ne51 (neon) bayonet base lamps.  Also, running an incandescent bulb in parallel off the b+ would pull too much, so these were meant to be neon I think. 

My thoughts are the possible overvoltage protection that neon provides, but I don’t have the math to back that up.

Or maybe they wanted an indicator that shows you’re getting at least (n)VDC.  n being whatever the firing point of the neon bulb is.  Maybe that’s just a more useful indicator?

But if no one has seen this before, and nobody chimes in with strong support and reasoning for this mod, I will revert to stock.

Update:  Dennis from Tab Funkenwerk informed me that Oliver had come across a similar mod a couple of times and always reverted it to stock, sighting the impedance and need for purity of the b+.

So it's unanimous.  I'm heating up the soldering iron...  Thanks Klaus and Uwe.
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #13 on: July 12, 2019, 01:09:33 am »

Does a stock NG connect the lamp socket leads to the two terminals I’ve indicated on the terminal block in the attached photo?  (Not a photo of my PSU)

The Ac wires from the lamp socket in the photo I found at not twisted.  Would you advise twisting?

I’m assuming that if switching from 110v to 220v main power, I would need to switch to 220v lamp as well?  Or when the jumpers are set to 220v does the lamp see half the voltage as well?

Sorry for all the questions. Thanks for helping me get these back to original design.
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #14 on: July 13, 2019, 01:13:57 am »

Ok. I removed the wiring to the lamp from the DC signal. I rewired the lamp socket to the AC with a 100kohm resistor in series with the ne-51 neon lamp as per schematic.  It glows!

Then I measured b+. (First I drained the caps, as I was doing the other work with the PSU unloaded.)

With my DMM set to measure the b+ VDC, (presently 0V) I connected the mic and powered up.  The first thing I noticed was that it got up to about 80VDC faster than I think it did before? About 2 seconds or less.  Is that normal? Then it climbed a little more slowly up to 105.  Then it kept climbing to 107 at which point I powered off.   Time to redo the dropping resistors.

I originally had all 1kohm dropping resistors. I substituted a pot for R2 and found that 1.3kohm brought the b+ down to 105VDC.  Is it ok to have that much of a mismatch on R2 (the first resistor after rectifier)?  Or do they all need to be the same?

Right now it’s:
R2 = 1.3kohm
R3 = 1kohm
R4 = 1kohm
R5 = 1kohm

Or would adding 300ohm 10W between rectifier and first cap be better? (heatsinking it to chassis as well.)


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