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Author Topic: NG Supply indicator lamps  (Read 1443 times)

afterlifestudios

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Re: NG Supply indicator lamps
« Reply #15 on: July 13, 2019, 04:12:24 pm »

So I ended up with this dropping resistor configuration

R2 1.2k ohm
R3 1k ohm
R4 1.2k ohm
R5 1k ohm

B+ is 105.4 after 10 mins.
Mains voltage at wall is 120 VAC today.

The B+ ramp up to about 80 VDC is only a couple of seconds but then it takes about 60 seconds to slowly climb from 80VDC up to 105 VDC.   Is that about right?

Is there any problem with this dropping resistor configuration? 

I rewired the lamp socket in the other NG supply to return it to stock as well.  Indicator is working. B+ is now at 107.3 on that supply.  Should I leave that alone?  Mains voltage is 120 VAC today and is usually lower. (115VAC) and Iíve never seen it above 120 VAC.  So it maybe ok to leave the dropping resistors alone in the 2nd supply?

Interestingly, I must have a very well matched pair of VF14ís in these u47ís.  They both measure almost exactly the same depending on which PSU they are plugged into.

Ie. The Neumann badged u47 is 105.4 on PSU #1 and 107.4 on PSU#2
And the Tele badged u47 is 105.3 on PSU #1 and 107.4 on PSU #2

Lucky me?

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uwe ret

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Re: NG Supply indicator lamps
« Reply #16 on: July 14, 2019, 04:14:12 pm »

Try not to overthink the NG power supply, it is a rather simple affair with no need or intention of regulation. The power indicator is connected across one half of the power transformer primaries. Thus it sees nominal 110 Vac regardless whether the mains settings are for 110 Vac or 220 Vac. The slightly excessive 105 Vdc (under load of a microphone with properly functioning VF14 tube or 2625 Ω/5W dummy load) is properly due to your AC-mains at 117 V rather than the nominal 110 V. Typically the tolerance permitted iss +/- 5% from the nominal 105 Vdc. No harm can be expected from adjusting the dropping resistor values to get closer to this target. Congratulations!
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #17 on: July 14, 2019, 04:56:11 pm »

Excellent.  Thanks Uwe.  Good points.  Could you describe which terminals to connect the lamp socket to so that is only connected across 1/2 of the primaries in the PT?  I think I did that correctly, but would like to know for sure. Working great at 110VAC.

The only other thing Iím still seeking confirmation of is the speed at which the b+ rises to about 80VDC before it more slowly climbs to 105. Is it normal for those first 80VDC to happen so quickly like that?  (2 seconds).  I donít want to do anything to potentially shorten VF14 life.

Then I promise Iíll stop posting every day whenever my b+ changes by one tenth of a volt! 😉

Thanks again.

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uwe ret

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Re: NG Supply indicator lamps
« Reply #18 on: July 14, 2019, 05:39:21 pm »

The power indicator neon lamp and its 100 kΩ series resistors originally connect parallel to the blue and red transformer primary winding wires. BTW. alternately a connection to the brown and white transformer leads would work as well.
Several factors are attributing to the time it takes the operating to settle to the final value. Initially the tube filament is cold and no current is drawn. The voltage rise is primarily determined by the staggered time constants of the series R/C combinations of the filter chain plus the time constant formed by the series impedance of the transformer secondary and the internal resistance of the selenium rectifier with the first filter capacitor. It takes around 4 time constants to reach 98% of the ultimate charge of each R/C combination, and since they are staggered, 2 seconds to around 80% seems right. Once the tube starts conducting and current is drawn, it does slow the voltage rise proportionally.
I hope this helps ...
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afterlifestudios

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Re: NG Supply indicator lamps
« Reply #19 on: July 14, 2019, 08:03:11 pm »

Thanks Uwe.  Yes. Very helpful.  So I believe that would be where the arrows are in this picture?

(Transformer primary lead wire colours are not consistent with schematic.)

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klaus

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Re: NG Supply indicator lamps
« Reply #20 on: July 14, 2019, 08:20:40 pm »

You are getting the light to illuminate, so your connections are correct. No need to fuss with it further. Uwe (!) explained the mathematical calculation of the slow rise to nominal voltage. I concur from my experience.

Let's not focus on B+ or the heater supply alone (by the way, 107VDC will give you almost exactly 36V on the heater in many cases of NG + U47 + VF14 I have observed): a good portion of the health of the tube can be deduced from the plate voltage generated. Anything above 32VDC is fine, anything much below this will be audible as tube noise. Observe* that voltage 5 minutes after start-up, then again after a few hours.  After a long time on the shelf, sometimes a lower than nominal plate voltage of  VF14 will come back up, once the cathode is freed of deposits.

* Don't do it if you are not familiar with the mic's circuit. The voltage inside the mic can be deadly. Even experienced technicians will keep one hand behind their back when measuring tube mic voltages!
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Klaus Heyne
German Masterworksģ
www.GermanMasterworks.com

afterlifestudios

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Re: NG Supply indicator lamps
« Reply #21 on: July 14, 2019, 08:31:14 pm »

Thank you Klaus and Uwe. (So sorry Uwe!)

I was getting 40VDC at the plate when b+ was 102 last week, so I think Iím all set.  Most importantly, they sound incredible.

Unless any problems arise, I will check B+ at the PSU every 2 months just to keep an eye on things.

Aside: While I was searching for photos of insides of NG supplies online, I found this one.  Seems like red and brown PT leads are reversed from schematic?  I think itís ok because it is strapped for 220VAC so they are still in series.  But if this thing got strapped for 110VAC, it would go poof, right?

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