Lamp in HV Centre Tap - why? - diyAudio
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Old 12th January 2012, 03:28 AM   #1
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Default Lamp in HV Centre Tap - why?

I am working on 2 1950's amp restorations at the moment.
One a SE 6V6 and one a PP 807 Output.
Both use Tube rectifiers and both have a standby switch between the HV centretap and 0V AND a 6.3V 300mA lamp in series with the switch to 0V.

These lamps are not front panel indications but are "buried" inside.

I have been unable to formulate any reason why the lamps are included except perhaps that they provide a fuse function and an indication of when the High Voltage is ON.

Is there any other reason why this might have been done?

Thanks,
Ian
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Old 12th January 2012, 03:43 AM   #2
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inrush current resistance?
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Old 12th January 2012, 04:02 AM   #3
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aardvarkash10,
I don't think so - I considered that, but as lamps have much lower cold resistance (at switch on) than hot I concluded that it would not limit the in-rush current all that much.
Cheers,
Ian
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Old 12th January 2012, 04:06 AM   #4
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yep, I tought that too, but it was the only semi-logical answer I could see...
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Old 12th January 2012, 04:10 AM   #5
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Thinking about it some more I think you are probably right - it was probably the best "cheap" solution to inrush current limiting available in those days before negative Temperature Coefficient (NTC) Thermistors were invented. It would help a bit with rectifier tube life.
Cheers,
Ian

I like your tag line - I do a fair bit of time and budget estimation for designs in the day job and I always tell folk that the first half of the project takes 90% of the time and the other half takes the other 90% of the time.

Last edited by gingertube; 12th January 2012 at 04:13 AM.
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Old 12th January 2012, 08:08 AM   #6
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Yes, a simple inrush limiter seems like a reasonable explanation, though resistors in series with rectifier plates would be a better alternative, spec sheet for the rectifier (min series resistance incl resistance of HT winding) is kinder to rectifier.
Aardvarkash, agree with sentiment in your sig, I keep trying to explain to friends that there is no such thing as a "finished" amp, it is always a moving target!
Built a new amp during holiday on a scruffy used chassis, strictly proof-of-concept, sounds decent, but I am not ready to post it for comment until I am over the initial "I made it so it must be OK" moment - sorry if I am threadjacking
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Old 12th January 2012, 08:19 AM   #7
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lol - got THREE of those poc things on the lounge floor as we speak...

Has anyone fond how to get solder burns out of carpet?
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Old 12th January 2012, 09:07 AM   #8
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Haha, not this week, hope you have wool carpet, when splattered it just smells bad, pick the splats out with tweezers, synthetic not so good...(it melts) I try to keep soldering activities in spare room with a mat to catch the splats, mostly works, most days...

Last edited by radiotron; 12th January 2012 at 09:10 AM. Reason: bad grammar
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Old 12th January 2012, 11:30 AM   #9
Enzo is offline Enzo  United States
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How about a... well, "outrush" limiter? (I made it up) If draw gets excessive on the supply, the lamp's current heats it, increasing its resistance. That tends to help soft limit B+ current.

If the lamps are not readily visible from the outside, that pretty much eliminates the lamp as indicator. The 1950s gear wasn't usually designed with diagnostic internal circuits, so we'd assume the lamp served the circuit in some other way than that.
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Old 12th January 2012, 11:42 AM   #10
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Quote:
Originally Posted by Enzo View Post
If draw gets excessive on the supply, the lamp's current heats it, increasing its resistance. That tends to help soft limit B+ current.
You mean it works as a crude voltage stabilisator ?
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