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Old 26th August 2008, 10:34 PM   #1
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Default Thermistor B+ or primary

One of the projects on the docket will use SS voltage doubler. I had intended to use a thermistor on the B+ to limit initial current at turn on but I have noticed several cases where putting it on the primary side. It would seem that this would do little to allow heater warmup before the application of significant B+. It would help the heaters of course.

So am I correct in putting the thermistor in the B+ line rather than tranny primary? Before or after the rectifier/doubler?
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Old 27th August 2008, 12:28 AM   #2
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Mike,

If a bias supply is powered from the same trafo as B+, it is ESSENTIAL that inrush current limiting be associated with the B+ winding. Even the slight delay in B+ rise the thermistor(s) yield is enough for the "instant on" SS rectified bias supply to electrostatically protect the "finals" against cathode stripping.
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Old 27th August 2008, 12:49 AM   #3
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I always thought that thermistors on the primary were to protect against blowing fuses. You could use a slo-blo or a thermistor on the primary, same effect.
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Old 27th August 2008, 02:06 AM   #4
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Quote:
Originally posted by SpreadSpectrum
I always thought that thermistors on the primary were to protect against blowing fuses. You could use a slo-blo or a thermistor on the primary, same effect.

LARGE caps. are the norm in a doubler B+ PSU. That's where the bulk of the turn surge occurs. Putting the inrush limiting thermistor(s) on the rectifier winding stops power trafo clang and slightly delays B+ rise. Please refer to a McShane modified H/K Cit. 2.
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Old 27th August 2008, 04:03 AM   #5
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Hmm. I had heard that blowing fuses can sometimes be a problem with large transformers due to large currents in initially magnetizing the core or something. I've never had the problem myself, but had heard that rationalization before, probably from someone like me who just repeated it even though he had no real knowledge.

I don't know. I just thought that the other designs that mashaffer had run into were trying to accomplish this with the thermistor on the primary. I completely agree with Eli's appraisal and have thermistors in the B+ path on my own amplifier.
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Old 27th August 2008, 04:55 AM   #6
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Quote:
Originally posted by Eli Duttman
[B]


Putting the inrush limiting thermistor(s) on the rectifier winding stops power trafo clang and slightly delays B+ rise.
Disagree ? Yup... From my service bench I used to get more equipment returns with a defect thermistor fitted in the B+. Why ? because so much equipment these days is quickly turned on/off while thermistor is still hot with a long time constant and then eventually goes zap with the next inrush. TV's were typical..There are better solutions using more components, and those of us who live in "brown out" areas, know the problem too well. The relay switching out a wirewound power resistor with quick reset sorts most problems out.

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Old 27th August 2008, 11:42 PM   #7
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I'm not aware of thermistors being used to delay B+. The normal applicaton is in the PT primary, to guard against inrush current caused by th elow initial resistance of cold heaters (for the first fraction of a second) and, with SS rectifiers, the surge of charging smoothing capacitors from zero volts. It normally operates over a relatively short time period.

If you want to delay B+ until negative bias is high enough, use a voltage-controlled switch (e.g. relay driven by a Schmitt trigger), which will do double duty of killing B+ if the bias ever fails.

If you want to give a delay + slow start to SS-rectified B+, so it will climb slowly to operating voltage only after the tube heaters in the amp are hot, either put a damper diode in series with B+ or use a series MOSFET with a delay on the gate.
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Old 27th August 2008, 11:50 PM   #8
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Once again:

http://amfone.net/Amforum/index.php?topic=4842.0

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Old 28th August 2008, 12:43 AM   #9
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Ahh, clever idea Ray. The main problem is the use of a tranny that requires a doubler. Thus the need for SS rectification. Adding a damper after the doubling still gives us the gradual startup.

I suppose a tube doubler could be done but most rectifier tubes are single cathode I think and the diodes I have seen with separate cathodes can't take the current.

Of course my particular project won't have a lot of space for rectifiers or dampers.
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Old 28th August 2008, 09:07 AM   #10
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You might like to try regulation using a MOSFET, which gives you the opportunity to delay the gate voltage for as long as you like. My favorite approach is to use all three safeguards: thermistor on the primary, B+ disconnect if bias fails AND B+ delay. Can't do any harm and it's fun! (Of course, if you're using cathode bias on the OP tubes, then the bias supply doesn't enter into it.)
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