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Old 3rd March 2010, 02:02 PM   #1
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Default tubelab se R3

Question: I have 5.1 V on the filaments of the drivers of my tubelab SE. Filaments of the 300B is correct 4.9something V.

Shouldn't be 6.3 ? I lose 1 V across R3, and this is also correct 2.2 ohm, 0.6 A, makes 1V drop. What's the role of R3 ? What if I shorten it ? What if I put a 6.3 V regulator instead ?

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Davide
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Old 3rd March 2010, 02:24 PM   #2
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No your on the money at 5v. The SE can be setup for 2.5v or 5v heater.

Ok go with me on this:

Yes most heaters and the transformer are 6.3 vac
They key is vac
once rectified it turns to 8.88vdc less diode drop
The limit of a 300b is like 5.3v and you can run them at lower 4.3v with longer life but reduced output, 5vcd is the sweet spot

http://www.emissionlabs.com/datasheets/EML300B.htm

So depending on your build you want to see 5vcd or 2.5vcd based on what jumpers you installed.

Last edited by nightanole; 3rd March 2010 at 02:38 PM.
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Old 3rd March 2010, 02:44 PM   #3
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Is it a general rule ? From the data-sheet of the 5842 I understand that the heater should be fed with 6.3 VAC or 6.3VDC. At the moment I am feeding them with 5.2VDC +0.3V ripple.
The jumpers set the voltage of the power tubes, the drivers should stay at 6.3V.

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Davide
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Old 3rd March 2010, 03:40 PM   #4
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8.88p - .7 for diode = 8.18
63.7% for average voltage = 5.21vdc

I dont know the value a hot heater so for all i know after the 2.2ohm it should be about 5.2v unloaded.

Eh i might not know what im talking about...
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Old 3rd March 2010, 03:41 PM   #5
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That sounds low, yes. I never measured mine. I wonder if it's your transformer or maybe those 1N5401s. They ran smoking hot on mine until I put Schottky diodes in there. What is the voltage relative to ground going into U1? What is the AC voltage of your 6.3V winding under this load? What transformer are you using?
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Old 4th March 2010, 03:43 AM   #6
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Actually I have the Schottky as you, with snubber caps. Wondering if it's a side effect of these tweaks. I have 6.4 VDC at the input of the regulator (same as R3). Doing some test now.

D.
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Old 4th March 2010, 04:42 AM   #7
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Nop, everything works fine. As it was obvious Schottky diodes should raise the voltage (and they do). Should I leave it like this, or bypass r3 ?

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Davide
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Old 4th March 2010, 10:49 AM   #8
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Oh yeah...sorry I forgot what R3 was. Should drop about 1.3V across there. Not sure why he did that, but there is that whole thing about improving linearity by running cathodes a little cooler. Maybe that's what this is. I never measured the 5842 heater voltage on my amp, but what you see makes sense. George?
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Old 4th March 2010, 12:47 PM   #9
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You always discover new things !

P.S.: I almost blowed everything today. George I think this is the most robust amp ever. I really mistreated.
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