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Novice Filament Advise Needed.
Novice Filament Advise Needed.
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Old 24th November 2021, 10:10 PM   #1
WntrMute2 is offline WntrMute2  United States
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Location: Detroit area
Default Novice Filament Advise Needed.

I'd like to modify my 300B Radiotorn amps to use DC rather than the AC I've been using. I have both 2.5V and 5V versions of the 300B and would like to use both with minor adjustments or switching from outside the amp. I've done this with AC as I just switch between the 5V transformer and its center tap. Works well in this configuration but I would like to get the hum lower.
I have both Rod Coleman regulators as well as Pete Millett's available. I've tried Millett's with a 6.3V 8A transformer and while I can adjust the voltage down from 5V to 2.5V the temperature of the regulator goes up to 90 C when going to the lower voltage. I think that may be pushing things. I have tried the center tap of a 6.3V 8A transformer but I cannot get the voltage needed - 1.9V is the maximum. I have some bigger heatsinks but oddly they have a larger C/W rating - 4.8 vs 3.8. I am hesitant to try those as a remedy.
I need some expertise as what avenue to explore. Should I just use two different transformers and switch between the two? That method adds some pretty hefty iron and expense.

Last edited by WntrMute2; 25th November 2021 at 11:54 AM.
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Old 25th November 2021, 03:03 AM   #2
Tom Bavis is offline Tom Bavis
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Dissipation in a series regulator can be decreased by using a series or shunt resistor. A series resistor can be added for 2.5V output, dropping some voltage before the regulator. Depending on the value, there may be regulator drop-out at turn-on, which would give a softer start.

A shunt resistor bypasses some current - it should be possible to choose one which works across the adjustment range. The disadvantage of a shunt is that voltage will rise below a minimum current, but a tube heater is not a variable load. So you obviously can't measure the voltage unloaded.
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Old 25th November 2021, 09:48 AM   #3
euro21 is online now euro21  Hungary
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Novice Filament Advise Needed.
R.C. regulator requires at least 4-5V headroom between raw DC and regulated DC voltage.So You must use 9V AC -more than 25VA- transformer to achieve 9..10V raw DC.

The dissipation of R.C.= headroom voltage*I heater.
Attached Images
File Type: jpg 300B raw supply for R.C.jpg (85.0 KB, 59 views)

Last edited by euro21; 25th November 2021 at 09:53 AM.
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