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Posted by Triode_Kingdom (A) on July 24, 2015 at 22:15:48

In Reply to: RE: Low Voltage to Heaters posted by Caucasian Blackplate on July 19, 2015 at 10:59:37:

The OP has resolved the problem now, but I want to expand on what you've said. I still don't see a problem substituting the Schottky diodes.

"One could get less forward voltage drop from a diode rated for lower voltages."

The Schottky replacements exhibit a voltage drop roughly the same as the diodes in the original bridge. Lower drop diodes could be used, but this filament supply isn't regulated. It would seem beneficial not to install parts that might raise the voltage above that of the original design.

"The 502 will pop up about 70C per Watt, so one would expect a temperature rise of more than 100C, which will simply cook the diodes where they sit."

Unless I'm misinterpreting the modification, each bridge that was replaced is powering two 6SN7s. That's 1.2A. Each diode of the bridge sees less than that (average), but even using the full current for calculations, that's about 0.8W per diode. This will cause the VT5202 junction to rise to about 69C in free air if ambient is 25C. The junction is rated to 175C.

"A 1N5820 will exhibit just a hair more than half these losses."

The 1N5820 will drop 0.475V at it rating of 3A. That's a dissipation of 1.425W. The VT5202 Schottky datasheet shows a dissipation of about 1.95W at that current. That means the 1N5820 will dissipate roughly 73% of the power of the Schottky. That doesn't seem a very significant difference to me, particularly given that A) the bridge is operating well under 3A, so real dissipation differences are very small, and B) the VT5202 Schottkys will exhibit power dissipation virtually identical to the bridge they replaced.

If you think any of this is in error, let me know. I just don't see anything here that justifies telling the OP he selected the wrong diodes.

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Follow Ups Full Thread

Topic - Low Voltage to Heaters - mashley 17:01:28 07/17/15 (17)

Low Voltage to Heaters - mashley 15:27:17 07/21/15 (2)

RE: Low Voltage to Heaters - danlaudionut 15:51:40 07/21/15 (1)

Low Voltage to Heaters - mashley 12:25:33 07/23/15 (0)

Low Voltage to Heaters - mashley 11:59:39 07/21/15 (1)

RE: Low Voltage to Heaters - danlaudionut 12:16:32 07/21/15 (0)

RE: Low Voltage to Heaters - Caucasian Blackplate 11:22:10 07/18/15 (9)

Low Voltage to Heaters - mashley 16:45:19 07/18/15 (8)

RE: Low Voltage to Heaters - Caucasian Blackplate 16:54:49 07/18/15 (7)

RE: Low Voltage to Heaters - Triode_Kingdom 08:27:31 07/19/15 (5)

RE: Low Voltage to Heaters - Caucasian Blackplate 10:59:37 07/19/15 (4)

FOLLOWUP... - Triode_Kingdom 22:15:48 07/24/15 (0)

Low Voltage to Heaters - mashley 19:34:42 07/19/15 (2)

RE: Low Voltage to Heaters - Triode_Kingdom 20:19:31 07/19/15 (1)

Low Voltage to Heaters - mashley 22:28:00 07/19/15 (0)

Low Voltage to Heaters - mashley 17:07:12 07/18/15 (0)

RE: Low Voltage to Heaters - Triode_Kingdom 19:18:40 07/17/15 (1)

Low Voltage to Heaters - mashley 15:05:12 07/18/15 (0)

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FOLLOWUP... - Triode_Kingdom 22:15:48 07/24/15 (0)

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