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Old 17th December 2019, 12:10 AM   #11
rayma is offline rayma  United States
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Try doubling the filter capacitance. This will both raise the DCV, and lower the noise.
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Old 17th December 2019, 01:22 AM   #12
Tubelab_com is offline Tubelab_com  United States
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I only have 6.6 VDC on the capacitor side of R3
I get a bit over 7.5 volts but my line voltage is 124 and the Hammond transformer I'm using makes around 7 volts on the 6.3 volt winding. I am currently in a hotel about 1000 miles from my lab so I don't have access to my notes or the workbench.

Quote:
A 6.3 to 6.4 VAC supply through two diodes should give me about 7-1/2 VDC.
When configured for 300B's or any 5 volt tube the heater supply becomes a 4 diode full wave bridge. One pair of diodes are the dual Schottky. The part number shown in the BOM was chosen for it's low voltage drop. These are the only diodes in the path to the LDO when configured for a 2.5 volt tube. This is important for 2A3's where the total current draw is 5 amps. Some other dual Schottky's may drop over 0.6 volts

The other pair are ordinary silicon diodes and can drop almost a volt when run near the maximum ratings. These can be swapped out for Schottky's to gain half a volt or so. I do not have the part number handy right now.

The value of R3 can be changed as needed. When I built amps for sale many years ago with whatever power transformer I had, the customer wanted, I adjusted R3 to put 6.0 to 6.3 volts on the 5842's.

The 5842's run from the raw heater DC supply. 79 mV of ripple is not excessive, and it is a bit better than I remember with my boards. DC with 100+ mV of ripple is quieter than 6.3 volts of AC.

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Try doubling the filter capacitance. This will both raise the DCV, and lower the noise.
The board has provisions for up to 3 X 15,000uF caps. In this case I would populate all three. I have built 300B based boards with only one cap for testing, but I usually use two. All three are needed with 2A3 tubes where 5 amps of heater current will put the LDO near dropout under low line conditions (108 VAC or less).
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Old 17th December 2019, 11:54 AM   #13
justmikey is offline justmikey  United States
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I think I will simply replace R3 with whatever I can find in my junk box that's between 1/2 and 1 ohm of suitable wattage. That should get me to something between 6 and 6.3 VDC on the 5842 heaters. Then I'll rig up a small fan to blow across the IC3 area of the board.
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Old 17th December 2019, 03:39 PM   #14
rayma is offline rayma  United States
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Bear in mind that the required wattage of R3 will be higher than you might think from the DC voltage drop,
since most of the ripple is also dropped across R3. Often the dissipation due to the AC drop is about the same as,
or higher than, the DC drop.
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Old 17th December 2019, 03:41 PM   #15
justmikey is offline justmikey  United States
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I believe the BOM part is 3W and I probably have some 5W Mills resistors around here somewhere.
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