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6.3 V heater supply problem

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I have my TSE-II board built and running on the bench. It works well. But I have a problem with the heater circuit for the 5842 tubes.

With a normal 6.4 VAC supplied from the PT to the board, I only have 6.6 VDC on the capacitor side of R3 and 5.5 VDC on the heater side of R3.

I should see 7 or 8 VDC on the high side of R3 and 6.3 VDC on the heaters for the 5842 tubes.

Any ideas? A bad diode?

I hate to run 6.3 V tubes on 5.5 V.
More data

The PT heater winding has at least 4 amp capacity. The 6.36 VAC supplied by the PT to the PCB remains steady and the transformer shows no sign of stress.

The voltage drop across 2.2 ohm R3 is as expected for a .55 to .6 amp current draw for two 5842 tubes. Tube data sheets spec .3 amp each.

Here are some exact measurements I just took this evening, repeating almost exactly the measurements I took yesterday:

6.36 V AC entering the PCB

4.97 V DC on the 300B heaters
3 mV AC on the 300B heaters

5.45 V DC on the 5842 heaters
79 mV AC on the 5842 heaters

The voltage regulator IC3 heatsink feels quite hot after only short time in service. I can hold a finger on it, but not comfortably. It has plenty of air space around it on the current workbench setup, but no fan.

It seems to me that I have good measurements for the regulated 300B heaters.

It seems to me that the 5842 heaters have low DC voltage and too much AC noise.

I will pursue a better heatsink and a little fan for the IC3 heat situation. But IC3 is mainly here for the 300B's - and the heater supply to them is looking good.
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.

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.

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).
Joined 2011
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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