Hi All,
I’ve gotten into tube-based gear the last few years, starting with building others’ designs and now working more on my own. I built the Bob Heil Pine board AM transmitter (Jan 2018 QST, also removed link ), with matching power supply and always noticed a lot of hum on the received signal, even when using 6V DC on the filaments. I built a copy of the power supply for more tube projects/experiments and have the same hum on the transmitter, and noticed a lot of very strange behavior with other circuits. A W5LDO 6AQ5 transmitter ( removed link ) won’t oscillate and has large drops in voltage when keyed. Also tried a simple circuit as an emission tester for a tube and that was giving very strange results.
My question/concern is around the filter network (CRC with bleeder resistor). 33K seems high for the resistors, but I can’t quite articulate why. Does anything this this is contributing to the hum, possible ripple coming off the power supply? I have a basic understanding of how voltage dividers work, and the function of bleeder resistor. I’m struggling to understand the choice of resistor values in the CRC network, and if this might be contributing to the large hum on received AM signal. When using solid state rectification, the schematics all say 33K 2W, and most of the BoMs also mention 33K 2W, while the text of the ARRL article (first link) mentions 3.3K 2W.
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From article: “bridge rectifier provides +350 V dc from the full-wave rectifier circuit. This is followed by two 20 µF capacitors along with a voltage divider comprised of two 3.3 kΩ, 2 W resistors. One hundred and twenty volts feeds the mic preamp”
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I have a decent oscilloscope Keysight DSOX1102G, but it only can handle 150V max on the probes and I’m kinda worried about frying it. Also just received a Heathkit EUW-15 from eBay that needs some restoration love and may try that as a separate high voltage power supply to isolate that issue. Regardless, I would love to get my Heil design power supplies working. Any suggestions would be appreciated!
Welcome, several things from my view. What probe are you using on the scope? If using a 1X probe, then yes the voltage seen by the scope will be just that. Using a 10X or 100X will reduce the signal seen by the scope, these are used when measuring strong signals or when you need to minimize circuit loading from the probe.
Next, it is very unlikely a design issue if you followed a predesigned layout and schematic. Also, if you have more than one device having issues that could point to something else making noise. As with anything homemade, go back and triple check all connections and components. Swap things around, trace signals, etc. There are many possibilities on where the noise is coming from, try to isolate each device and determine if it is operating as it should.
For the resistors, 33K is pretty high for being between two caps for a power supply. At most this would lower the output voltage as far as I can think. It really all depends on the design of the circuit as to what the optimal values are. Two 20uf caps is not much filtering if there is no other form of filtering (choke/inductor) and if it is a halfwave design even more lack of filtering. 3.3K does sound more normal for a pie filter setup over 33K.
That is my thoughts, not a straight forward answer unfortunately, more so a direction with things to think about.
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