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Hum Problem

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I did a search and could find anything that could help my situation.

Anyway,I have built a line level preamp using 6SN7's,and I have built this amp with a solid state external power supply,and it has worked great,very low noise that was only audible with the gain turned all the way up,and then it was just a slight buzz,well I decided to go with a tube based power supply using a 5AR4,I plugged it all in and powered up the preamp and then the amp and right off the bat, a very noticable hum,even with the gain turned all the way down on the preamp. I disconnected everything checked my voltages and everything measured just fine,I checked for possible ground loops in the preamp and none could be found,I'm at a loss as to the problem.


The power supply consists of a 275-0-275 50mA transformer connected to a 5AR4 rectifier it then passes through a LCLC filter consisting of a 2- 47uf 450V capacitors,and 2- 15H 30mA chokes which then passes on to 2-12K ohm plate resistors connected to 2-100uF caps,and also I am using a DC supply for the filaments.

Thanks

Denny
 
Jeb-D. said:
Those component choices sound like they would have a tendency to ring. Download PSUD2 and model a power supply first. Run a search for "PSUD2" Here. So you can get tips, such as using stepped loads to look for instabilities.

After doing some reading and trying the software,I think my component choices were a little off I think my caps are too small,and my chokes could be a little larger,could this be the problem?
 

cbutterworth

Member
2006-10-03 11:02 pm
Denny,

When using a LC filter, there has to a minimum current draw for your chokes to operate correctly. Take your B+ voltage and divide it by the inductance - this is minimum that the current draw should be. This means that for 25mA and a 15H choke, your B+ should measure at something like 375VDC. If it is less than this, then the choke will be less likely to filter sufficiently.

Try adding between 0.6uF and 6uF in parallel in front of your first choke. This would make you PSU more of a cap input filter - CLCLC. The effect of this will be to increase your B+, which will be higher based on the value of the "tuning" cap. However, with a lower value, PSUD will show more ripple.

For example, I have a PSU that is very similar to yours: 275-0-275 into a 5V4GA; 5uF; 10H; 45uF; 10H; 80uF. When I run four 6SN7's for a total of around 24mA, I get something like 450V at B+ (I am running an Aikido, so the 6SN7's only see something like 220 V on each plate). Hum is very low.

When I swap out the inputs for 6SL7's, the total current draw drops to around 18mA and the hum, especially the 120Hz hum increases but it is still very, very low.

If you do not want to increase your B+ dramatically, then I would try initially a 0.6uF cap and increase it from there. When I used 0.6uF, my B+ was around 430V, so I got only around 20V increase when I moved up to a 5uF cap.

If you're careful with your settings in PSUD, it can model very close to empirical values. It predicts very close for me (to within 4V or so). When I add the trafo, I input the off-load voltage and current capacity of the trafo and use 10% for efficiency.

Good Luck,

Charlie
 
Around 25mA.

I'll try to model it for you if I get a chance. Can you measure the primary and secondary DC resistance of your transformer? Generally when you strand togeather multiple LC stages a power supply can be prone to ringing and instabilities due to phase shifting and resonance. Especially when the inductors in each stage are of equal value and the caps are equal value.

Your grounding scheme hasn't changed since switching to tube rectification right?
 
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