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Rectifier Tubes - Exceeding input filter capacitor
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Author:  HT Performance [ 20 Aug 2011, 13:37 ]
Post subject:  Rectifier Tubes - Exceeding input filter capacitor


I´m using the 50DC4 tube for the audio amplifier 50C5 stage in my HiFi FM Tuner.

It happens that there is still some HUM, which is only noticed with a good speaker or with headphones.

The 50DC4 rectifier tube data says 40uF for the input filter capacitor.

I am using 33uF and have nearly 25v ripple, but when I place another capacitor (total 55uF) the hum is very very low. And if running 66uF in total for input, the is gone for good!

I cannot play very much with the filter stage as i´m getting the exact 115v that the driver tube needs (50C5). This circuit was some sort of add-on to the project, as it was only intended for a receiver (rca outputs), but later I added the small amplifier. I don´t have enough voltage / current to play with...

How bad can this be for the rectifier tube?

Any opinions or suggestions are very welcome! :)

Maybe I can discard some of the audio power output and run the 50C5 tube at 90v instead of 110V?


Author:  gofar99 [ 20 Aug 2011, 14:41 ]
Post subject:  Re: Rectifier Tubes - Exceeding input filter capacitor

Hi, Nothing a bit of smoke, fire, sparks or arcs ..... The 50C5 will work at a lower B+. What I would do is separate the capacitors with a 50-100 ohm resistor and turn it into a two stage filter. This will give you much cleaner B+ and keep the rectifier from becoming toast. The reduction in power output will not be all that much. BTW... are you running this directly off the AC mains? :o I hope not as it is rather dangerous.

Good Listening

Author:  HT Performance [ 20 Aug 2011, 15:17 ]
Post subject:  Re: Rectifier Tubes - Exceeding input filter capacitor

Hi Bruce!

I just changed the filter and now the 50C5 is running at 90v with 5.5v on cathode.

The 90v allowed me to minimize the AC ripple.

There is almost no hum right now, but the output power decreased a little bit.

This is a trade, and I gladly accepted it.

In fact I didn´t need full power for a kitchen radio...

This makes a wonderfull kitchen radio, it is much better than lots of mini hi-fi stereos that "people" run on other places.

The radio is working with isolating transformers. It is 100% safe! :)

Like the Oddwatt I built, this radio also uses AC heaters (in series) and there is no hum!

I am very happy! Thanks for your input! :)


P.S. - I´m happy to know about Ben!

Author:  Suncalc [ 20 Aug 2011, 16:06 ]
Post subject:  Re: Rectifier Tubes - Exceeding input filter capacitor

HT Performance wrote:
How bad can this be for the rectifier tube?
The only number you really need to worry about is the maximum steady-state peak plate current limit of 720mA.

In general, the typical conditions given in most rectifier data sheets result in a peak steady state plate current of approximately four times the stated load current. You'll notice that the "typical" data given in the data sheet (without panel lamp) has a 110mA load current with the 40µf capacitor. The peak steady state plate current in this case is probably about 450mA. This is far below the 720mA limit giving lots of margin. As you increase the size of the first filter cap, the conduction angle will decrease and the peak plate current will increase. You just need to make sure you don't violate the 720mA limit. We also need to keep in mind that when these data sheets were written, a 40µf electrolytic capacitor was +/- 20% or even +/-50%, so lots of margin in the design point was required.

If you have any series resistance in line with the plate, examining the voltage waveform across this known resistance with an oscilloscope will let you determine the peak steady state plate current. Then you can just increase your first filter capacitor until you get close to the 720mA limit.

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